Author: Daniel B. Fambro, Kay Fitzpatrick, Rodger J. Koppa

Date of Publication:  1997

Sponsoring Agency:  American Association of State Highway and Transportation Officials in cooperation with the U.S. Department of Transportation, Federal Highway Administration

Abstract:

This report describes the development of recommended revisions to the stopping sight
distance (SSD) design policy that appears in portions of Chapters II and III of the 1994 AASHTO publication, A Policy on Geometric Design of Highways and Streets (referred to as the Green Book). It also proposes modifications to other sections of the Green Book that currently reference stopping sight distance. The contents of this report are, therefore, of immediate interest to highway designers; highway-operations, capacity, and traffic-control personnel; and others concerned with highway safety. The report’s conclusions are derived from field observations of driver performance, driver visual capacity, driver eye heights, and vehicle heights, as well as safety and operational studies.

The current AASHTO stopping sight distance model has two components: (1) perception-reaction time, which is equated to the distance a vehicle travels at a fixed speed while these actions occur, and (2) braking distance, the distance the vehicle travels during the braking maneuver. This model has been altered only slightly since its inception in the 1940s, and it continues to result in well-designed roads. However, the hypothesis that the worst-case scenario-with its conservative assumptions of reaction time and pavement friction values and unproven driver visual capabilities - combined with an assumed below average driver, results in a model that provides a considerable margin of safety but is difficult to justify or defend as representative of either a real-life environment or a safe driving behavior.

The Texas Transportation Institute (TTI) at Texas A&M University was awarded NCHRP Project 3-42, Determination of Stopping Sight Distances, to evaluate, on the basis of the impact on vertical and horizontal curve design, the current AASHTO methodology and alternative approaches to establishing stopping sight distance. TTI produced five working papers describing their research, under controlled testing environments, into the different aspects that make up the components of the SSD: (1) Driver Braking Performance, which studied drivers’ perception and reaction times in unexpected situations, deceleration characteristics of unexpected braking, and braking distances associated with those events; (2) Driver Visual Capacity, which measured driver capability in detecting and recognizing objects of various sizes and contrasts under different lighting conditions; (3) Driver Eye and Vehicle Heights, which collected real-world data to construct a cumulative distribution of driver eye, headlight, taillight, and vehicle heights as determined by a more current (1994) vehicle fleet; (4) Safety Effects, which collected and analyzed accident data for identified roadway segments containing limited sight distance crest vertical curves; and (5) Operational Effects, which evaluated the relationship between design and operating speeds at crest vertical curves with limited stopping sight distance. The information developed in these working papers is the basis for this report.

The recommended SSD model remains conceptually the same as the existing AASHTO model, that is, SSD equals reaction distance plus braking distance, but with initial speed equal to design speed and design deceleration substituted for friction coefficient. As with the current model, the minimum SSD, driver eye height, and object height values are used to calculate required minimum length of vertical curve (VC) and required minimum rate of curvature or lateral clearance on horizontal curves. The recommended changes result in proposed SSDs that fall between current AASHTO minimum and desirable design values, crest K values (K is the VC length divided by the algebraic difference of adjoining grades) that are slightly below AASHTO current minimum design values, sag K design values that fall between current AASHTO minimum and desirable design values, and horizontal curve offsets that are also between AASHTO current minimum and desirable design values. This research was undertaken, not because of safety concerns with the current AASHTO SSD model, but to propose scientifically-based, reproducible, rational SSD design values reflecting driver capabilities and performance.

No. of Pages:  134