Chapter 1260 Sight Distance Design Manual M 22 01 2

User Manual: 1260-2

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WSDOT Design Manual M 22-01.13 Page 1260-1
July 2016
Chapter 1260 Sight Distance
1260.01 General
1260.02 References
1260.03 Stopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)
1260.04 Passing Sight Distance (Eye height – 3.5 ft, Object height – 3.5 ft)
1260.05 Decision Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)
1260.06 Documentation
1260.01 General
Sight distance allows the driver to assess developing situations and take actions appropriate
for the conditions. Sight distance relies on drivers being aware of and paying attention to their
surroundings and driving appropriately for conditions presented. For the purposes of design,
sight distance is considered in terms of stopping sight distance, passing sight distance, and
decision sight distance.
For additional information, see the following chapters:
Chapter Subject
1250 Sight distance at railroad crossings
1310 Sight distance at intersections at grade
1320 Sight distance at roundabouts
1340 Sight distance at driveways
1515 Sight distance for shared-use paths
1260.02 References
1260.02(1) Design Guidance
Manual on Uniform Traffic Control Devices for Streets and Highways, USDOT, FHWA; as
adopted and modified by Chapter 468-95 WAC “Manual on uniform traffic control devices
for streets and highways” (MUTCD)
1260.02(2) Supporting Information
A Policy on Geometric Design of Highways and Streets (Green Book), AASHTO
Passing Sight Distance Criteria, NCHRP 605
1260.03 Stopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)
1260.03(1) Design Criteria
Stopping sight distance is provided when the sight distance available to a driver equals or exceeds
the stopping distance for a passenger car traveling at the design speed.
Stopping distance for design is very conservatively calculated, with lower deceleration and
slower perception reaction time than normally expected from the driver. Provide design stopping
sight distance at all points on all highways and on all intersecting roadways, unless a design
analysis is deemed appropriate.
Sight Distance Chapter 1260
Page 1260-2 WSDOT Design Manual M 22-01.10
July 2013
1260.03(1)(a) Stopping Sight Distance
Stopping sight distance is
the sum of two distances:
the distance traveled during
perception and reaction time
and the distance to stop the
vehicle. The perception and
reaction distance used in
design is the distance traveled
in 2.5 seconds at the design
speed.
The design stopping sight
distance is calculated using the
design speed and a constant
deceleration rate of 11.2
feet/second2. For stopping
sight distances on grades less
than 3%, see Exhibit 1260-1;
for grades 3% or greater, see
Exhibit 1260-2.
1260.03(1)(b) Design Stopping Sight Distance
Exhibit 1260-1 gives the design stopping sight distances for grades less than 3%, the minimum
curve length for a 1% grade change to provide the stopping sight distance for a crest (Kc) and
sag (Ks) vertical curve, and the minimum length of vertical curve for the design speed (VCLm).
For stopping sight distances when the grade is 3% or greater, see Exhibit 1260-2.
Design Speed
(mph)
Design Stopping
Sight Distance (ft)
Kc Ks VCLm (ft)
25
155
12
26
75
30
200
19
37
90
35
250
29
49
105
40
305
44
64
120
45
360
61
79
135
50
425
84
96
150
55
495
114
115
165
60
570
151
136
180
65
645
193
157
195
70
730
247
181
210
75
820
312
206
225
80
910
384
231
240
Design Stopping Sight Distance
Exhibit 1260-1
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-3
July 2013
1260.03(2) Effects of Grade
The grade of the highway has an effect on the stopping sight distance. The stopping distance is
increased on downgrades and decreased on upgrades. Exhibit 1260-2 gives the stopping sight
distances for grades of 3% and steeper. When evaluating sight distance with a changing grade,
use the grade for which the longest sight distance is needed.
Design Speed
(mph)
Stopping Sight Distance (ft)
Downgrade
-3%
-6%
-9%
3%
6%
9%
25
158
165
173
147
143
140
30
205
215
227
200
184
179
35
257
271
287
237
229
222
40
315
333
354
289
278
269
45
378
400
427
344
331
320
50
446
474
507
405
388
375
55
520
553
593
469
450
433
60
598
638
686
538
515
495
65
682
728
785
612
584
561
70
771
825
891
690
658
631
75
866
927
1,003
772
736
704
80
965
1,035
1,121
859
817
782
Design Stopping Sight Distance on Grades
Exhibit 1260-2
For stopping sight distances on grades between those listed, interpolate between the values
given or use the equation in Exhibit 1260-3.
±
+=
100
347826.030
)5.2(47.1 2
G
V
VS
Where:
S = Stopping sight distance on grade (ft)
V = Design speed (mph)
G = Grade (%)
Stopping Sight Distance on Grades
Exhibit 1260-3
1260.03(3) Crest Vertical Curves
When evaluating an existing roadway, refer to 1260.03(7).
Use Exhibit 1260-4 or the equations in Exhibit 1260-5 to find the minimum crest vertical curve
length to provide stopping sight distance when given the algebraic difference in grades. Exhibit
1260-4 does not show the sight distance greater than the length of curve equation.
Sight Distance Chapter 1260
Page 1260-4 WSDOT Design Manual M 22-01.10
July 2013
When the sight distance is greater than the length of curve and the length of curve is critical,
the S>L equation given in Exhibit 1260-5 shall be used to find the minimum curve length.
When a new crest vertical curve is built or an existing one is rebuilt with grades less than 3%,
provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, provide
stopping sight distance from 1260.03(2).
Eye height h
1
=3.5'
S
Object height h
2
=2.0'
The minimum length can also be determined by multiplying the algebraic difference in grades by the
KC value from Exhibit 1260-1 (L=KC*A). Both the exhibit and the equation give approximately the
same length of curve. Neither use the S>L equation.
Length of Vertical Curve, L (ft)
Algebraic Difference in Grade, A (%)
0
0500 1000 1500 2000
2
4
6
8
10
12
14
16
25 mph, S=155 ft.
30 mph, S=200 ft.
35 mph, S=250 ft.
40 mph, S=305 ft.
45 mph, S=360 ft.
50
mph, S=425 ft.
55 mph, S=495 ft.
60 mph, S=570 ft.
65 mph, S=645 ft.
70 mph, S=730 ft.
75 mph, S=820 ft.
80 mph, S=910 ft.
Stopping Sight Distance: Crest Vertical Curves
Exhibit 1260-4
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-5
July 2013
When S>L
A
SL 2158
2−=
2
2158
+
=A
L
S
When S<L
2158
2
AS
L=
A
L
S2158
=
Where:
L = Length of vertical curve (ft)
S = Sight distance (ft)
A = Algebraic difference in grades (%)
Sight Distance: Crest Vertical Curve
Exhibit 1260-5
1260.03(4) Sag Vertical Curves
When evaluating an existing roadway, refer to 1260.03(7).
Sight distance is not restricted by sag vertical curves during the hours of daylight. Therefore,
headlight sight distance is used for the sight distance design criteria at sag vertical curves.
In some cases, a lesser length may be allowed. For guidance, see Chapter 1220.
Refer to Exhibit 1260-6 or the equations in Exhibit 1260-7 to find the minimum length for
a sag vertical curve to provide the headlight stopping sight distance when given the algebraic
difference in grades. The value for S is shown as the distance between the vehicle and the
point where a 1-degree angle upward of the headlight beam intersects with the roadway.
The sight distance greater than the length of curve equation is not used in Exhibit 1260-6. When
the sight distance is greater than the length of curve and the length of curve is critical, the S>L
equation given in Exhibit 1260-7 shall be used to find the minimum length of curve.
When a new sag vertical curve is built or an existing one is rebuilt with grades less than 3%,
provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, provide
stopping sight distance from 1260.03(2).
Sight Distance Chapter 1260
Page 1260-6 WSDOT Design Manual M 22-01.10
July 2013
S
1
°
2.0'
The minimum length can also be determined by multiplying the algebraic difference in grades by the KS
value from Exhibit 1260-1 (L=KS*A). Both the exhibit and equation give approximately the same length
of curve. Neither use the S>L equation.
Length of Curve, L (ft)
0
0200 400 600 800 1000 1200 1400 1600 1800 2000
2
4
6
8
10
12
14
16
Algebraic Difference in Grade, A (%)
25 mph, S=155 ft
30 mph, S=200 ft
35 mph, S=250 ft
40 mph, S=305 ft
45 mph, S=360 ft
50 mph, S=425 ft
55 mph, S=495 ft
60 mph, S=570 ft
65 mph, S=645 ft
70 mph, S=730 ft
75 mph, S=820 ft
80 mph, S=910 ft
Stopping Sight Distance for Sag Vertical Curves
Exhibit 1260-6
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-7
July 2013
When
S>L
A
S
SL 5.3400
2+
−=
5.32
400
−
+
=A
LA
S
When
S<L
S
AS
L5.3400
2
+
=
( )
A
ALLL
S2
16005.35.3
2
+±
=
Where:
L = Curve length (ft)
A = Algebraic grade difference (%)
S = Sight distance (ft)
Note:
Values for A less than 1.75 are within the 1-degree diverge
of the headlight beam and therefore do not need to be
evaluated for SSD on sag curves.
Sight Distance: Sag Vertical Curve
Exhibit 1260-7
1260.03(5) Horizontal Curves
When evaluating an existing roadway, see 1260.03(7).
Use Exhibit 1260-8 or the equation in Exhibit 1260-9 to check stopping sight distance where
sightline obstructions are on the inside of a curve. A stopping sight distance sightline obstruction
is any roadside object within the
horizontal sightline offset (M)
distance (such as median barrier,
guardrail, bridges, walls, cut
slopes, buildings or wooded
areas), 2.0 feet or greater above
the roadway surface at the
centerline of the lane on the inside
of the curve (h0). Exhibit 1260-8
and the equation in Exhibit 1260-9
are for use when the length of
curve is greater than the sight
distance and the sight restriction is
more than half the sight distance
from the end of the curve. Where
the length of curve is less than the
stopping sight distance or the sight
restriction is near either end of the curve, the desired sight distance may be available with a lesser
M distance. When this occurs, the sight distance can be checked graphically.
Sight Distance Chapter 1260
Page 1260-8 WSDOT Design Manual M 22-01.10
July 2013
A sightline obstruction is any
roadside object within the
horizontal sightline offset (M)
distance, 2.0 feet or greater
above the roadway surface at
the centerline of the lane on
the inside of the curve.
Line of
sight
Sight distance (S)
Center of lane
M
Obstruction or
backslope Radius
30 mph,
S=200 ft
25 mph,
S=155 ft 50 mph,
S=425 ft
45 mph,
S=360 ft
0 1,000 2,000 3,000 4,000 5,000 6,000
0
10
20
30
40
50
60
Curve Radius, R (ft)
Lateral Clearance to Obstruction, M (ft)
40 mph,
S=305 ft 55 mph,
S=495 ft
60 mph, S=570 ft
65 mph, S=645 ft
70 mph, S=730 ft
75 mph, S=820 ft
35 mph,
S=250 ft
80 mph, S=910 ft
Horizontal Stopping Sight Distance
Exhibit 1260-8
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-9
July 2013
When the road grade is less than 3%, provide design stopping sight distance from Exhibit 1260-1.
When the grade is 3% or greater, provide stopping sight distance from 1260.03(2).
Roadside objects with a height (h0) between 2.0 feet and 2.75 feet might not be a stopping sight
distance sightline obstruction. Objects with an h0 between 2.0 feet and 2.75 feet can be checked
graphically to determine whether they are stopping sight distance sightline obstructions.
Where a sightline obstruction exists and site characteristics preclude design modifications to meet
criteria, consult with the region Traffic Engineer and Assistant State Design Engineer for a
determination of appropriate action.
−= R
S
RM 65.28
1cos
−
=−
R
MRR
S1
cos
65.28
Where:
M = Horizontal sightline offset measured from the centerline of
the inside lane of the curve to the sightline obstruction (ft)
R = Radius of the curve (ft)
S = Sight distance (ft)
Sight Distance: Horizontal Curves
Exhibit 1260-9
1260.03(6) Overlapping Horizontal and Vertical Curves
Vertical curves on a horizontal curve have an effect on which roadside objects are sightline
obstructions. Crest vertical curves make roadside objects more likely to become sightline
obstructions. Sag vertical curves make roadside objects less likely to be sightline obstructions.
Exhibit 1260-10 can be used to determine the sight distance for crest vertical curves on horizontal
curves with:
•Sightline
obstructions
inside the
M distance.
•Sightline
obstruction
height (h0)
of 2.0 feet
or less.
For other
locations, the
sight distance
can be checked
graphically.
h
2
=2.0'
h
1
=3.5'
h
O
Edge of roadway
IMI
L of lane
C
Roadside sightline
obstruction 2.0' or
less above L of lane
C
Line of
Sight
Sight Distance Chapter 1260
Page 1260-10 WSDOT Design Manual M 22-01.10
July 2013
The following equation may be used to determine the sight distance for roadside sightline
obstructions inside the horizontal sightline offset (M) distance (see Exhibit 1260-9) with
a height of 2.0 feet or less above the centerline of the lane on the inside of the curve on
overlapping horizontal and crest vertical curves.
( ) ( )
[ ]
A
hhhhL
S
2
0201
22100 −+−
=
Where:
L = Length of vertical curve (ft)
S = Sight distance (ft)
A = Algebraic difference in grades (%)
h1 = Eye height (3.5 ft)
h2 = Object height (2.0 ft)
h0 = Height of roadside sightline obstructions above the
centerline of the inside curve lane (2.0 ft or less)
Note:
The above equation cannot be used for sightline obstruction
height (h0) more than 2.0 ft above the centerline of the lane on
the inside of the curve. The available sight distance must be
checked graphically for these sightline obstructions.
Sight Distance: Overlapping Horizontal and Crest Vertical Curves
Exhibit 1260-10
1260.03(7) Existing Stopping Sight Distance
Existing stopping sight distance values from Exhibit 1260-11 may be used at all horizontal
and vertical curves where all of the following are met at the curve:
•There is no identified collision trend.
•The existing vertical and horizontal alignment is retained.
•The existing roadway pavement is not reconstructed.
•The roadway will not be widened, except for minor shoulder widening requiring no work
past the bottom of the ditch.
•The sightline obstruction is existing.
•Roadside improvements to sight distance are within existing right of way.
Crest Vertical Curves – The minimum length of an existing crest vertical curve may be found
using the equations in Exhibit 1260-5 or using the KC values from Exhibit 1260-11.
Sag Vertical Curves – The minimum length of an existing sag vertical curve may be found using
the equations in Exhibit 1260-7 or using the KS values from Exhibit 1260-11. In some cases,
when continuous illumination is provided, a lesser length may be allowed. For guidance, see
Chapter 1220.
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-11
July 2013
Design Speed
(mph)
Existing Stopping
Sight Distance (ft)
KC KS
20
115
6
16
25
145
10
23
30
180
15
31
35
220
22
41
40
260
31
52
45
305
43
63
50
350
57
75
55
400
74
89
60
455
96
104
65
495
114
115
70
540
135
127
75
585
159
140
80
630
184
152
Existing Stopping Sight Distance
Exhibit 1260-11
1260.04 Passing Sight Distance (Eye height – 3.5 ft, Object height – 3.5 ft)
1260.04(1) Design Criteria
Minimum passing sight distance
is the distance (on a two-lane
highway) used for a driver
to execute a normal passing
maneuver based on design
conditions and design speed.
The potential for passing maneuver
conflicts is ultimately determined
by the judgments of the driver and
the conditions present at the time
of the maneuver. Exhibit 1260-12
gives the passing sight distances
for various design speeds.
Passing Sight Distance
Exhibit 1260-12
Design Speed (mph)
Minimum Passing Sight
Distance (ft)
20
400
25
450
30
500
35
550
40
600
45
700
50
800
55
900
60
1000
65
1100
70
1200
75
1300
80
1400
Sight Distance Chapter 1260
Page 1260-12 WSDOT Design Manual M 22-01.10
July 2013
On two-lane two-way highways, provide passing opportunities to meet traffic volume demands.
This can be accomplished with roadway sections that provide passing sight distance or by adding
passing lanes at locations that would provide the greatest benefit to passing (see Chapter 1270).
In the design stage, passing sight distance can be provided by adjusting the alignment either
vertically or horizontally to increase passing opportunities.
These considerations also apply to multilane highways where staged construction includes a
two-lane two-way operation as an initial stage. Whether auxiliary lanes are provided, however,
depends on the time lag proposed between the initial stage and the final stage of construction.
1260.04(2) Passing Sight Distance Vertical Curves
Exhibit 1260-14 gives the length of crest vertical curve used to provide passing sight distance for
two lane highways. The distance from Exhibit 1260-12 and the equations in Exhibit 1260-13 may
also be used to determine the minimum length of vertical curve to meet the passing sight distance
criteria.
Sag vertical curves are not a restriction to passing sight distance.
A
Eye height h1=3.5' Object height h2=3.5'
S
When
S>L
A
SL 2800
2−=
A
L
S1400
2+=
When
S<L
2800
2
AS
L=
A
L
S2800
=
Where:
L = Length of vertical curve (ft)
A = Algebraic grade difference (%)
S =
Sight distance (ft)
Passing Sight Distance: Crest Vertical Curve Calculations
Exhibit 1260-13
Chapter 1260 Sight Distance
WSDOT Design Manual M 22-01.10 Page 1260-13
July 2013
Passing Sight Distance: Crest Vertical Curves
Exhibit 1260-14
1260.04(3) Passing Sight Distance Horizontal Curves
Passing sight distance can be restricted on the inside of a horizontal curve by sightline
obstructions that are 3.5 feet or more above the roadway surface. Use the distance from
Exhibit 1260-12 and the equation in Exhibit 1260-9 to determine whether the object is close
enough to the roadway to be a restriction to passing sight distance. The equation assumes that
the curve length is greater than the sight distance. Where the curve length is less than the sight
distance, the desired sight distance may be available with a lesser sightline offset (M) distance.
Sight Distance Chapter 1260
Page 1260-14 WSDOT Design Manual M 22-01.10
July 2013
1260.05 Decision Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft)
Decision sight distance values are greater than stopping sight distance values because they
give the driver an additional margin for error and afford sufficient length to maneuver at the
same or reduced speed rather than to just stop.
Consider decision sight distances (see Exhibit 1260-15) at locations where there is high
likelihood for driver error in information reception, decision making, or control actions.
If site characteristics and budget allow, locate these highway features where decision sight
distance can be provided. If this is not practicable, use suitable traffic control devices and
positive guidance to give advanced warning of the conditions.
Design Speed
(mph)
Decision Sight Distance for Maneuvers (ft)
A
B
C
D
E
30
220
490
450
535
620
35
275
590
525
625
720
40
330
690
600
715
825
45
395
800
675
800
930
50
465
910
750
890
1030
55
535
1030
865
980
1135
60
610
1150
990
1125
1280
65
695
1275
1050
1220
1365
70
780
1410
1105
1275
1445
75
875
1545
1180
1365
1545
80
970
1685
1260
1455
1650
Decision Sight Distance
Exhibit 1260-15
The maneuvers in Exhibit 1260-15 are as follows:
A = Rural stop
B = Urban stop
C = Rural speed/path/direction change
D = Suburban speed/path/direction change
E = Urban speed/path/direction change
Use the equations in Exhibits 1260-5, 1260-7, and 1260-9 to determine the available decision
sight distance for crest vertical curves, sag vertical curves, and horizontal curves.
1260.06 Documentation
It is recognized that some designs do not allow all criteria and guidelines to be followed as
outlined in this chapter.
Refer to Chapter 300 for design documentation requirements.

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