Effective Shoulder Width on Rural Highway System Related to Roadway Departure Crashes

"Roadway design plays a crucial role in traffic safety, particularly on rural roads. This research investigates the safety effectiveness of shoulder width on Indiana’s rural two-lane highways, and to use this understanding to provide a basis for more effective shoulder improvement programs. Although previous studies have established the benefits of shoulder widening in other states, limited research has been conducted in Indiana conditions. This research aims to evaluate the safety effectiveness of shoulders on rural roads in Indiana and to propose a practical and systemic approach to identifying road segments that require additional attention and possible need for shoulder improvements. A negative binomial model with random effects was implemented to evaluate run off road crash frequency on two lane rural roads in relation to shoulder width. Nine years of crash data from 2015 to 2023 from over 5,000 miles of Indiana rural highways was used. This model incorporates geometric factors including shoulder width, traffic exposure, and the presence of rumble strips to quantify the impact of shoulder width on crash frequency. Crash Modification Factors (CMFs) were developed based on the existing combinations of shoulder and lane width, and proposed design alternatives. This allows for a comparison among different configurations. Findings indicated that increasing shoulder width generally reduces crash frequency. Installing a 5- or 6-foot shoulder in a location that previously did not have a shoulder resulted in the highest reduction in crashes according to CMFs. Shoulders equal to or exceeding 7 feet showed diminishing safety benefits when compared to shoulders of 5–6 feet. This is potentially due to the additional risk-taking by drivers under seemingly safer conditions. The presence of roadside rumble strips was found to reduce run-off-road (ROR) crashes by 9.1%."