Estimation of Safety Effectiveness of Widening Shoulders and Adding Passing Lanes on Rural Two-Lane Roads

Abstract

There is a need to evaluate the costs and benefits of potential safety improvements so that limited budgets for low-volume roads may be used most effectively. Increasing the width of shoulders and adding passing lanes have been regarded as economical and effective safety improvements. The state of Kansas has more than 2,000 mi (3,219 km) of highway that may need these improvements, most of which are rural two-lane roads. This study considered how historical data and crash modification factors (CMFs) were combined to estimate the safety benefits of widening shoulders and adding passing lanes on rural two-lane roads. The objective was to determine specific CMFs for the state of Kansas and states with similar rural environments as well as to estimate the number of crashes avoided. Safety and traffic volume data for rural two-lane roads from 2000 to 2008 were obtained from the State of Kansas and combined with existing models from earlier studies to create state-specific CMFs to more accurately estimate safety benefits. It was determined in this study that CMFs for shoulder widening and the addition of passing lanes for low-volume roads are 0.95 and 0.65, respectively. These CMFs are based on shoulder widening from 2 ft (0.61 m) to 8 ft (2.44 m) or construction of a short four-lane passing section. From these results, 20-year projections were developed for the benefits in crash reductions that could be achieved through implementing these safety improvements.