Diss. Stockholm : Kungliga tekniska högskolan. Institutionen för Transportvetenskap, 2015

The majority of the road infrastructure in developing countries consists of two-lane highways with one lane in each travel direction. Operational efficiency of these highways is derived from intermittent passing zones where fast vehicles are permitted by design to pass slow vehicles using the opposite traffic lane. Passing zones contribute to reduction of travel delay and queuing of fast vehicles behind slow vehicles. This show ever increases crash risks between passing and opposite vehicles especially at high traffic volumes due to reduction of passing opportunities. Reduction of passing-related crash risks is therefore a primary concern of policy makers, planners, and highway design engineers. Despite the wide application of passing zones on two-lane highways, there is limited knowledge on the underlying causal mechanisms that exacerbate crash risks, and the essential tools to assess safety of the passing zones. This thesis presents a methodology to appraise safety of two-lane rural highways based on observed operation of passing zones. The proposed methodology takes into account the impact of traffic and geometric factors on the rate passing maneuvers end inside passing zones and in the no-passing zones, adequacy of the design passing sight distance, and time-to-collision at the end of passing maneuvers. The thesis is comprised of five papers addressing capacity and safety aspects of passing zones on two-lane rural highways. Paper I presents a review of the literature on capacity and safety of passing zones. Paper II discusses adequacy of the design passing sight distance based on the sight distance required to complete a passing maneuver using observed data. Paper III discusses formulation, estimation, and application of a model to predict the passing rate using geometric and traffic factors, and applications. Paper IV discusses risk appraisal of the passing process based on the probability to complete passing maneuvers with time-to-collision less than 3.0 seconds taking into account the accepted gap in the opposite direction and the passing duration. Paper V discusses formulation and estimation of models to predict the probability and the rate at which passing maneuvers end in a no-passing zone, and applications.