PASSING SIGHT DISTANCE ON 2-LANE HIGHWAYS - REVIEW AND REVISION

Several models have been developed to determine the minimum passing si ght distance required for safe and efficient operation on two-lane hig hways. The American Association of State Highway and Transportation Of ficials has developed a model assuming that once the driver begins a p ass, he/she has no opportunity but to complete it. This assumption is believed to result in exaggerated passing sight distance requirements. Considerably shorter passing sight distance values are presented in t he Manual of Uniform Traffic Control Devices and are used as the marki ng standards in Canada and the U.S.A. More appropriate models have bee n developed considering the driver's opportunity to abort the pass, an d are based on a critical sight distance which produces the same facto r of safety whether the pass is completed or aborted. However, these m odels need to be revised to determine the passing sight distance requi rements more accurately and to closely match field observations. In th is paper, a revised model for determining the minimum required passing sight distance was developed, based on the concept of critical sight distance and considering the kinematic interaction between the passing , passed, and opposing vehicles. The results of the revised model were compared with field data and showed that the revised model simulates the passing manoeuvre better than the currently-available models which are either too conservative or too liberal. The results showed that t he passing sight distance requirements recommended in the Manual of Un iform Traffic Control Devices are sufficient at low design speeds (50- 60 k.p.h.) and for manoeuvres involving passenger cars only. For highe r design speeds, the Manual of Uniform Traffic Control Devices standar ds are less than the passing sight distance required for safe and comf ortable passes. The deficiency was found to increase with the increase in design speed, and reaches about 36% at a 120-k.p.h. design speed. Based on these results, major revisions to the current Manual of Unifo rm Traffic Control Devices marking standards are recommended. Copyrigh t (C) 1996 Elsevier Science Ltd