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User Manual: Roundabout

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CONTENTS

Glossary
85th-percentile speed—a speed value obtained from a set of field-measured speeds where
only 15 percent of the observed speeds are greater (source: HCM 2000).
AADT—see average annual daily traffic.

AASHO—American Association of State Highway Officials. Predecessor to AASHTO.
AASHTO—American Association of State Highway and Transportation Officials.
accessible—describes a site, building, facility, or portion thereof that complies with the Americans with Disabilities Act Accessibility Guidelines (source: ADAAG).
accessible route—a continuous, unobstructed path connecting all accessible elements and
spaces of a building or facility. Exterior accessible routes may include parking access aisles,
curb ramps, crosswalks at vehicular ways, walks, ramps, and lifts (source: ADAAG).
accident—see crash.
ADA—Americans with Disabilities Act.
ADAAG—Americans with Disabilities Act Accessibility Guidelines.
all-way stop control—all approaches at the intersections have stop signs where all drivers
must come to a complete stop. The decision to proceed is based in part on the rules of the
road, which suggest that the driver on the right has the right-of-way, and also on the traffic
conditions of the other approaches (source: HCM 2000).
angle, entry—see entry angle.
approach—the portion of a roadway leading into a roundabout.
approach capacity—the capacity provided at the yield line during a specified period of time.
approach curvature—a series of progressively sharper curves used on an approach to slow
traffic to a safe speed prior to reaching the yield line.
approach road half-width—term used in the United Kingdom regression models. The approach half width is measured at a point in the approach upstream from any entry flare, from
the median line or median curb to the nearside curb along a line perpendicular to the curb. See
also approach width. (source: UK Geometric Design of Roundabouts)
approach speed—the posted or 85th-percentile speed on an approach prior to any geometric
or signing treatments designed to slow speeds.
approach width—the width of the roadway used by approaching traffic upstream of any
changes in width associated with the roundabout. The approach width is typically no more
than half the total roadway width.
apron—the mountable portion of the central island adjacent to the circulatory roadway. Used
in smaller roundabouts to accommodate the wheel tracking of large vehicles.
average annual daily traffic—the total volume passing a point or segment of a highway
facility in both directions for one year divided by the number of days in the year (source: HCM
2000).
average effective flare length—term used in the United Kingdom regression models. Defined by a geometric construct and is approximately equivalent to the length of flare that can
be effectively used by vehicles. (source: UK Geometric Design of Roundabouts)
AWSC—see all-way stop control.
back of queue—the distance between the yield line of a roundabout and the farthest reach of
an upstream queue, expressed as a number of vehicles. The vehicles previously stopped at
the front of the queue may be moving (adapted from HCM 2000).

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benefit-cost analysis—a method of economic evaluation that uses the benefit-cost ratio as
the measure of effectiveness.
benefit-cost ratio—the difference in benefits between an alternative and the no-build scenario, divided by the difference in costs between the alternative and the no-build scenario.
See also incremental benefit-cost ratio.
bulb-out—see curb extension.

capacity—the maximum sustainable flow rate at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a specified
time period under a given roadway, geometric, traffic, environmental, and control conditions.
Usually expressed as vehicles per hour, passenger cars per hour, or persons per hour (source:
HCM 2000).
capacity, approach—see approach capacity.
capacity, roundabout—see roundabout capacity.
capital recovery factor—a factor that converts a present value cost into an annualized cost
over a period of n years using an assumed discount rate of i percent.
central island—the raised area in the center of a roundabout around which traffic circulates.
CFR—Code of Federal Regulations.
channelization—the separation or regulation of conflicting traffic movements into definite
paths of travel by traffic islands or pavement marking to facilitate the safe and orderly movements of both vehicles and pedestrians (source: 1994 AASHTO Green Book).
circle, inscribed—see inscribed circle.
circular intersection—an intersection that vehicles traverse by circulating around a central
island.
circulating flow—see circulating volume.
circulating path radius—the minimum radius on the fastest through path around the central
island.
circulating traffic—vehicles located on the circulatory roadway.
circulating volume—the total volume in a given period of time on the circulatory roadway
immediately prior to an entrance.
circulatory roadway—the curved path used by vehicles to travel in a counterclockwise fashion around the central island.
circulatory roadway width—the width between the outer edge of the circulatory roadway
and the central island, not including the width of any apron.
circulating speed—the speed vehicles travel at while on the circulatory roadway.
community enhancement roundabout—a roundabout used for aesthetic or community
enhancement reasons, rather than as a solution to traffic problems. When used, often located
in commercial and civic districts.
conflict point—a location where the paths of two vehicles, or a vehicle and a bicycle or
pedestrian, merge, diverge, cross, or queue behind each other.
conflict, crossing—see crossing conflict.
conflict, diverge—see diverge conflict.
conflict, merge—see merge conflict.
conflict, queuing—see queuing conflict.
conflicting flows—the two paths that merge, diverge, cross, or queue behind each other at
a conflict point.
control delay—delay experienced by vehicles at an intersection due to movements at slower
speeds and stops on approaches as vehicles move up in the queue.

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crash—a collision between a vehicle and another vehicle, a pedestrian, a bicycle, or a fixed
object.
crash frequency—the average number of crashes at a location per period of time.
crash rate—the number of crashes at a location or on a roadway segment, divided by the
number of vehicles entering the location or by the length of the segment.
CRF—see capital recovery factor.
crossing conflict—the intersection of two traffic streams, including pedestrians. Crossing
conflicts are the most severe type of conflict.
curb extension—the construction of curbing such that the width of a street is reduced. Often
used to provide space for parking or a bus stop or to reduce pedestrian crossing distances.
curb ramp—a short ramp cutting through a curb or built up to it (source: ADAAG).
curvature, approach—see approach curvature.
D factor—the proportion of the two-way traffic assigned to the peak direction.

deflection—the change in trajectory of a vehicle imposed by geometric features of the roadway.
degree of saturation—see volume-to-capacity ratio.
delay—additional travel time experienced by a driver, passenger, or pedestrian beyond what
would reasonably be desired for a given trip.
delay, control—see control delay.
delay, geometric—see geometric delay.
demand flow—the number of vehicles or persons that would like to use a roadway facility
during a specified period of time.
departure width—the width of the roadway used by departing traffic downstream of any
changes in width associated with the roundabout. The departure width is typically no more
than half the total roadway width.
design user—any user (motorized or nonmotorized) that can be reasonably be anticipated to
use a facility.
design vehicle—the largest vehicle that can reasonably be anticipated to use a facility.
detectable warning surface—a standardized surface feature built in or applied to walking
surfaces or other elements to warn visually impaired people of hazards on a circulation path
(source: ADAAG).
diameter, inscribed circle—see inscribed circle diameter.
distance, set-back—see set-back distance.
diverge conflict—the separation of two traffic streams, typically the least severe of all conflicts.
double-lane roundabout—a roundabout that has at least one entry with two lanes, and a
circulatory roadway that can accommodate more than one vehicle traveling side-by-side.
downstream—the direction toward which traffic is flowing (source: HCM 2000).
entering traffic—vehicles located on a roundabout entrance.

entering volume—the total volume in a given period of time on an entrance to a roundabout.
entry angle—term used in the United Kingdom regression models. It serves as a geometric
proxy for the conflict angle between entering and circulating streams and is determined through
a geometric construct. (source: UK Geometric Design of Roundabouts)
entry flare—the widening of an approach to multiple lanes to provide additional capacity at
the yield line and storage.
entry flow—see entering volume.

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entry path curvature—term used in the United Kingdom to describe a measure of the amount
of entry deflection to the right imposed on vehicles at the entry to a roundabout. (source: UK
Geometric Design of Roundabouts)
entry path radius—the minimum radius on the fastest through path prior to the yield line.
entry radius—the minimum radius of curvature of the outside curb at the entry.
entry speed—the speed a vehicle is traveling at as it crosses the yield line.
entry width—the width of the entry where it meets the inscribed circle, measured perpendicularly from the right edge of the entry to the intersection point of the left edge line and the
inscribed circle.
entry, perpendicular—see perpendicular entry.
exit path radius—the minimum radius on the fastest through path into the exit.
exit radius—the minimum radius of curvature of the outside curb at the exit.
exit width—the width of the exit where it meets the inscribed circle, measured perpendicularly from the right edge of the exit to the intersection point of the left edge line and the
inscribed circle.
exiting traffic—vehicles departing a roundabout by a particular exit.
extended splitter island—see splitter island, extended.

FHWA—Federal Highway Administration.
flare—see entry flare.
flare, entry—see entry flare.
flow, circulating—see circulating volume.
flow, demand—see demand flow.
flow, entry—see entry volume.
flows, conflicting—see conflicting flows.

geometric delay—the delay caused by the alignment of the lane or the path taken by the
vehicle on a roadway or through an intersection.
geometric design—a term used in this document to describe the design of horizontal and
vertical alignment and cross-sectional elements of a roadway.
give way—term used in the United Kingdom and Australia for yield.
“give way” rule—rule adopted in the United Kingdom in November 1966 which required that
all vehicles entering a roundabout give way, or yield, to circulating vehicles.

H
I

HCM—Highway Capacity Manual.
IES—Illuminating Engineers Society.
incremental benefit-cost ratio—the difference in benefits between two alternatives, divided
by the difference in costs between the two alternatives. See also benefit-cost ratio.
inscribed circle—the circle forming the outer edge of the circulatory roadway.
inscribed circle diameter—the basic parameter used to define the size of a roundabout,
measured between the outer edges of the circulatory roadway. It is the diameter of the largest circle that can be inscribed within the outline of the intersection.
interchange—a grade-separated junction of two roadways, where movement from one roadway to the other is provided for.

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intersection—an at-grade junction of two or more roadways.
intersection sight distance—the distance required for a driver without the right-of-way to
perceive and react to the presence of conflicting vehicles.
island, central—see central island.
island, median—see splitter island.
island, separator—see splitter island.
island, splitter—see splitter island.
ITE—Institute of Transportation Engineers.
K factor—the proportion of the AADT assigned to the design hour.

left-turn path radius—the minimum radius on the fastest path of the conflicting left-turn
movement.

level of service—a qualitative measure describing operational conditions within a traffic stream,
generally described in terms of service measures such as speed and travel time, freedom to
maneuver, traffic interruptions, comfort, and convenience.
line, yield—see yield line.
locking—stoppage of traffic on the circulatory roadway caused by queuing backing into the
roundabout from one of the exits, resulting in traffic being unable to enter or circulate.
LOS—see level of service.

maximum service volume—the maximum hourly rate at which vehicles, bicycles, or persons can be reasonably expected to traverse a point or uniform section of a roadway during
an hour under specific assumed conditions while maintaining a designated level of service.
(source: HCM 2000)

measures of effectiveness—a quantitative parameter whose value is an indicator of the
performance of a transportation facility or service from the perspective of the users of the
facility or service.
median island—see splitter island.
merge conflict—the joining of two traffic streams.
mini-roundabout—small roundabouts used in low-speed urban environments. The central
island is fully mountable, and the splitter islands are either painted or mountable.
model, crash prediction—see crash prediction model.
modern roundabout—a term used to distinguish newer circular intersections conforming to
the characteristics of roundabouts from older-style rotaries and traffic circles.
m.o.e.—see measures of effectiveness.
mountable—used to describe geometric features that can be driven upon by vehicles without damage, but not intended to be in the normal path of traffic.
multilane roundabout—a roundabout that has at least one entry with two or more lanes,
and a circulatory roadway that can accommodate more than one vehicle traveling side-byside.
MUTCD—Manual on Uniform Traffic Control Devices.

neighborhood traffic circle—a circular intersection constructed at the intersection of two
local streets for traffic calming and/or aesthetic purposes. They are generally not channelized,
may be uncontrolled or stop-controlled, and may allow left turns to occur left (clockwise) of
the central island.

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nonconforming traffic circle—see traffic circle.
nontraversable—see raised.

O&M costs—operations and maintenance costs.

peak hour factor—the hourly volume during the maximum-volume hour of the day divided by
the peak 15-minute flow rate within the peak hour; a measure of traffic demand fluctuation
within the peak hour.
pedestrian refuge—an at-grade opening within a median island that allows pedestrians to
safely wait for an acceptable gap in traffic.
perpendicular entry—an entry angle of 70 degrees or more.
PHF—see peak hour factor.
platoon—a group of vehicles or pedestrians traveling together as a group, either voluntarily
or involuntarily because of signal control, geometrics, or other factors.
point, conflict—see conflict point.
priority—the assignment of right-of-way to a particular traffic stream or movement.
progression, signal—see signal progression.

queue—a line of vehicles, bicycles, or persons waiting to be served by the system in which
the flow rate from the front of the queue determines the average speed within the queue.
Slowly moving vehicles or persons joining the rear of the queue are usually considered a part
of the queue. The internal queue dynamics may involve a series of starts and stops. (source:
HCM 2000)
queuing conflict—a conflict that arises within a traffic stream between a lead vehicle and a
following vehicle, when the lead vehicle must come to a stop.

radius, circulating path—see circulating path radius.
radius, entry—see entry radius.
radius, entry path—see entry path radius.
radius, exit—see exit radius.
radius, exit path—see exit path radius.
radius, left-turn path—see left-turn path radius.
radius, right-turn path—see right-turn path radius.
raised—used to describe geometric features with a sharp elevation change that are not intended to be driven upon by vehicles at any time.
ramp, wheelchair—see wheelchair ramp.
refuge, pedestrian—see pedestrian refuge.
right-of-way—(1) an intersection user that has priority over other users. (2) Land owned by a
public agency for transportation uses.
right-turn bypass lane—a lane provided adjacent to, but separated from, the circulatory
roadway, that allows right-turning movements to bypass the roundabout. Also known as a
right-turn slip lane.
right-turn path radius—the minimum radius on the fastest path of a right-turning vehicle.
right-turn slip lane—see right-turn bypass lane.
roadway, circulatory—see circulatory roadway.

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rotary—a term used particularly in the Eastern U.S. to describe an older-style circular intersection that does not have one or more of the characteristics of a roundabout. They often have
large diameters, often in excess of 100 m (300 ft), allowing high travel speeds on the circulatory roadway. Also known as a traffic circle.
roundabout—a circular intersection with yield control of all entering traffic, channelized approaches, counter-clockwise circulation, and appropriate geometric curvature to ensure that
travel speeds on the circulatory roadway are typically less than 50 km/h (30 mph).
roundabout capacity—the maximum number of entering vehicles that can be reasonably
expected to be served by a roundabout during a specified period of time.
roundabout, community enhancement—see community enhancement roundabout.
roundabout, modern—see modern roundabout.
roundabout, multilane—see multilane roundabout.
roundabout, rural double-lane—see rural double-lane roundabout.
roundabout, rural single-lane—see rural single-lane roundabout.
roundabout, single lane—see single-lane roundabout.
roundabout, urban compact—see urban compact roundabout.
roundabout, urban single-lane—see urban single-lane roundabout.
rural double-lane roundabout—a roundabout located in a rural area that has at least one
entry with two lanes, and a circulatory roadway that can accommodate more than one vehicle
traveling side-by-side. They incorporate approach curvature to slow entering traffic to a safe
speed.
rural single-lane roundabout—a roundabout located in a rural area that has single lanes on
all entries and one circulatory lane. This form typically has larger diameters and more tangential exits than urban forms.
separator island—see median island.

service volume—the hourly rate at which vehicles, bicycles, or persons can be reasonably
expected to traverse a point or uniform section of a roadway during an hour under specific
assumed conditions. See also maximum service volume. (Adapted from HCM 2000)
set-back distance—the distance between the edge of the circulatory roadway and the sidewalk.
sharpness of flare—a measure of the rate at which extra width is developed in the entry
flare. (source: UK Geometric Design of Roundabouts)
sight distance, intersection—see intersection sight distance.
sight distance, stopping—see stopping sight distance.
sight triangle—an area required to be free of obstructions to enable visibility between conflicting movements.
signal progression—the use of coordinated traffic signals along a roadway in order to minimize stops and delay to through traffic on the major road.
single-lane roundabout—a roundabout that has single lanes on all entries and one circulatory lane.
speed table—an extended, flat-top road hump sometimes used at pedestrian crossings to
slow traffic and to provide a better visual indication of the crosswalk location.
speed, approach—see approach speed.
speed, circulating—see circulating speed.
speed, entry—see entry speed.

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splitter island—a raised or painted area on an approach used to separate entering from exiting traffic, deflect and slow entering traffic, and provide storage space for pedestrians crossing that intersection approach in two stages. Also known as a median island or a separator
island.
splitter island, extended—a raised splitter island that begins some distance upstream of the
pedestrian crossing to separate entering and exiting traffic. A design feature of rural roundabouts.
stopping sight distance—the distance along a roadway required for a driver to perceive and
react to an object in the roadway and to brake to a complete stop before reaching that object.

traffic calming—geometric treatments used to slow traffic speeds or to discourage the use
of a roadway by nonlocal traffic.
traffic circle—a circular intersection that does not have one or more of the characteristics of
a roundabout. Also known as a rotary.
traffic circle, neighborhood—see neighborhood traffic circle.
traffic circle, nonconforming—see traffic circle.
traffic design—a term used in this document to describe the design of traffic control devices,
including signing, pavement markings, and construction traffic control.
traffic, circulating—see circulating traffic.
traffic, entering—see entering traffic.
truck apron—see apron.
two-stage crossing—a process in which pedestrians cross a roadway by crossing one direction of traffic at a time, waiting in a pedestrian refuge between the two traffic streams if
necessary before completing the crossing.
two-way stop-control—stop signs are present on the approach(es) of the minor street. Drivers on the minor street or drivers turning left from the major street wait for a gap in the major
street traffic in order to complete a maneuver.
TWSC—see two-way stop control.

U-turn—a turning movement at an intersection in which a vehicle departs the intersection
using the same roadway it used to enter the intersection.
upstream—the direction from which traffic is flowing (source: HCM 2000).
urban compact roundabout—a small roundabout with a raised central island and splitter
islands, with perpendicular approaches that require vehicles to make a distinct right turn into
the circulatory roadway.
urban double-lane roundabout—an urban roundabout with at least one entry with two
lanes, and a circulatory roadway that can accommodate more than one vehicle traveling sideby-side. They have similar speed characteristics as urban single-lane roundabouts.
urban single-lane roundabout—a roundabout with single lane entries on all legs and one
circulatory lane. Entries are less perpendicular than the urban compact roundabout, allowing
somewhat higher speeds with higher capacities.
UVC—Uniform Vehicle Code.

vehicle, design—see design vehicle.
volume, circulating—see circulating volume.
volume, entering—see entering volume.
volume, service—see service volume.
volume-to-capacity ratio—the ratio of flow rate to capacity for a transportation facility.

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wheelchair ramp—see curb ramp.

width, approach—see approach width.
width, circulatory roadway—see circulatory roadway width.
width, departure—see departure width.
width, entry—see entry width.
width, exit—see exit width.

yield—an intersection control in which controlled traffic must stop only if higher priority traffic
is present.

yield line—a pavement marking used to mark the point of entry from an approach into the
circulatory roadway and generally marked along the inscribed circle. If necessary, entering
traffic must yield to circulating traffic before crossing this line into the circulatory roadway.

zebra crossing—a crossing marked by transverse white stripes where vehicles are required
to yield to pedestrians.

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Bibliography
5.1 U.S. References
American Association of State Highway Officials (AASHO). A Policy on Design of Urban Highways and Arterial Streets. Washington, D.C.: AASHO, 1973.
American Association of State Highway and Transportation Officials (AASHTO). Guide for Development of Bicycle Facilities. Washington, D.C.: AASHTO, 1991.
—. An Information Guide for Roadway Lighting. Washington, D.C.: AASHTO, 1985.
—. A Manual on User Benefit Analysis of Highway and Bus Transit Improvements. Washington, D.C.: AASHTO, 1977.
—. A Policy on Geometric Design of Highways and Streets. Washington, D.C.: AASHTO, 1994.
—. Roadside Design Guide. Washington, D.C.: AASHTO, 1989.
—. Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic
Signals. Washington, D.C.: AASHTO, 1994.

Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities (ADAAG).
36 CFR Part 1191. As amended through January 1998.
Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. Washington, D.C.: FHWA, 1988.
—. Older Driver Highway Design Handbook. Publication No. FHWA-RD-97-135. Washington,
D.C.: FHWA, January 1998.
—. Standard Highway Signs. Washington, D.C.:, FHWA, 1979.
—. Railroad-Highway Grade Crossing Handbook, 2nd edition. Report number FHWA-TS-86215, September 1986.
Illuminating Engineering Society (IES). American National Standard Practice for Roadway Lighting. Standard RP-8. December 1982.
Institute of Transportation Engineers. Manual of Transportation Engineering Studies (H.D.
Robertson, J.E. Hummer, and D.C. Nelson, ed.). Englewood Cliffs, N.J.: Prentice-Hall, 1994.
—. Transportation Planning Handbook (J. Edwards, Jr., ed.). Englewood Cliffs, New Jersey:
Prentice Hall, 1992.
National Committee on Uniform Traffic Laws and Ordinances (NCUTLO). Uniform Vehicle Code
and Model Traffic Ordinance. Evanston, Illinois: NCUTLO, 1992.
Pein, W.E. Trail Intersection Design Guidelines. Prepared for State Bicycle/Pedestrian Program,
State Safety Office, Florida Department of Transportation. Highway Safety Research Center,
University of North Carolina. September 1996.

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Transportation Research Board. Highway Capacity Manual. Special Report 209. Washington,
D.C.: Transportation Research Board, National Research Council, 1994.
—. Highway Capacity Manual. Special Report 209. Washington, D.C.: Transportation Research
Board, National Research Council, July 1999 (draft).

5.2 Roundabout Design Guides

5.2.1 United States
Florida Department of Transportation. Florida Roundabout Guide. Florida Department of Transportation, March 1996
Maryland Department of Transportation. Roundabout Design Guidelines. State of Maryland
Department of Transportation, State Highway Administration, 1995.
Ourston & Doctors, Inc. Roundabout Design Guidelines. 1995.

5.2.2 Australia/New Zealand
Australia/New Zealand Standard. Road lighting. Part 1.3: Vehicular traffic (Category V) lighting—Guide to design, installation, operation and maintenance. Report no. AS/NZS 1158.1.3:1997.
Published jointly by Homebush, New South Wales (Australia): Standards Australia and
Wellington (New Zealand): Standards New Zealand. 1997.
Austroads. Guide to Traffic Engineering Practice, Part 6—Roundabouts. Sydney, Australia:
Austroads, 1993.
Queensland Department of Main Roads (QDMR), Road Planning and Design Guidelines (Draft),
Chapter 12, Section 12.4—Roundabouts. Brisbane, Australia: QDMR, 1999.
Queensland Department of Main Roads (QDMR). Relationships between Roundabout Geometry and Accident Rates. Queensland, Australia: Infrastructure Design of the Technology Division of QDMR, April 1998.
Roads and Traffic Authority (RTA), New South Wales (Australia). Roundabouts—Geometric
Design Method. January 1997.

Roundabouts: A Design Guide, National Association of Australian State Road Authorities, 1986.

5.2.3 Germany
Small Roundabouts: Recommendations for Application and Design, Nordrhein-Westfalen Department of City Development and Traffic (MSV), prepared by Werner Brilon, Ruhr-University
Bochum; translated from German by Daniel J. Parrish, 1993.
Haller, et al. Merkblatt für die Anlage von kleinen Kreisverkehrsplätzen (Guideline for the Construction of Small Roundabouts). Cologne, Germany: Forschungsgesellschaft für Straßen- und
Verkehrswesene. V., August 1998.
Department of Transport of Northrhine-Westfalia, Germany. Empfehlungen zum Einsatz und
zur Gestaltung von Mini-Kreisverkehrsplaetzen (Guidelines for the Use and Design of MiniRoundabouts). Dusseldorf, Germany, 1999.

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5.2.4 The Netherlands
C.R.O.W. Eenheid in rotondes (Uniformity in roundabouts). Publication 126. Ede, The Netherlands: C.R.O.W., March 1998.
Centrum voor Regelgeving en Onderzoek in de Grond-, Water- en Wegenbouw en de
Verkeerstechniek (C.R.O.W). Rotondes (Roundabouts). Ede, The Netherlands: C.R.O.W. December 1993.

5.2.5 United Kingdom
Department of Transport (United Kingdom). Geometric Design of Roundabouts. TD 16/93.
September 1993.
Sawers, C. Mini-roundabouts: Getting them right!. Canterbury, Kent, United Kingdom: EuroMarketing Communications, 1996

5.2.6 France
Service d’Etudes Techniques des Routes et Autoroutes (SETRA—Center for Technical Studies of
Roads and Highways). Aménagement des Carrefours Interurbains sur les Routes Principales (Design of Rural Intersections on Major Roads). Ministry of Transport and Housing, December 1998.
Centre d’Etudes sur les Réseaux, les Transports, l’Urbanisme et les constructions publiques
(CERTU). L’Éclairage des Carrefours à Sens Giratoire (The Illumination of Roundabout Intersections). Lyon, France: CERTU, 1991.
Centre d’Etudes sur les Réseaux, les Transports, l’Urbanisme, et les Constructions Publiques
(CERTU) (Center for Studies on Transportation Networks, Urban Planning, and Public Works).
Carrefours Urbains (Urban Intersections) Guide. Lyon, France: CERTU, January 1999.

5.2.7 Spain
Ministerio de Fomento. Recomendaciones sobre glorietas (Recommendations on roundabouts).
Ministerio de Fomento, Dirección General de Carreteras, 1996.

5.3 Books and Reports

5.3.1 United States
Bauer, K.M., and D.W. Harwood. Statistical Models of At-Grade Intersection Crashes. Report
No FHWA-RD-99-094. Washington, D.C.: Federal Highway Administration, 1999.
Fambro, D.B., et al. NCHRP Report 400: Determination of Stopping Sight Distances. National
Cooperative Highway Research Program, Transportation Research Board, National Research
Council. Washington, D.C.: National Academy Press, 1997.
Garder, P. The Modern Roundabouts: The Sensible Alternative for Maine. Maine Department
of Transportation, Bureau of Planning, Research and Community Services, Transportation Research Division, 1998.
Glauz, W.D., and D.J. Migletz. NCHRP Report 219: Application of Traffic Conflict Analysis at
Intersections. National Cooperative Highway Research Program, Transportation Research Board,
National Research Council. Washington, D.C.: National Academy Press, 1980.

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Harwood, D.W., et al. NCHRP Report 383: Intersection Sight Distances. National Cooperative
Highway Research Program, Transportation Research Board, National Research Council. Washington, D.C.: National Academy Press, 1996.
Institute of Transportation Engineers. Use of Roundabouts, prepared by ITE Technical Council
Committee 5B-17, February 1992.
Jacquemart, G. Synthesis of Highway Practice 264: Modern Roundabout Practice in the United
States. National Cooperative Highway Research Program. Washington, D.C: National Academy Press, 1998.
Krammes, R., et al. Horizontal Alignment Design Consistency for Rural Two-Lane Highways. Publication No. FHWA-RD-94-034. Washington, D.C.: Federal Highway Administration, January 1995.
Migletz, D.J., W.D. Glauz, and K.M. Bauer. Relationships between Traffic Conflicts and Crashes.
Report No. FHWA-RD-84-042. Washington, D.C.: Federal Highway Administration, 1985.
National Safety Council. Accident Facts, 1998 Edition.
Ourston & Doctors, Inc. Designs of Modern American Roundabouts. September 1996.
Prince George’s County, Maryland. Neighborhood Traffic Management Program. Prince George’s
County (Maryland), Department of Public Works and Transportation, November 1995.

The Design and Evaluation of Roundabout Layouts, source unknown
Transportation Research Board. Review of International Practices Used to Evaluate Unsignalized
Intersections. Transportation Research Circular 468. Washington, D.C.: Transportation Research
Board, National Research Council, April 1997.

5.3.2 United Kingdom
Brown, M. TRL State of the Art Review—The Design of Roundabouts. London: HMSO, 1995.
Department of Transport (United Kingdom). The Highway Code. Department of Transport and
the Central Office of Information for Her Majesty’s Stationery Office, 1996.
Kimber, R.M., and E.M. Hollis. Traffic queues and delays at road junctions. TRRL Laboratory
Report LR 909. Crowthorne, England: Transport and Road Research Laboratory, 1979.
Kimber, R.M. The traffic capacity of roundabouts. TRRL Laboratory Report LR 942. Crowthorne,
England: Transport and Road Research Laboratory, 1980.
Maycock, G., and R.D. Hall. Crashes at four-arm roundabouts. TRRL Laboratory Report LR
1120. Crowthorne, England: Transport and Road Research Laboratory, 1984.

5.3.3 Australia
Australia. Traffic Act, Part 6A, 1962.
Troutbeck, R.J. Evaluating the Performance of a Roundabout. SR 45. Australian Road Research Board, August 1989.
VicRoads. Victorian Traffic Handbook, Fourth Edition. Melbourne, Australia: Roads Corporation, 1998.

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243

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5.3.4 Germany
Brilon, W., B. Stuwe, and O. Drews. Sicherheit und Leistungsfähigkeit von Kreisverkehrsplätzen
(Safety and Capacity of Roundabouts). Research Report. Ruhr-University Bochum, 1993.

Empfehlungen zum Einsatz und zur Gestaltung kleiner Kresverkehrsplatze, Freistaat Sahsen
Einsatz-Und Gestaltung von Kreisverkehrsplatzen an Bundersstrassen Ausserhalb Bebauter
Gebiete, Brilon, W. and Bondzio, L., Ruhr-Universitat Bochum (Juni 1995).

5.3.5 France
Acts du Seminaire: “Giratoires 92” SETRA, CETUR.

5.3.6 The Netherlands
Schoon, C.C., and J. van Minnen. Accidents on Roundabouts: II. Second study into the road
hazard presented by roundabouts, particularly with regard to cyclists and moped riders. R-9316. The Netherlands: SWOV Institute for Road Safety Research, 1993.

5.4 Articles
Akçelic, R., and R.J. Troutbeck, “Implementation of the Australian roundabout analysis method
in SIDRA,” In Highway Capacity and Level of Service: Proceedings of the International Symposium on Highway Capacity (U. Brannolte, ed.), Karlsruhe, Germany. Rotterdam, Germany:
Balkema Publisher, 1991, pp. 17–34.
Akçelik, R., and M. Besley, SIDRA 5 User Guide. Melbourne, Australia: Australian Road Research Board Transport Research Ltd., January 1999.
Akçelik, R., “Lane-by-Lane Modeling of Unequal Lane Use and Flares at Roundabouts and
Signalized Intersection: the Sidra Solution,” Traffic Engineering & Control, Vol. 38, No. 7/8, July/
August 1997.
Akçelik, R., E. Chung, and M. Besley, “Performance of Roundabouts under Heavy Demand
Conditions,” Road & Transport Research, Vol. 5, No. 2, June 1996, pp. 36–50.
Akçelik, R., E. Chung, and M. Besley, “Getting Around Better,” Pc-Trans, winter quarter 1997,
pp. 14–19.
Alphand, F., U. Noelle, and B. Guichet, “Evolution of Design Rules for Urban Roundabouts in
France,” In Intersection without Traffic Signals II, Springer-Verlag, Germany (W. Brilon, ed.),
1991, pp. 126–140.
Alphand, F., U. Noelle, and B. Guichet, “Roundabouts and Road Safety: State of the Art in
France,” In Intersections without Traffic Signals II, Springer-Verlag, Germany (W. Brilon, ed.),
1991, pp. 107–125.
Arem, Bart van, “Capacities and Delays at Roundabouts in The Netherlands,” Proceedings of
Seminar H held at the PTRC Transport, Highways and Planning Summer Annual Meeting,
University of Manchester Institute of Science and Technology, England, from 14–18 September 1992, pp. 257–267.
Armitage, D.J., and M. McDonald, “Roundabout Capacity,” Traffic Engineering & Control, October 1974.

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Arndt, O, “Road Design Incorporating Three Fundamental Safety Parameters,” Technology Transfer Forums 5 & 6, Transport Technology Division, Main Roads Department, Queensland, Australia, August 1998.
Avent, A.M., and R.A. Taylor, “Roundabouts—Aspects of their Design and Operations,”
Queensland Division Technical Papers, Vol. 20, No. 17, 1979, pp. 1–10.
Bared, J.G., W. Prosser, and C. Tan Esse, “State-of-the-Art Design of Roundabouts,” In Transportation Research Record 1579. Washington, D.C.: Transportation Research Board, National
Research Council, 1997.
Bergh,T., “Intersections WithoutTraffic Signals—Swedish Experience on Capacity andTraffic Safety,”
Intersection without Traffic Signals II, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 192–213.
Brilon, W., N. Wu, and L. Bondzio, “Unsignalized Intersections in Germany—A State of the Art
1997”
, In Proceedings of the Third International Symposium on Intersections without Traffic
Signals (M. Kyte, ed.), Portland, Oregon, U.S.A. University of Idaho, 1997.
Brilon, W., and B. Stuwe, “Capacity and Design of Traffic Circles in Germany,” In Transportation
Research Record 1398. Washington, D.C.: Transportation Research Board, National Research
Council, 1993.
Brilon, W., and L. Bondzio, Untersuchung von Mini-Kreisverkehrsplaetzen (Investigation of
Mini-Roundabouts). Ruhr-University Bochum, Germany, 1999.
Brilon, W., and L. Bondzio, White Paper: “Summary of International Statistics on Roundabout
Safety” (unpublished), July 1998.
Brilon, W., and M. Vandehey, “Roundabouts—The State of the Art in Germany,” In ITE Journal,
November 1998.
Brilon, Werner, “Traffic Engineering and the New German Highway Capacity Manual,” Transportation Research A, Vol. 28 A, No. 6, 1994, pp.469–481.
Brilon, Werner, and Birgit Stuwe, “Capacity and Safety of Roundabouts in West Germany,”
Proceedings, 15th ARRB Conference, Vol. 15, Part 5, 1990, pp. 275–281.
Brilon, Werner, Michael Grossmann, and Birgit Stuwe, “Toward a New German Guideline for Capacity of Unsignalized Intersections,” Transportation Research Record 1320, 1991, pp.168–174.
Brude, U., and J. Larsson, The Safety of Cyclists at Roundabouts—A Comparison Between
Swedish, Danish and Dutch Results. Swedish National Road and Transport Research Institute
(VTI), Nordic Road & Transport Research No. 1, 1997.
Cassidy, Michael J., Samer M. Madanat, Wang Mu-han, and Fan Yan, “Unsignalized Intersection Capacity and Level of Service: Revisiting Critical Gap,” Transportation Research Record
1484 (1995) pp. 16–23.
Cedersund, H.A., “Traffic Safety at Roundabouts,” Intersection without Traffic Signals I, SpringerVerlag (Werner Brilon, ed.), 1991, pp. 305–318.
Centre d’Etude des Transports Urbains (CETUR), “Safety of Roundabouts in Urban and Suburban Areas,” Paris, 1992.
Chang, Stanley H., “Overcoming Unbalanced Flow Problems at a Roundabouts by Use of
Part-Time Metering Signals,” Master’s Thesis, Monash University, January 1994.

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Chin, H.C., “SIMRO: A Model to Simulate Traffic at Roundabouts,” Traffic Engineering & Controls.
Chung, Edward, “Comparison of Roundabout Capacity and Delay Estimates from Analytical
and Simulation Models,” Proceedings, 16th ARRB Conference, Vol. 16, Part 5, 1992.
Courage, Kenneth G., “Roundabout Modeling in CORSIM,” presented at the Third International Symposium on Intersections without Traffic Signals, Portland, Oregon, U.S.A.,1997.
CROW. Sign Up for the Bike: Design Manual for a Cycle-Friendly Infrastructure. The Netherlands: Center for Research and Standardization in Civil Engineering (CROW), 1993.
Crown, B., An Introduction to Some Basic Principles of U.K. Roundabout Design, presented at
the ITE District 6 Conference on Roundabouts, Loveland, Colorado, October 1998.
Department of Transport (United Kingdom), “Killing Speed and Saving Lives,” as reported in
Oregon Bicycle and Pedestrian Plan, Oregon Department of Transportation, 1995.
Department of Transport, “Determination of Size of Roundabouts at Major/Minor Junctions,”
Departmental Advice Note TA 23/81, 1981.
Dinwoodie, J., “Surveying Traffic Delays at a Roundabout near Plymouth,” Mathematics In
Transport Planning and Control (J.D. Criffiths, ed.), 1992, pp. 227–286.
Duncan, G., “Paramics Technical Report: Car-Following, Lane-Changing and Junction Modelling.” Edinburgh, Scotland: Quadstone, Ltd., 1997.
Fisk, C.S., “Traffic Performance Analysis at Roundabouts,” Transportation Research Board, Vol.
25B, Bi. 2/3, 1991, pp. 89–102.
Flannery, A., and T.K. Datta, “Modern Roundabouts and Traffic Crash Experience in the United
States.” In Transportation Research Record 1553. Washington, D.C.: Transportation Research
Board, National Research Council, 1996.
Flannery, A., L. Elefteriadou, P. Koza, and J. McFadden, “Safety, delay and capacity of singlelane roundabouts in the United States.” In Transportation Research Record 1646. Washington,
D.C.: Transportation Research Board, National Research Council, 1998, pp. 63–70.
Flannery, Aimee, and Tapan K. Datta, “Operational Performance Measures of American Roundabouts,” Transportation Research Board Annual Meeting, January 1997, Washington D.C., January 1997.
Flannery, Aimee, and Tapan K. Datta, “Operational Performance Measures of American Roundabouts,” 1996 ITE Compendium of Technical Papers, 1996.
Gambard, J.M., “Safety and Design of Unsignalized Intersections in France,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp.48–61.
Guichet, B., “Roundabouts In France: Development, Safety, Design, and Capacity.” In Proceedings of the Third International Symposium on Intersections Without Traffic Signals (M.
Kyte, ed.), Portland, Oregon, U.S.A. University of Idaho, 1997.
Hagring, Ola, “The Use of the Cowan M3 Distribution for Modeling Roundabout Flow,” Traffic
Engineering & Control, May 1996, pp. 328–332.

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Hakkert, A.S., D. Mahalel, and S.A. Asante, “A Comparative Study of Roundabout Capacity
Procedures,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991,
pp. 93–106.
Hallworth, M.S., “Signalling Roundabouts,” In Traffic Engineering + Control, Vol. 33, No. 6,
June 1992.
Harders, J., Grenz- und Folgezeitlücken als Grundlage für die Berechnung der Leistungsfähigkeit
von Landstrassen (Critical gaps and follow-up times for capacity calculations at rural roads),
Schriftenreihe Strassenbau und Strassenverkehrstechnik, Vol. 216, 1976.
Heidemann, D., “Queue lengths and waiting-time distributions at priority intersections.” In
Transportation Research B, Vol. 25B, 4, pp. 163–174, 1991.
Herms, B.F., “Some Visual Aspects of Pedestrian Crosswalks.” In Proceedings, 22nd California Street and Highway Conference, Institute of Transportation and Traffic Engineering, University of California, Los Angeles, January 1970.
Hoglund, Paul G., “Case Study. Performance Effects of Changing a Traffic Signal Intersection
to Roundabout,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.),
1991, pp. 141–158.
Horman, C.B., “Design and Analysis of Roundabouts,” Proceedings, 7th ARRB Conference,
Vol. 7, Part 4, 1974, pp. 58–82.
Hughes, B.P., “So You Think You Understand Gap Acceptance!,” Australian Road Research
Board, 19(3), 1989, pp. 195–204.
Innovative Transportation Concepts, LLC, VISSIM—User Manual. Program Version 2.32–2.36.
November 10, 1997.
ITE Technical Council Committee 5B-17, “Use of Roundabouts,” ITE Journal, February 1992,
pp. 42–45.
Jessen, G.D., Ein Richtlinienvorschlag für die Behandlung der Leistungsfähigkeit von
Knotenpunkten ohne Signalregelung (A guideline suggested for capacity calculations for
unsignalized intersections). Strassenverkehrstechnik, Nr. 7/8, 1968.
Jones, S.E., “Signalling Roundabouts 2. Controlling the Revolution,” Traffic Engineering & Control, Vol. 33, No. 11, November 1992, pp. 606–613.
Kimber, R.M., “Gap-Acceptance and Empiricism in Capacity Prediction,” Transportation Science, Vol. 23, No. 2, 1989.
Kimber, R.M., “The Design of Unsignalized Intersections in the UK,” Intersections Without
Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 20–34.
Lalani, N., “The impact on accidents of the introduction of mini, small and large roundabouts
at major/minor priority junctions.” Traffic Engineering + Control, December 1975.
Lamm, R., and E. M. Choueiri, “Recommendations for Evaluating Horizontal Design Consistency Based on Investigations in the State of New York.” In Transportation Research Record
1122. Washington, D.C.: Transportation Research Board, National Research Council, 1987.
Layfield, R.E., and G. Maycock, “Pedal-Cyclists at Roundabouts,” Traffic Engineering + Control, June 1986, pp. 343–349.

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247

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List, George, Siew Leong, Yursi Embong, Azizan Naim, and Jennifer Conley, “Case Study Investigation of Traffic Circle Capacity,” Transportation Research Record 1457, 1994, pp.118–126.
Little, J.D.C., A Proof of the Queueing Formula L = W • λ. Operations Research 9, S. 383–387,
1961.
McDonald, M.; and D.J. Armitage, “The capacity of roundabouts,” Traffic Engineering & Control, Vol. 19, October 1978, pp. 447–450.
Minnen, J. van, “Roundabouts,” Institute for Road Safety Research SWOV, The Netherlands,
1986.
Myers, Edward J., “Modern Roundabouts for Maryland,” ITE Journal, October 1994, pp. 18–22.
Niederhauser, M.E., B.A. Collins, E.J. Myers, “The Use of Roundabouts: Comparison with
Alternate Design Solution,” Compendium of Technical Papers, 67th Annual Meeting, Institute
of Transportation Engineers, August 1997.
Ourston, Leif, “British Interchanges, Intersections, and Traffic Control Devices,” Westernite,
Vol. XXXV, No. 5, September–October 1992.
Ourston, Leif, “Wide Nodes and Narrow Roads,” paper presented to the Transportation Research Board 72nd Annual Meeting, January 10–14, 1993.
Ourston, Leif, and Joe G. Bared, “Roundabouts: A Direct Way to Safer Highways,” Public
Roads, Autumn 1995, pp. 41–49.
Ourston, Leif, and Gregory A. Hall, “Modern Roundabout Interchanges come to America,”
1996 ITE Compendium of Technical Papers, 1996.
Paramics, Ltd., “Comparison of Arcady and Paramics for Roundabout Flows,” Version 0.3,
August 23, 1996.
Pearce, C.E.M., “A Probabilistic Model for the Behavior of Traffic at Roundabout,” Transportation Research Board, Vol. 21B, No. 3, 1987, pp. 207–216. Washington, D.C., January 1997.
Rahman, Mountasser A., “Design Criteria for Roundabouts,” 1995 ITE Compendium of Technical Papers, 1995.
Redington, T., “The Modern Roundabout Arrives in Vermont,” AASJTP Quarterly Magazine,
Vol. 75, No. 1, 1995, pp. 11–12.
Schoon, C., and J. van Minnen, “The Safety of Roundabouts in The Netherlands,” Traffic Engineering & Control, Vol. 35, No. 3, 1994, pp. 142–148.
Seim, K., “Use, Design and Safety of Small Roundabouts in Norway,” In Intersections Without
Traffic Signals II, Springer-Verlag, Germany (W. Brilon, ed.), 1991, pp. 270–281.
Service d’Etudes Techniques des Routes et Autoroutes (SETRA), Carrefours Giratoires: Evolution des Characteristiques Geometriques, Ministère de l’Equipement, du Logement, de
l’Aménagement du Territoire et des Transports, Documentation Technique 44, SETRA, August
1997, and 60, SETRA, May 1988.
SETRA/CETE de l’Ouest, “Safety Concerns on Roundabouts,” 1998.

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Simon, Michael J., “Roundabouts in Switzerland,” Intersection without Traffic Signals II, SpringerVerlag (Werner Brilon, ed.), 1991, pp. 41–52.
Smith, Mark J., “Improved Signing for Traffic Circles,” New Jersey Department of Transportation, FHWA/NJ-91-003 91-003-7350, 1990.
Smith, S.A., and R. L. Knoblauch, “Guidelines for the Installation of Crosswalk Markings.” In
Transportation Research Record 1141, Transportation Research Board, National Research Council, Washington, D.C., 1987.
Stuwe, Birgit, “Capacity and Safety of Roundabouts—German Results,” Intersection Without
Traffic Signals II, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 1–12.
Tan, Jian-an, “A Microscopic Simulation Model of Roundabout Entry Operations,” Intersection
without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 159–176.
Taylor, Marie C., “UK Techniques for the Prediction of Capacities, Queues, and Delays At
Intersections Without Traffic Signals,” Intersection without Traffic Signals I, Springer-Verlag
(Werner Brilon, ed.), 1991, pp. 274–288.
Technical Research Centre of Finland (VTT), “Traffic effects of a roundabout,” Nordic Road &
Transport Research, No. 1, 1993, pp. 9–11.
Todd, K., “A history of roundabouts in Britain,” Transportation Quarterly, Vol. 45, No. 1, January
1991.
Troutbeck, R.J., “Changes to Analysis and Design of Roundabouts Initiated in the Austroads
Guide” Proceedings, 16th ARRB Conference, Vol. 16, Part 5, 1992.
Troutbeck, R.J., “Capacity and Delays at Roundabouts—A Literature Review,” Australian Road
Research Board, 14(4), 1984, pp. 205–216.
Troutbeck, R.J., “Capacity and Design of Traffic Circles in Australia,” Transportation Research
Record 1398, 1993, pp. 68–74.
Troutbeck, R.J., “Current and Future Australian Practices for the Design of Unsignalized Intersections,” Intersection without Traffic Signals I, Springer-Verlag (Werner Brilon, ed.), 1991, pp.
1–19.
Troutbeck, R.J., “Does Gap Acceptance Theory Adequately Predict the Capacity of a Roundabout,” Proceedings, 12th ARRB Conference, Vol. 12, Part 4, 1985, pp. 62–75.
Troutbeck, R.J., “Effect of Heavy Vehicles at Australian Traffic Circles and Unsignalized Intersections,” Transportation Research Record 1398, 1993, pp. 54–60.
Troutbeck, R.J., “Recent Australian Unsignalized Intersection Research and Practices,” Intersection without Traffic Signals II, Springer-Verlag (Werner Brilon, ed.), 1991, pp. 238–257.
Troutbeck, R.J., “Traffic Interactions at Roundabouts,” Proceedings, 15th ARRB Conference,
Vol. 15, Part 5, 1990.
Tudge, R.T., “Accidents at Roundabouts in New South Wales,” Proceedings, 15th ARRB Conference, Vol. 15, Part 5, 1990.
Unknown source, “Roundabouts—Implications of U.S. Implementation.”

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Van Minnen, J., “Safety of Bicyclists on Roundabouts Deserves Special Attention,” SWOV
Institute of Road Safety Research in the Netherlands, Research Activities 5, March 1996.
Vogt, A., Crash Models for Rural Intersections: 4-Lane by 2-Lane Stop-Controlled and 2-Lane
by 2-Lane Signalized. Washington, D.C.: Federal Highway Administration, 1999.
Wallwork, Michael J., “Roundabouts,” Genesis Group, Inc.
Wong, S.C., “On the Reserve Capacities of Priority Junctions and Roundabouts,” Transportation Research Board, Vol. 30, No. 6, 1996, pp. 441–453.
Worthington, J.C., “Roundabout Design: A Comparison of Practice in the UK and France,”
Proceedings of Seminar H Held at the PTRC Transport, Highways and Planning Summer Annual Meeting, University of Manchester Institute of Science And Technology, England, 14–18
September 1992, pp. 269–279.
Wu, N., “An Approximation for the Distribution of Queue Lengths at Unsignalized Intersections,” In Proceedings of the Second International Symposium on Highway Capacity (R. Akçelik,
ed.). Sydney, Australia: Australian Road Research Board, 1994.

5.5 Video Cassettes
Ourston & Doctors, Inc. “I-70/Vail Road.”
—. “Nonconforming Traffic Circle Becomes Modern Roundabout.”
—. “Snow at Roundabouts.”
Maryland DOT. “Modern Roundabouts.”

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Appendix A Operational Analysis Formulas

This appendix presents the assumptions used to develop the graphs and charts in
the operational analysis presented in Chapter 4.

A.1 Single-Lane Roundabout

A.1.1 Equations
Qe = k (F − fc Qc ),
= 0,

fcQc ≤ F

(A-1)

fcQc > F

Qe= entry capacity, pce/h
Qc= circulating flow, pce/h

where:

1

k = 1− 0.00347(φ − 30) − 0.978  − 0.05 
r


(A-2)

F = 303x 2

(A-3)

fc = 0.210tD (1+ 0.2x 2 )

(A-4)

tD = 1+

0.5
 D − 60 
1+ exp 
 10 

x2 = v +
S=

e −v
1+ 2S

(A-6)

1.6 (e − v )
l′

where: e
v
l’
S
D
φ
r

=
=
=
=
=
=
=

(A-5)

(A-7)
entry width, m
approach half width, m
effective flare length, m
sharpness of flare, m/m
inscribed circle diameter, m
entry angle, degrees
entry radius, m

Roundabouts: An Informational Guide • Appendix A: Operational Analysis Formulas

251

CONTENTS

A.1.2 Parameter assumptions
For design purposes, when e = v
then l’ is effectively zero.
However, setting l’ = 0 results in
S being undefined. Therefore a
non-zero value of l’ has been
selected. When e = v, any
non-zero value of l’ results in
S = 0 and x2 = v.

D = 40 m
re = 20 m
φ = 30 degrees
v=4m
e=4m
l’ = 40 m
S=

1.6(e − v ) 1.6( 4 − 4)
=
=0
40
l′

tD = 1+

0.5
= 1.4404
 D − 60 
1+ exp 
 10 

x2 = v +

4−4
e −v
=4
= 4+
1+ 2(0)
1+ 2S

F = 303x 2 = 303(4) = 1212
fc = 0.210tD (1+ 0.2x 2 ) = 0.5447
1

k = 1− 0.00347 (φ − 30) − 0.978  − 0.05  = 1
r

A.1.3 Final equation

Qe = 1212 − 0.5447Qc

(A-8)

A.2 Double-Lane Roundabout

A.2.1 Equations
See Section A.1.1.
A.2.2 Parameter assumptions
For design purposes, when e = v
then l’ is effectively zero.
However, setting l’ = 0 results in
S being undefined. Therefore a
non-zero value of l’ has been
selected. When e = v, any
non-zero value of l’ results in
S = 0 and x2 = v.

252

D = 55 m
re = 20 m
φ = 30 degrees
v=8m
e=8m

l’ = 40 m

Federal Highway Administration

CONTENTS

1.6 (e − v ) 1.6 (8 − 8)
=
=0
40
l′

tD = 1+

0.5
= 1.3112
 D − 60 
1+ exp 
 10 

x2 = v +

8−8
e −v
=8
= 8+
1+ 2(0)
1+ 2S

F = 303x 2 = 303(8) = 2424
fc = 0.210tD (1+ 0.2x 2 ) = 0.7159
1

k = 1− 0.00347(φ − 30) − 0.978  − 0.05  = 1
r

A.2.3 Final equation

Qe = 2424 − 0.7159Qc

(A-9)

A.3 Urban Compact Roundabout
The capacity curve for the urban compact roundabout is based on the capacity
curves developed for roundabouts in Germany with single-lane entries and a singlelane circulatory roadway. This equation, developed by Brilon, Wu, and Bondzio is as
follows:

Qe = 1218 − 0.74Qc

(A-10)

where: Qe = entry capacity, pce/h
Q = circulating flow, pce/h
c

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253

CONTENTS

A.4 Short Lanes
The effect of short lanes (flare) on capacity has been documented by Wu (3). Page
321 of Wu’s paper states that for a right flared approach,

kf , right =

1
nF, right +1

nF, right +1

( xL + xT )

+ xRF, right

(A-11)

Dropping some subscripts,
k=

1
n +1

n +1

( x LT )

+ ( x R )n +1

(A-12)

Noting that the capacities of each lane are the same and that the flows are the
same (that is, the entries are constantly fed with vehicles), this gives:

1
x

n +1

(A-13)

with xLT = xR. Capacity qmax is then
(A-14)

qmax = k qi
where qi is flow in lane i and q1=q2

qmax =

2q
x

n +1

(A-15)

qmax2 is the capacity of a two-lane roundabout, the capacity of each entry lane is
qmax2/2 and this is equal to the flow, q, divided by the degree of saturation, x.

q max =

q max 2
n +1

(A-16)

The results of Equation A-16 can be compared with the results from the British
equations. The TRL equations are listed above. The results are listed for four circulating flow conditions: 500 veh/h,1000 veh/h, 1500 veh/h, and 2000 veh/h.

254

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CONTENTS

Qc = 500 veh/h Qc=1000 veh/h Qc=1500 veh/h

Qc=2000 veh/h

TRL

940

668

395

123

1447

1461

1151

1208

855

955

559

702

1636

1640

1321

1356

1006

1072

691

787

1737

1411

1436

1086

1135

761

834

1799

1468

1487

1136

1175

805

864

1841

1506

1522

1170

1203

835

884

1872

1871

1534

1547

1195

1223

857

899

1896

1895

1555

1566

1214

1238

873

910

1914

1913

1571

1581

1229

1250

886

919

1929

1928

1585

1594

1240

1260

896

926

1941

1940

1596

1604

1250

1268

905

932

1951

1950

1605

1612

1258

1274

912

936

1960

1959

1612

1619

1265

1280

918

941

1967

1966

1619

1626

1271

1285

923

944

1974

1973

1625

1631

1276

1289

928

947

1979

1978

1630

1636

1281

1293

931

950

1984

1983

1635

1640

1285

1296

935

952

1989

1988

1639

1644

1288

1299

938

955

1993

1992

1642

1647

1292

1302

941

957

1996

1645

1650

1294

1304

943

958

2000
2066

1999
2066

1648
1708

1653
1708

1297
1350

1306
1350

946
992

960
992

Exhibit A-1. Tabular
comparison of TRL and Wu
short-lane methodologies.

Exhibit A-2. Graphical
comparison of TRL and Wu
short-lane methodologies.

Roundabouts: An Informational Guide • Appendix A: Operational Analysis Formulas

255

CONTENTS

A.5 References
1. Kimber, R.M. The traffic capacity of roundabouts. TRRL Laboratory Report LR
942. Crowthorne, England: Transport and Road Research Laboratory, 1980.
2. Brilon , W., N. Wu, and L. Bondzio. “Unsignalized Intersections in Germany – A
State of the Art 1997.” In Proceedings of the Third International Symposium on
Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.
3. Wu, N. “Capacity of shared/short lanes at unsignalized intersections.” In
Proceedings of the Third International Symposium on Intersections without Traffic Signals (ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.

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Appendix B Example Roundabout Designs

The purpose of this Appendix is to provide examples for each of the six roundabout
categories. Exhibit B-1 lists typical inscribed circle diameter ranges for each roundabout category. Note that the flared-entry roundabout uses the same range of
inscribed circle diameters as the double-lane roundabouts. Note that the dimensions of roundabouts may vary considerably within each category, depending on
site-specific characteristics, including number of legs, approach angles, design
vehicle requirements, and so on. Refer to Chapter 6 for more discussion of specific
dimensions.

Exhibit B-1. Typical inscribed
circle diameter ranges by
roundabout category.

Site Category

Inscribed Circle
Diameter Range

Mini-roundabout

13–25 m (45–80 ft)

Urban compact

25–30 m (80–100 ft)

Urban single lane

30–40 m (100–130 ft)

Urban double lane

45–55 m (150–180 ft)

Rural single lane

35–40 m (115–130 ft)

Rural double lane

55–60 m (180–200 ft)

The following pages show examples for each of the roundabout categories:

•
•
•
•
•
•
•

Exhibit B-2: Typical mini-roundabout.
Exhibit B-3: Typical urban compact roundabout.
Exhibit B-4: Typical urban single-lane roundabout.
Exhibit B-5: Typical urban double-lane roundabout.
Exhibit B-6: Typical flared-entry roundabout.
Exhibit B-7: Typical rural single-lane roundabout.
Exhibit B-8: Typical rural double-lane roundabout.

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Exhibit B-2. Example of a
typical mini-roundabout.

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Exhibit B-3. Example of a
typical urban compact
roundabout.

Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs

259

CONTENTS

Exhibit B-4. Example of a
typical single-lane roundabout.

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Exhibit B-5. Example of a
typical urban double-lane
roundabout.

Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs

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CONTENTS

Exhibit B-6. Example of a
typical flared-entry roundabout.

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Exhibit B-7. Example of a
typical rural single-lane
roundabout.

Roundabouts: An Informational Guide • Appendix B: Example Roundabout Designs

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CONTENTS

Exhibit B-8. Example of a
typical rural double-lane
roundabout.

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Appendix C MUTCD Recommendation
The purpose of this Appendix is to provide the rationale behind recommended
deviations from the current (1988 edition) or proposed (2000 edition) Manual on
Uniform Traffic Control Devices (MUTCD). The following devices are discussed:

•
•

YIELD Sign
Roundabout Ahead Sign

C.1 Yield Sign
The proposed use of the YIELD sign in the Guide is generally consistent with the
MUTCD. However, the MUTCD contains language that generally discourages the
use of YIELD signs for controlling the major flow at an intersection and the use of
YIELD signs on more than one approach (MUTCD, §2B-8). This language predates
the consideration of roundabouts and should be modified in the next edition of the
MUTCD.

C.2 Roundabout Ahead Sign
As an alternative to the Circular Intersection sign, a Roundabout Ahead sign has
been proposed. This sign, along with a supplemental advisory speed plate (W13-1),
is shown in Exhibit C-1.
Exhibit C-1. Roundabout
Ahead sign with advisory
speed plate (W13-1).

This sign should be used on all approaches to a roundabout. The purpose of a
Roundabout Ahead sign is to convey to a driver that the driver is approaching an
intersection with the form of a roundabout. The intent of this sign is to be similar in
function to the other intersection warning signs (e.g., CROSS ROAD (W2-1) signs),
for example, which convey that the driver is approaching intersections of those
forms. Unlike those signs, however, the Roundabout Ahead sign is recommended
for all roundabouts, not just visually obscured locations.
C.2.1 Need
The 1988 edition of the MUTCD provides no sign related to roundabouts. The closest applicable sign is the YIELD AHEAD sign, either in word message or symbolic
form (W3-2 or W3-2a, respectively). While this sign is necessary for indicating an
upcoming traffic control device, it does not provide any information to the driver
that the upcoming yield sign is for a roundabout. Driver behavior, lane assignments,

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and driver expectation are much different for roundabouts than for traditional yieldcontrolled locations (typically low-volume streets or right-turn bypass lanes). Identification that a roundabout is upcoming is particularly important for multilane approaches so that drivers can anticipate and move into the proper lane in advance of
the roundabout. Therefore, some indication that a driver is approaching a roundabout is essential, especially given the relative rarity of roundabouts in the United
States.
The National Committee on Uniform Traffic Control Devices (NCUTCD) has adopted
the Circular Intersection sign shown in Exhibit C-2, and this sign is being considered for adoption by FHWA.
Exhibit C-2. Circular
Intersection sign.

C.2.2 Existing Practice
Due to the lack of a standard Roundabout Ahead sign, jurisdictions in the U.S.
have experimented with a variety of warning signs, sometimes with multiple variations within the same jurisdiction. Examples of these are shown in Exhibit C-3. As
can be seen from the figure, the lack of standardization from jurisdiction to jurisdiction is evident.
Exhibit C-3. Sample of
existing Roundabout Ahead
signs in United States.
Bradenton Beach, FL
Mary Esther, FL
Mary Esther, FL
Lisbon, MD
Leeds, MD
Lothian, MD
Naples, FL
West Boca Raton, FL

(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)

(c)

(f)

266

(b)

(a)

(d)

(g)

(e)

(h)
(g)

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Exhibit C-3 (continued).
(i) Santa Barbara, CA
(j) Tallahassee, FL
(k) Taneytown, MD
(l) Tavares, FL
(m) Vail, CO
(n) West Vail, CO

(i)

(j)

(k)

(l)

(m)

(n)

International practice varies from country to country but is generally more consistent than current U.S. practice. Sign shapes and coloration vary depending on the
standards of that country, but the one consistent feature is a simple ring of arrows,
oriented to the direction of traffic flow. Examples from the United Kingdom and
Australia are given in Exhibit C-4.
Exhibit C-4. Sample of
Roundabout Ahead signs used
internationally.

United Kingdom

Australia

C.2.3 Recommendation
Based on a review of existing signs in the U.S. and current international practice,
a recommended Roundabout Ahead sign was developed, as presented previously
in Exhibit C-1. This sign is similar in concept to those shown in (b), (c), and (j) of
Exhibit C-3 and is shown fully dimensioned in Exhibit C-5. This sign has been
developed based on the following criteria:

•
•

The recommended sign is symbolic, consistent with current MUTCD practice.

•

The recommended sign gives advanced notice of the proper direction of circulation. The NCUTCD-adopted sign in Exhibit C-2 does not convey this information and could give the driver the incorrect impression that the circulatory roadway is bidirectional.

The recommended sign uses the internationally recognized circular ring of arrows to represent a roundabout and is almost an exact mirror image of the sign
used in Australia (Exhibit C-4).

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•

The recommended sign can be used for roundabouts with any number of legs,
including intersections with one-way approaches. Many of the signs in Exhibit
C-3 and the NCUTCD-recommended sign in Exhibit C-2 are unique to four-leg
roundabouts with legs at right angles and would be inappropriate for roundabouts with three or five legs, for example.

•

The recommended sign can be supplemented by an advisory speed plate. An
advisory speed plate would not be appropriate for a YIELD AHEAD sign because
of the need for the driver to proceed only when clear.

•

The recommended sign is simple with no extraneous or distracting elements to
confuse a driver. Some of the signs in Exhibit C-3 are perhaps too complex for
higher speed environments.

•

Mini-roundabouts cannot be easily signed to show the proper direction of circulation. The recommended sign provides guidance to the driver as to the proper
direction of circulation.

Exhibit C-5. Dimensions of
Roundabout Ahead sign.

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