01658310

Component

The AASHTO stopping sight distance equation has two terms; a brake reaction term describing the distance travelled by the vehicle from the time the driver sees an obstacle on the roadway until the moment braking is initiated, and a braking term describing the distance travelled while the vehicle is slowed to a stop. For a vehicle on a grade, AASHTO recommends the braking term be modified, however the result is an equation that does not make sense because the brake reaction term represents a distance along the grade while the modified braking term represents a horizontal distance. And yet, such a modified equation has been repeated in design manuals, textbooks and research articles. The purpose of this paper was to propose a stopping sight distance equation that produces a horizontal distance suitable for the geometric design of vertical alignments. The distances calculated by the modified AASHTO equation and the proposed equation were compared. At design speeds up to 70 mph and grades up to ±12 %, the difference was insignificant. Therefore, using the proposed equation will not make any practical impact on future designs or cause the safety of existing designs to be questioned. However, the proposed equation better represents the physics of a vehicle braking on a grade. If the Green Book were updated to include the proposed equation or at least a discussion of the impact of grade on brake reaction distance, then perhaps this topic would be better addressed in engineering education and research.

This paper was sponsored by TRB committee AFB10 Standing Committee on Geometric Design.

18-00949

English

10p

2018

Transportation Research Board 97th Annual Meeting

Digital/other

Figures; References; Tables

Braking; Highway design; Reaction time; Stopping sight distance

American Association of State Highway and Transportation Officials

Design; Highways

Transportation Research Board Annual Meeting 2018 Paper #18-00949

TRIS, TRB, ATRI

Jan 8 2018 10:14AM