Safety Effectiveness of Various Types of Shoulders on Rural Two-Lane Roads in Winter and Non-winter Periods

There has been growing recognition of the quantitative effects of various roadway designs and traffic control strategies on safety. Meanwhile, there is increasing interest in measuring the variances of safety effectiveness in different periods of the year for similar roadway designs or similar traffic control strategies. This study tried to address the variances of safety effectiveness between the winter and non-winter periods for the ten most common shoulder designs in Kansas. Traffic and geometric data were collected on 6,510 miles (10,477 km) of rural two-lane highways in Kansas. A cross-sectional approach was applied to develop winter period safety performance functions (SPFs), non-winter period SPFs and SPFs aggregated at an annual level in which shoulder designs were treated as independent variables. A variance test was conducted based on these SPFs to investigate the variances of safety effectiveness between the two different periods. It was found that wider and upgraded shoulders offer significant less safety benefit in reducing total crash number during winter periods than during non-winter periods. The indexes of safety effectiveness for the winter period are larger than those for the non-winter period by between 13 to 25 percent. However, winter weather appears not to significantly diminish wider and/or upgraded shoulders’ safety benefit in reducing crash severity and the number of shoulder related crashes. The results demonstrate that treating the winter and non-winter data equally is likely to bias a shoulder’s estimated safety effectiveness in total crashes.