What's Wrong with Applying the
New AASHTO Opposing Left-Turn
Intersection Sight Distance Design
Standard to Existing Roadways?

John C. Glennon, D. Engr., P.E.
March 2005 (copyright)

Since 1940, the American Association of State Highway and Transportation Officials (AASHTO) has been looked to as the most authoritative source of highway design standards. AASHTO has published updated design standards at frequent intervals since that time.

In the 2001 Policy on Geometric Design for Streets and Highways, AASHTO added a previously unaddressed standard for sight distance to opposing left-turn movements. In other words, how much sight distance to an opposing head-on vehicle does a left-turn driver need to turn off of the major highway.

Because the AASHTO Policy has a stated disclaimer that their design standards only apply to new construction, and because the application of longstanding stopping and intersection sight distance standards almost always provide adequate sight distance for the opposing left-turn maneuvers, the only possible motivation for this new standard seems to be for providing safety on divided roadways where objects such as vegetation or other vehicles in the median might potentially obstruct the view to opposing vehicles.

With all of the above discussion set aside, however, it is clear that AASHTO design standards are used worldwide to judge the safety of existing roadways. In this context, the application of the AASHTO opposing left-turn sight distance standard can come up short when considering intersections where sharp vertical crests or objects on the inside of sharp horizontal curves create a substantial sight obstruction.

Because sharp horizontal curve and hillcrest obstructions are similar in their potential deleterious effects, I will simplify this discussion by only addressing hillcrests. Similarly, because intersections can be situated before, at, or beyond a sharp hillcrest, I will further simplify the discussion by only considering a side road that forms the stem of a T-intersection. All of the following discussion using these simplifications can be easily modified to analyze similar intersections that are not directly covered.

AASHTO Opposing Left Turn Sight Distance Defined

The main discussion about opposing left-turn sight distance, on pages 678-9 of the 2001 Policy on Geometric Design for Streets and Highway, is stated as follows:

Case F-Left Turns From the Major Road

All locations along a major highway from which vehicles are permitted to turn left across opposing traffic, including intersections and driveways, should have sufficient sight distance to accommodate the left-turn maneuver. Left-turning drivers need sufficient sight distance to decide when it is safe to turn left across the lane(s) used by opposing traffic. 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. The sight distance along the major road to accommodate left turns is the distance traversed at the design speed of the major-road in the travel time for the design vehicle given in Exhibit 9-66.


Design Vehicle Time Gap at Design Speed
of Major Road
Passenger Car 5.5 seconds
Single-unit Truck 6.5
Combination Truck 7.5
Adjustment for Multilane Highways: For left-turning vehicles that cross more than one opposing lane, add 0.5 seconds for passenger cars and 0.7 seconds for trucks for each additional lane to be crossed.

Exhibit 9-66. Time Gap for Case F - Left Turns From the Major Road




The table also contains appropriate adjustment factors for the number of major-road lanes to be crossed by the turning vehicle. The unadjusted time gap in Exhibit 9-66 for passenger cars was used to develop the sight distances in Exhibit 9-67.


Distance (ft)
Intersection Sight
(Passenger Cars)
15 80 121.3 125
20 115 161.7 165
25 155 202.1 205
30 200 242.6 245
35 250 283.0 285
40 305 323.4 325
45 360 363.8 365
50 425 404.3 405
55 495 444.7 445
60 570 485.1 490
65 645 525.5 530
70 730 566.0 570
75 820 606.4 610
80 910 646.8 650
Note: Intersection sight distance shown is for a passenger car making a left turn from an undivided highway. For other conditions and design vehicles, the time gap should be adjusted and the sight distance recalculated.

Exhibit 9-67. Intersection Sight Distance - Case F - Left Turn From Major Road



Evaluating Existing Intersection with the New AASHTO Standard

Thousands of intersections on rural 2-lane secondary roads in the U.S. are at or near a sharp hillcrest. When evaluating a roadway using the AASHTO Stopping Sight Distance Standard, these existing sharp hillcrests are often found to have effective "design speeds" (safe speeds) of 30 mph or less. However, when using AASHTO to measure for opposing left-turn sight distance on these hillcrests, that analysis can be misleading depending on how the intersection lines up with the actual crest.. Because AASHTO concludes that available opposing left-turn sight distance should be measured from a stopped vehicle (presumably at or near the center of the intersection), the available left-turn sight distance could be judged adequate for those intersections that are near or beyond the sharp crest where the sight distance opens up.

What the AASHTO assumption fails to recognize is that most drivers making left-turns on rural 2-lane roadways will begin turning well before the center of the intersection, will slow but not stop during the turn, and will usually enter the opposing lane before reaching the intersection. Understanding these realities, the safe left turn sight distance needs to be available not only at the intersection, but also a distance before the intersection of, say 50 feet. There are three factors that possibly compensate for one another. With the average speed of these approaching vehicles higher than the average speed of vehicles turning from a stop, with the left-lane exposure distance greater, and with the turning vehicle contributing to the reduction of the available gap as it approaches the intersection, the time gaps shown in Exhibit 9-66 may also nominally apply to the approaching vehicle.

The conclusion of all of this discussion is that the standard opposing left-turn sight distances at sharp hillcrests and other similarly sight-restricted conditions need to be available both at the intersection and for some reasonable distance along the approach to that intersection for the left-turn vehicle.

About the Author

Dr. John C. Glennon is a traffic engineer with over 45 years experience. He has over 120 publications. He is the author of the book "Roadway Safety and Tort Liability" and is frequently called to testify both about roadway defects and as a crash reconstructionist.


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