Roundabouts Roundabout 000679

User Manual: Roundabout

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Glossary
Bibliography
Appendix A - Operational Analysis Formulas
Appendix B - Example Roundabout Designs
Appendix C - MUTCD Recommendation

231

Roundabouts: An Informational Guide • Glossary

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 Ameri-

cans 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 ap-

proach 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. De-

fined 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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Federal Highway Administration

232

benefit-cost analysis—a method of economic evaluation that uses the

benefit-cost ratio

the measure of effectiveness.

benefit-cost ratio—the difference in benefits between an alternative and the no-build sce-

nario, 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 reason-

ably 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

years using an assumed discount rate of

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 move-

ments 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 fash-

ion 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

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.

CONTENTS

233

Roundabouts: An Informational Guide • Glossary

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 road-

way.

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 con-

flicts.

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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Federal Highway Administration

234

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 perpen-

dicularly 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 perpendicu-

larly 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.

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 larg-

est circle that can be inscribed within the outline of the

intersection

interchange—a grade-separated junction of two roadways, where movement from one road-

way to the other is provided for.

CONTENTS

235

Roundabouts: An Informational Guide • Glossary

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 per-

sons 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 with-

out 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-by-

side.

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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Federal Highway Administration

236

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 in-

tended 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.

CONTENTS

237

Roundabouts: An Informational Guide • Glossary

rotary—a term used particularly in the Eastern U.S. to describe an older-style

circular inter-

section

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

circula-

tory roadway.

Also known as a

traffic circle.

roundabout—a

circular intersection

with yield control of all entering traffic, channelized ap-

proaches, 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 tangen-

tial 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 side-

walk.

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 con-

flicting movements.

signal progression—the use of coordinated traffic signals along a roadway in order to mini-

mize stops and delay to through traffic on the major road.

single-lane roundabout—a

roundabout

that has single lanes on all entries and one circula-

tory 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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Federal Highway Administration

238

splitter island—a raised or painted area on an approach used to separate entering from exit-

ing traffic, deflect and slow entering traffic, and provide storage space for pedestrians cross-

ing 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 round-

abouts.

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

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 direc-

tion 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. Driv-

ers 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 side-

by-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.

CONTENTS

239

Roundabouts: An Informational Guide • Glossary

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.

CONTENTS

Federal Highway Administration240

Bibliography

5.1 U.S. References

American Association of State Highway Officials (AASHO).

A Policy on Design of Urban High-

ways and Arterial Streets

. Washington, D.C.: AASHO, 1973.

American Association of State Highway and Transportation Officials (AASHTO).

Guide for De-

velopment of Bicycle Facilities

. Washington, D.C.: AASHTO, 1991.

—.

An Information Guide for Roadway Lighting

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—.

A Manual on User Benefit Analysis of Highway and Bus Transit Improvements

. Washing-

ton, 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

. Wash-

ington, 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-86-

215, September 1986.

Illuminating Engineering Society (IES).

American National Standard Practice for Roadway Light-

ing

. 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.

CONTENTS

241

Roundabouts: An Informational Guide • Bibliography

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 Trans-

portation, 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) light-

ing—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 Geom-

etry and Accident Rates

. Queensland, Australia: Infrastructure Design of the Technology Divi-

sion 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 De-

partment 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 Con-

struction 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 Mini-

Roundabouts

). Dusseldorf, Germany, 1999.

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Federal Highway Administration242

5.2.4 The Netherlands

C.R.O.W.

Eenheid in rotondes

(

Uniformity in roundabouts

). Publication 126. Ede, The Nether-

lands: 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. De-

cember 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: Euro-

Marketing 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

(

De-

sign 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).

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(

The Illumination of Roundabout Inter-

sections

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(CERTU) (Center for Studies on Transportation Networks, Urban Planning, and Public Works).

Carrefours Urbains

(

Urban Intersections

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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.

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Cooperative Highway Research Program, Transportation Research Board, National Research

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Garder, P.

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Glauz, W.D., and D.J. Migletz.

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National Research Council. Washington, D.C.: National Academy Press, 1980.

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5.3.3 Australia

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CONTENTS

Federal Highway Administration244

5.3.4 Germany

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CONTENTS

251

Roundabouts: An Informational Guide • 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

(A-1)

where:

= entry capacity, pce/h

= circulating flow, pce/h

=− − − −











1 0 00347 30 0 978 1005.(). .

(A-2)

Fx=303 2

(A-3)

ftx

=+0 210 1 0 2

.(.)

(A-4)

+−











105

160

exp

(A-5)

212

=+ −

+(A-6)

Sev

=−

′

16.( )

(A-7)

where:

= entry width, m

= approach half width, m

l’

= effective flare length, m

= sharpness of flare, m/m

= inscribed circle diameter, m

= entry angle, degrees

= entry radius, m

QkFfQ fQF

fQ F

ecccc

=− ≤

(),

Appendix A Operational Analysis Formulas

CONTENTS

Federal Highway Administration252

A.1.2 Parameter assumptions

= 40 m

= 20 m

= 30 degrees

= 4 m

l’

= 40 m

Sev

=−

′=−=

16 164 4

40 0

.( ) .( )

+−











=105

160

1 4404

exp

12 444

120 4=+ −

+=+ −

()

Fx===303 303 4 1212

2()

ftx

=+=0 210 1 0 2 0 5447

.(.).

=− − − −









=1 0 00347 30 0 978 1005 1.(). .

A.1.3 Final equation

=−1212 0 5447.

(A-8)

A.2 Double-Lane Roundabout

A.2.1 Equations

See Section A.1.1.

A.2.2 Parameter assumptions

= 55 m

= 20 m

= 30 degrees

= 8 m

l’

= 40 m

For design purposes, when

then

l’

is effectively zero.

However, setting

l’

= 0 results in

being undefined. Therefore a

non-zero value of

l’

has been

selected. When

any

non-zero value of

l’

results in

= 0 and

For design purposes, when

then

l’

is effectively zero.

However, setting

l’

= 0 results in

being undefined. Therefore a

non-zero value of

l’

has been

selected. When

any

non-zero value of

l’

results in

= 0 and

CONTENTS

253

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

Sev

=−

′=−=

16 168 8

40 0

.( ) .( )

+−











=105

160

1 3112

exp

212 888

120 8=+ −

+=+ −

()

Fx===303 303 8 2424

()

ftx

=+=0 210 1 0 2 0 7159

.(.).

=− − − −









=1 0 00347 30 0 978 1005 1.(). .

A.2.3 Final equation

=−2424 0 7159.

(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 single-

lane circulatory roadway. This equation, developed by Brilon, Wu, and Bondzio is as

follows:

=−1218 0 74.

(A-10)

where:

= entry capacity, pce/h

= circulating flow, pce/h

CONTENTS

Federal Highway Administration254

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,

xx x

f right

F right F right

F right

()

=++

(A-11)

Dropping some subscripts,

() ()

(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:

(A-13)

with

xLT

. Capacity

qmax

is then

qkq

imax =

(A-14)

where

is flow in lane

and

max

(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,

, divided by the degree of saturation,

max

max 2

(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 circu-

lating flow conditions: 500 veh/h,1000 veh/h, 1500 veh/h, and 2000 veh/h.

CONTENTS

255

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

Qc = 500 veh/h Qc=1000 veh/h Qc=1500 veh/h Qc=2000 veh/h

n TRL Wu TRL Wu TRL Wu TRL Wu

0 940 940 668 668 395 395 123 123

1 1447 1461 1151 1208 855 955 559 702

2 1636 1640 1321 1356 1006 1072 691 787

3 1737 1737 1411 1436 1086 1135 761 834

4 1799 1799 1468 1487 1136 1175 805 864

5 1841 1841 1506 1522 1170 1203 835 884

6 1872 1871 1534 1547 1195 1223 857 899

7 1896 1895 1555 1566 1214 1238 873 910

8 1914 1913 1571 1581 1229 1250 886 919

9 1929 1928 1585 1594 1240 1260 896 926

10 1941 1940 1596 1604 1250 1268 905 932

11 1951 1950 1605 1612 1258 1274 912 936

12 1960 1959 1612 1619 1265 1280 918 941

13 1967 1966 1619 1626 1271 1285 923 944

14 1974 1973 1625 1631 1276 1289 928 947

15 1979 1978 1630 1636 1281 1293 931 950

16 1984 1983 1635 1640 1285 1296 935 952

17 1989 1988 1639 1644 1288 1299 938 955

18 1993 1992 1642 1647 1292 1302 941 957

19 1996 1996 1645 1650 1294 1304 943 958

20 2000 1999 1648 1653 1297 1306 946 960

2066 2066 1708 1708 1350 1350 992 992

Exhibit A-2. Graphical

comparison of TRL and Wu

short-lane methodologies.

Exhibit A-1. Tabular

comparison of TRL and Wu

short-lane methodologies.

CONTENTS

Federal Highway Administration256

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. Uni-

versity of Idaho, 1997.

3. Wu, N. “Capacity of shared/short lanes at unsignalized intersections.” In

Proceedings of the Third International Symposium on Intersections without Traf-

fic Signals

(ed: M. Kyte), Portland, Oregon, U.S.A. University of Idaho, 1997.

CONTENTS

257

Roundabouts: An Informational Guide • 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 round-

about category. Note that the flared-entry roundabout uses the same range of

inscribed circle diameters as the double-lane roundabouts. Note that the dimen-

sions 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.

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)

Inscribed Circle

Site Category Diameter Range

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.

Exhibit B-1. Typical inscribed

circle diameter ranges by

roundabout category.

Example Roundabout Designs

Appendix B

CONTENTS

Federal Highway Administration258

Exhibit B-2. Example of a

typical mini-roundabout.

CONTENTS

259

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

Exhibit B-3. Example of a

typical urban compact

roundabout.

CONTENTS

Federal Highway Administration260

Exhibit B-4. Example of a

typical single-lane roundabout.

CONTENTS

261

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

Exhibit B-5. Example of a

typical urban double-lane

roundabout.

CONTENTS

Federal Highway Administration262

Exhibit B-6. Example of a

typical flared-entry roundabout.

CONTENTS

263

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

Exhibit B-7. Example of a

typical rural single-lane

roundabout.

CONTENTS

Federal Highway Administration264

Exhibit B-8. Example of a

typical rural double-lane

roundabout.

CONTENTS

265

Roundabouts: An Informational Guide • Appendix C: MUTCD Recommendations

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 clos-

est 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,

MUTCD Recommendation

Appendix C

CONTENTS

Federal Highway Administration

266

and driver expectation are much different for roundabouts than for traditional yield-

controlled locations (typically low-volume streets or right-turn bypass lanes). Iden-

tification that a roundabout is upcoming is particularly important for multilane ap-

proaches 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 round-

about 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 consid-

ered 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 varia-

tions 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 juris-

diction is evident.

Exhibit C-3. Sample of

existing Roundabout Ahead

signs in United States.

(g) (g)

(b)

(a)

(h)

(f)

Bradenton Beach, FL

Mary Esther, FL

Lisbon, MD

Leeds, MD

Lothian, MD

Naples, FL

West Boca Raton, FL

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

CONTENTS

267

Roundabouts: An Informational Guide • Appendix C: MUTCD Recommendations

Exhibit C-4. Sample of

Roundabout Ahead signs used

internationally.

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 uses the internationally recognized circular ring of ar-

rows to represent a roundabout and is almost an exact mirror image of the sign

used in Australia (Exhibit C-4).

•The recommended sign gives advanced notice of the proper direction of circu-

lation. The NCUTCD-adopted sign in Exhibit C-2 does not convey this informa-

tion and could give the driver the incorrect impression that the circulatory road-

way is bidirectional.

International practice varies from country to country but is generally more consis-

tent 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.

(i)

(n)

(k)

(j)

Exhibit C-3 (continued).

(i) Santa Barbara, CA

(j) Tallahassee, FL

(k) Taneytown, MD

(l) Tavares, FL

(m) Vail, CO

(n) West Vail, CO

(m)

(l)

United Kingdom Australia

CONTENTS

Federal Highway Administration

268

Exhibit C-5. Dimensions of

Roundabout Ahead sign.

•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 round-

abouts 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 circu-

lation. The recommended sign provides guidance to the driver as to the proper

direction of circulation.

CONTENTS

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