Estimation of the Crash Modification Factor (CMF) for Shoulder Rumble Strips on Horizontal Curves of Two-Lane Two-Way Rural Highways Using the Cross-Sectional Method (Case Study: Lorestan Province)

Shoulder rumble strips, as a cost-effective and efficient safety countermeasure, play a crucial role in reducing Run-off-road crashes. However, the effectiveness of this measure is influenced by the geometric, traffic, and environmental conditions of each region. Directly applying Crash Modification Factors (CMFs) developed in other countries or regions may lead to unrealistic estimates. This study aims to develop a local CMF for the implementation of shoulder rumble strips on horizontal curves of two-lane two-way rural highways in Lorestan Province using the Cross-Sectional method. To this end, data were collected for 100 horizontal curves, including 42 curves with rumble strips and 58 curves without them, over the period from 2019 to 2023. Run-off-road crash data were obtained from the Traffic Police, while geometric and traffic information was acquired from the General Directorate of Road Maintenance and Transportation. In this study, the frequency of run-off-road crashes was considered the dependent variable, while the average annual daily traffic (AADT), curve radius, curve length, shoulder width, and speed limit were treated as independent variables. The presence or absence of rumble strips was included in the model as a dummy variable. Modeling was performed using negative binomial regression and the maximum likelihood method. The results indicated that the presence of rumble strips had a significant negative effect on the frequency of run-off-road crashes, with an estimated CMF of 0.68, representing an approximate 32% reduction in crashes following implementation.