Highway Stopping Sight Distance, Decision Sight Distance, and Passing Sight Distance Based on AASHTO Models

Abstract

Highway sight distance is a measure of roadway visibility, which is an important factor in the assessment of road safety. Greater visibility can provide motorists more time to avoid crashes and conflicts, facilitating safe and efficient operation. However, poor visibility can reduce the driver’s ability to react to changing conditions and is a significant factor in roadway crashes and near collisions. A driver’s ability to view ambient roadway conditions is necessary for safe operation of a vehicle. The roadway must have sufficient sight distance that drivers have the time to react to and avoid striking unexpected objects in their path. In addition, certain two-lane, two-way highways should also have adequate passing sight distance to enable drivers to use the opposing traffic lane for passing other vehicles without interfering with oncoming vehicles. This paper presents the concept and analysis of three different types of sight distance that are considered in highway geometric design based on AASHTO models: 1) the sight distances needed for stopping, applicable to all highway travels; 2) the sight distances needed for decisions at hazardous complex locations; and 3) the passing sight distance needed on two lane highways.