95-100 Hassan

95 THE CANADIAN HIGHWAY NETwork consists mainly of two-lane highways where passing slower cars requires traveling in the lane assigned to the opposite traffic. Therefore, studying the mechanism of the passing maneuver is important for maintaining traffic safety, as well enhancing the highway’s level of service. Models for passing maneuvers are available in the design guides1,2 while different values for the required passing sight distance (PSD) are recommended for marking passing and no-passing zones.3,4 Further models have been developed to account for the trade-offs between the needs to complete or abort the passing maneuver.5,6,7 However, these models do not consider the effect of the highway geometry and cannot represent special cases of passing maneuvers. An example of these special cases is a passenger car trailing a truck on an upgrade, where both vehicles have to stop then accelerate from a zero speed. Due to the low acceleration rate of the truck, the driver of the passenger car may get impatient and decide to pass the truck during proceeding. PSD requirements for this case are different from the traditional PSD requirements. The potential hazards that may result from introducing trucks on a road with such conditions cannot be quantified. This feature discusses the PSD needs for the special case of passing during acceleration. A stepwise modeling of the passing maneuver is used to determine the required PSD at any point, as well as the corresponding speeds of the passing and impeding vehicles. This modeling is used to study the effect of the highway geometry, in terms of longitudinal upgrade, on the required PSD and on the driver behavior. Also, the effect of the ratio between the accelerations of the passenger car and the truck is studied. CURRENT PSD REQUIREMENTS The required PSD in highway design guides1,2 is based on the sight distance needs to complete the pass at the moment the maneuver begins. This assumption ignores the ability of the passing driver to abort the pass during its early stages, thus tending to overestimate the required PSD. Other developed models,5,6,7 based on the concept of critical position, account for the trade-offs between the sight distance needs to complete or abort the pass. According to this concept, the sight distance required to complete the pass is maximum at the beginning and decreases as the pass evolves. The sight distance required to abort the pass is minimal at the beginning and increases as the pass evolves. Therefore, the required PSD during the pass early stages should correspond to the needs for aborting the maneuver. During the late stages, the required PSD should correspond to the needs for completing the maneuver. The point at which the sight distances required for completing or aborting the pass are equal is referred to as the critical position. The corresponding sight distance is the maximum required PSD and is referred to as the critical sight distance. The concept of the critical position produces PSD requirements that are less than those recommended in the design guides.