Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Trons/m zyxwvutsrqponmlkjihgfedcbaZYXWVU Rr s.-A. Vol. 30. No. 6, pp. 453-467. 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 096S-8x4/96 $lS.OO + 0.00 0!965-8564(%)00032-1 PASSING SIGHT DISTANCE ON TWO-LANE HIGHWAYS: REVIEW AND REVISION YASSER HASSAN Carleton University, Ottawa. Ontario, Canada, KIS 5B6 SAID M. EASA Lakehead University, Thunder Bay, Ontario, Canada, P7B 5El and A. 0. ABD EL HALIM Carleton University Ottawa, Ontario, Canada, KIS 5B6 (Received 13 February 1995: in revisedform 21 June 1995) Abstract-Several models have been developed to determine the minimum passing sight distance required for safe and efficient operation on two-lane highways. The American Association of State Highway and Transportation Officials has developed a model assuming that once the driver begins a pass, 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 the Manual of Uniform Traffic Control Devices and are used as the marking standards in Canada and the U.S.A. More appropriate models have been developed considering the driver’s opportunity to abort the pass, and are based on a critical sight distance which produces the same factor of safety whether the pass is completed or aborted. However, these models need to be revised to determine the passing sight distance requirements more accurately and to closely match field observations. In this 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 the passing sight distance requirements recommended in the Manual of Uniform Traffic Control Devices are sufficient at low design speeds (5&60 k.p.h.) and for manoeuvres involving passenger cars only. For higher design speeds, the Manual of Uniform Traffic Control Devices standards are less than the passing sight distance required for safe and comfortable 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 Uniform Traffic Control Devices marking standards are recommended. Copyright 0 1996 Elsevier Science Ltd INTRODUCTION Sight distance is one of the fundamental elements in achieving safe and efficient operation of highways. The American Association of State Highway and Transportation Officials (AASHTO) and the Transportation Association of Canada (TAC), formerly the Road and Transportation Association of Canada (RTAC), state that the designer should provide sufficient sight distance for the drivers to control the speed of their cars before striking an unexpected obstacle in the travelled way (RTAC. 1986; AASHTO. 1994). Subsequently, highway design standards are established so as to provide sufficient stopping sight distance (SSD) on all highways. Moreover, the capacity of a segment of a two-lane highway, as explained in the Highway Capacity Manual (HCM, 1994), depends significantly on the percentage of segment length where passing manoeuvres are prohibited because the existence of any slow vehicle will generate a queue of other slow vehicles trailing it. Therefore, a passing sight distance (PSD) sufficient for the drivers to pass slow vehicles should be provided at frequent intervals on two-lane highways. However, to apply this recommendation in 453