13.5 Intersection Sight Distance

Each intersection has the potential for several different types of vehicular conflicts. The possibility of these conflicts actually occurring can be greatly reduced through the provision of proper sight distances and appropriate traffic controls. The avoidance of conflicts and the efficiency of traffic operations still depend on the judgement, capabilities, and response of each individual driver.
The operator of a vehicle approaching an intersection should have an unobstructed view of the entire intersection and an adequate view of the intersecting highway to permit control of the vehicle to avoid a collision. This visibility is referred to as intersection sight distance. When designing an intersection, the following factors should be taken into consideration:
  • Minimum intersection sight distance based on the appropriate case should be provided along both highway approaches and across corners (consider the use of landscaping in the sight distance calculations);
  • Gradients of intersecting highways should be as flat as practical on sections that are to be used for storage of stopped vehicles;
  • Combination of vertical and horizontal curvature should allow adequate sight distance of the intersection;
  • Traffic lanes and marked pedestrian crosswalks should be clearly visible at all times;
  • Lane markings and signs should be clearly visible and understandable from a desired distance;
  • Intersections should eliminate, relocate, or modify conflict points to the extent allowable in order to improve safety; and
  • Intersections should be evaluated for the effects of barriers, rails, and retaining walls on sight distance.
The provision of adequate sight distance is of utmost importance in the design of intersections. Consideration should be given to avoid steep profile grades and limited horizontal or vertical sight distance. An intersection should not be situated just beyond a short crest vertical curve or a sharp horizontal curve. Where necessary, backslopes should be flattened and horizontal and vertical curves lengthened to provide additional sight distance.
Figure 13-9
shows typical clear sight triangles to the left and to the right for a vehicle approaching an uncontrolled or yield-controlled intersection.
Figure 13-10
shows typical departure sight triangles to the left and to the right of the location of a stopped vehicle on the minor road.
Approach Sight Triangles for Intersections ( click in image to see full-size image)
Figure 13-9: Approach Sight Triangles for Intersections
Source: AASHTO’s A Policy on Geometric Design of Highways and Streets
Departure Sight Triangles for Intersections ( click in image to see full-size image)
Figure 13-10: Departure Sight Triangles for Intersections
Source: AASHTO’s A Policy on Geometric Design of Highways and Streets
Intersection sight distance criteria is provided in the subsequent sections for the following types of intersection controls:
  • Case A-Intersections with no control;
  • Case B-Intersections with stop control on the minor road;
  • Case C-Intersections with yield control on the minor road;
  • Case D-Intersections with traffic signal control;
  • Case E-Intersections with all-way stop control;
  • Case F-Left turns from the major road; and
  • Case G-Roundabouts
Intersection sight distance can be determined with the following formula:
tg=ta + w +La0.88Vminor
b
 = 1.47
V
major
t
g
where:
t
g
 = travel time to reach and clear the major road (s)
b
 = length of leg of sight triangle along the major road (ft)
t
a
 = travel time to reach the major road from the decision point for a vehicle that does not stop (s)
w
 = width of intersection to be crossed (ft)
L
a
 = length of design vehicle (ft)
V
minor
= design speed of minor road (mph)
V
major
 = design speed of major road (mph)
Refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for additional information on intersection sight distance.
Table 13-1: Adjustment Factors for Intersection Sight Distance Based on Approach Grade
U.S. Customary
Approach Grade (%)
Design Speed (mph)
15
20
25
30
35
40
45
50
55
60
65
70
75
80
-6
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.2
1.2
1.2
1.2
1.2
1.2
-5
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.2
1.2
1.2
-4
1.0
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
-3 to +3
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
+4
1.0
1.0
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
+5
1.0
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
+6
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
Notes:
  1. Based on ratio of stopping sight distance on specified approach grade to stopping sight distance on level terrain.
  2. This table is used in determining intersection sight distance criteria for Case C.

13.5.1 Case A-Intersections with no control

Refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for additional guidance on Intersections with no control.

13.5.2 Case B-Intersections with stop control on the minor road

Intersections with stop control on minor roads should consider three cases:
  • Left turns from minor road;
  • Right turns from minor road; and
  • Crossing a major road from a minor-road approach.
Intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to allow the intersection to operate smoothly. Minor-road vehicle operators can wait until they can proceed safely without forcing a major-road vehicle to stop.
Multiple design vehicles and intersection layout, such as median width, should be considered when determining sight distance. If the design vehicle used for intersection sight distance is larger than a passenger car, then sight distance checks, for both larger vehicle and passenger vehicle, should be calculated.
Left turns from minor roads should use a time gap (
t
g
)(s) shown in
Table 13-2
. Right turns from a minor road should use a time gap (
t
g
)(s) shown in
Table 13-3
. Crossing the major road from a minor road approach should use a time gap (
t
g
)(s) shown in
Table 13-4
.
Table 13-2: Time-Gap for Left Turn from Stop
Design Vehicle
Time Gap (t
g
)(s) at Design Speed of Major Road
Passenger car
7.5
Single-unit truck
9.5
Combination truck
11.5
Notes:
Time gaps are for a stopped vehicle to turn left onto a two-lane highway with no median and with minor road approach grades of 3 percent or less. The time gaps are applicable to determining sight distance to the right in left-turn maneuvers. The table values should be adjusted as follows:
For multilane roadways or medians – For left turns onto two-way roadways with more than two lanes, including turn lanes, add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane, from the left, in excess of one, to be crossed by the turning vehicle. Median widths should be converted to an equivalent number of lanes in applying the 0.5 and 0.7 s criteria presented above.
For minor-road approach grades – If the approach grade is an upgrade that exceeds 3 percent, add 0.2 s for each percent grade by which the approach grade exceeds zero percent.
Table 13-3: Time Gap for Right Turn from Stop
Design Vehicle
Time Gap (t
g
)(s) at Design Speed of Major Road
Passenger car
6.5
Single-unit truck
8.5
Combination truck
10.5
Notes:
Time gaps are for a stopped vehicle to turn right onto or to cross a two-lane roadway with no median and with minor-road approach grades of 3 percent or less. The table values should be adjusted as follows:
For minor-road approach grades – If the approach grade is an upgrade that exceeds 3 percent, add 0.1 s for each percent grade by which the approach grade exceeds zero percent.
Table 13-4: Time Gap for Crossing Maneuver from the Minor Road
Design Vehicle
Time Gap (t
g
)(s) at Design Speed of Major Road
Passenger car
6.5
Single-unit truck
8.5
Combination truck
10.5
Notes:
Time gaps are for a stopped vehicle to cross a two-lane highway with no median and with minor-road approach grades of 3 percent or less. The table values should be adjusted as follows:
For multilane roadways or medians—For crossing maneuvers that cross roadways with more than two lanes, including turn lanes, add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane, from the left, in excess of two, to be crossed by the turning vehicle. Median widths should be converted to equivalent lanes.
For minor-road approach grades—If the approach grade is an upgrade that exceeds 3 percent, add 0.2 s for each percent grade by which the approach grade exceeds zero percent.

13.5.3 Case C-Intersections with yield control on the minor road

Intersections with yield control on the minor roads should have two separate pairs of approach sight triangles as seen in
Figure 13-9
. A four-leg intersection requires sight triangles for left, right, and crossing maneuvers, while a three-leg intersection only requires left and right sight triangles. Adjustment factors should be applied to intersection sight distance on approach grades based on
Table 13-1
. Where adjustment factors cannot be applied, consideration should be given to installing regulatory speed signing to reduce speed or installing stop signs on one or more approaches.
Intersections with yield control on minor roads should consider two cases:
  • Crossing maneuver from the minor road; and
  • Left and right-turn maneuvers.
Crossing maneuvers from the minor road should use or exceed a time gap (
t
g
)(s) shown in
Table 13-3
. Left and right turns should use a time gap (
t
g
)(s) shown in
Table 13-5
.
Yield-controlled approaches generally need greater sight distance than stop-controlled approaches, especially at four-leg yield-controlled intersections where the sight distance needs of the crossing maneuver should be considered. If sight distance sufficient for yield control is not available, use of a stop sign instead of a yield sign should be considered. In addition, at locations where the recommended sight distance cannot be provided, consideration should be given to installing regulatory speed signing or other traffic control devices at the intersection on the major road to reduce the speeds of approaching vehicles.
Table 13-5: Time Gap for Left or Right Turn at Yield-Controlled Intersections
Design Vehicle
Time Gap (t
g
)(s)
Passenger car
8.0
Single-unit truck
10.0
Combination truck
12.0
Notes:
Time gaps are for a vehicle to turn right or left onto a two-lane roadway with no median. The table values should be adjusted for multilane roadways:
For multilane roadways or medians—For left turns onto two-way roadways with more than two lanes, including turn lanes, add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane, from the left, in excess of one, to be crossed by the turning vehicle. Median widths should be converted to equivalent lanes.
For right turns, no adjustment is needed.

13.5.4 Case D-Intersections with traffic signal control

Signalized intersections require the first stopped vehicle on one approach to be visible to the first vehicle stopped on all other approaches. Left-turning vehicles should have sufficient sight distance to select gaps in oncoming traffic and complete left turns.
For traffic signal control intersections with a pattern of sight distance-related crashes due to limited visibility, an appropriate crash countermeasure is signalization. However, if the traffic signal is to be placed on two-way flashing operation (i.e., flashing yellow on the major-road approaches and flashing red on the minor-road approaches) under off-peak or nighttime conditions, then Case B sight triangles should be provided for the minor road approaches. In addition, if right turns on a red signal are permitted from any approach, then the appropriate departure sight triangle for Case B should be provided to accommodate right turns from that approach.

13.5.5 Case E-Intersections with all-way stop control

Intersections with all way stop control require the first stopped vehicle on one approach to be visible to the first stopped vehicle on all other approaches. No other sight distance criteria is applicable.

13.5.6 Case F-Left turns from the major road

A left turn from the major road across opposing traffic when permitted, including intersections and driveways, should have sufficient sight distance to accommodate the maneuver. Sight distance design should be based on a left turn by a stopped vehicle since a vehicle that turns left without stopping would need less sight distance.
Left turns from the major road should use a time gap (
t
g
)(s) shown in
Table 13-6
.
If stopping sight distance has been provided continuously along the major road and if sight distance for Case B (Stop control) or Case C (Yield control) has been provided for each minor-road approach, sight distance will generally be adequate for left turns from the major road. On three-leg intersections or driveways located near a horizontal curve, it is recommended to check the left turn sight distance for possibility of obstructions in the median.
Table 13-6: Time Gap for Left Turns from the Major Road
Design Vehicle
Time Gap (t
g
)(s) at Design Speed of Major Road
Passenger car
5.5
Single-unit truck
6.5
Combination truck
7.5
Notes:
Time gaps are for a stopped vehicle turning left from a two-lane highway with no median
For multilane and/or divided roadways—For left turns on two-way roadways across more than one opposing lane, including turn lanes, add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane to be crossed in the left-turn maneuver in excess of one lane. Where the left-turning vehicle must pass through a median, the median width should be converted to an equivalent number of lanes.

13.5.7 Case G-Roundabouts

See
Section 14.4.5.7
, and refer to
NCHRP Report 1043
, Section 9.5 for intersection sight distance.