Citation

Forbes TW, Greenshields BD. Highw. Res. Board Proc. 1939; 1939.

Copyright

(Copyright © 1939, National Research Council (U.S.A.), Highway Research Board)

DOI

unavailable

PMID

unavailable

Abstract

In determining the minimum sight distance which allows safe passing, it is necessary to know the interval needed by the driver to decide whether it is safe to attempt a pass. This interval must be added to the actual passing distance or time used in calculating the minimum tangent or straight stretch necessitated. Design engineers have recognized this judgment and perception time interval and various estimates as to its magnitude have been made. Two methods of measuring this interval are described. The first, which employs movies from directly above the test location, is capable of greater accuracy. The second method, which consists of timing by an observer with a stop watch, is rougher but easy and economical. The observer judges the physical layout to determine the beginning of the sight distance, and the driver registers his decision by stepping on the accelerator. A series of results are given for this rough check method. The results showed two judgment and perception time figures. Both were selected to include 80 percent of the cases. The first was 3.5 seconds and the second was 2.8 seconds. The former is more applicable to a tangent between curves and the latter may apply to other problems such as those relating to intersections.