Study Details

Study Title: Exploration and comparison of crash modification factors for multiple treatments on rural multilane roadways

Authors: Park et al.

Publication Date: SEP, 2014

Abstract: As multiple treatments (or countermeasures) are simultaneously applied to roadways, there is a need to assess their combined safety effects. Due to a lack of empirical crash modification factors (CMFs) for multiple treatments, the Highway Safety Manual (HSM) and other related studies developed various methods of combining multiple CMFs for single treatments. However, the literature did not evaluate the accuracy of these methods using CMFs obtained from the same study area. Thus, the main objectives of this research are: (1) develop CMFs for two single treatments (shoulder rumble strips, widening shoulder width) and one combined treatment (shoulder rumble strips + widening shoulder width) using before-after and cross-sectional methods and (2) evaluate the accuracy of the combined CMFs for multiple treatments estimated by the existing methods based on actual evaluated combined CMFs. Data was collected for rural multi-lane highways in Florida and four safety performance functions (SPFs) were estimated using 360 reference sites for two crash types (All crashes and Single Vehicle Run-off Roadway(SVROR) crashes) and two severity levels (all severity (KABCO) and injury (KABC)).The results of both before-after and cross-sectional methods show that the two single treatments and the combined treatment produced safety improvement. It was found that safety effects were higher for the roadway segments with shoulder rumble strips and wider shoulder width. It was also found that the treatments were more safety effective (i.e. lower CMF) for the roadway segments with narrower original shoulder width in the before period. However, although CMFs for multiple treatments were generally lower than CMFs for single treatments, they were similar for the roadway segments with shoulder width of 8-12 feet. More specifically, CMFs for single treatments were lower than CMFs for multiple treatments for the roadway segments with shoulder width of 9 feet or higher. Among different methods of combining CMFs, the HSM, Systematic Reduction of Subsequent CMFs, Applying only the most effective CMF, and Weighted average of multiple CMFs (Meta-Analysis) showed good estimates of the combined CMFs for multiple treatments with 2.2% difference between actual and estimated CMFs. The findings suggest that the existing methods of combining multiple CMFs are generally valid but they need to be applied for different crash types and injury levels separately. Lastly, an average of the combined CMFs from the best two methods was closer to the actual CMF than the combined CMF from only one best method. This indicates that it is better not to rely on only one specific existing method of combining CMFs for predicting CMF for multiple treatments.

Study Citation: Park, J., M. Abdel-Aty, and C. Lee. "Exploration and comparison of crash modification factors for multiple treatments on rural multilane roadways". Accident Analysis and Prevention, Vol. 70, (2014) pp. 167-177.


CMFs Associated With This Study

Category: Shoulder treatments

Countermeasure: Install shoulder rumble strips

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.763 23.7 4 Stars All All Not specified Rural
0.643 35.7 4 Stars All K,A,B,C Not specified Rural
0.651 34.9 3 Stars Run off road,Single vehicle All Not specified Rural
0.625 37.5 3 Stars Run off road,Single vehicle K,A,B,C Not specified Rural
0.614 38.6 2 Stars All All Not specified Rural
0.565 43.5 2 Stars All K,A,B,C Not specified Rural
0.792 20.8 2 Stars All All Not specified Rural
0.659 34.1 2 Stars All K,A,B,C Not specified Rural
CMF Equation CRF Equation 1 Star All All Not specified Rural
CMF Equation CRF Equation 1 Star All K,A,B,C Not specified Rural

Countermeasure: Install shoulder rumble strips and widen shoulder

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.608 39.2 4 Stars All All Not specified Rural
0.66 34 3 Stars All K,A,B,C Not specified Rural
0.541 45.9 4 Stars Run off road,Single vehicle All Not specified Rural
0.611 38.9 3 Stars Run off road,Single vehicle K,A,B,C Not specified Rural
0.351 64.9 3 Stars All All Not specified Rural
0.451 54.9 3 Stars All K,A,B,C Not specified Rural
0.807 19.3 3 Stars All All Not specified Rural
0.839 16.1 2 Stars All K,A,B,C Not specified Rural
CMF Equation CRF Equation 1 Star All All Not specified Rural
CMF Equation CRF Equation 1 Star All K,A,B,C Not specified Rural

Countermeasure: Widen shoulder

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.771 22.9 4 Stars All All Not specified Rural
0.688 31.2 4 Stars All K,A,B,C Not specified Rural
0.607 39.3 4 Stars Run off road,Single vehicle All Not specified Rural
0.566 43.4 4 Stars Run off road,Single vehicle K,A,B,C Not specified Rural
0.617 38.3 2 Stars All All Not specified Rural
0.5 50 2 Stars All K,A,B,C Not specified Rural
0.817 18.3 2 Stars All All Not specified Rural
0.814 18.6 2 Stars All K,A,B,C Not specified Rural
CMF Equation CRF Equation 1 Star All All Not specified Rural
CMF Equation CRF Equation 1 Star All K,A,B,C Not specified Rural