Study Details

Study Title: Safety Effects of Lane Width and Shoulder Width on Two-Lane Rural State Highways in Idaho

Authors: Abdel-Rahim and Sonnen

Publication Date:JUL, 2012

Abstract: This study provides a comprehensive evaluation of the relationship between crash rates and shoulder width and lane width for two-lane rural state highways in Idaho. Crash Modification Factors (CMFs) for shoulder width and lane width were developed using Idaho crash data covering the period from 1993 to 2010. The CMFs developed as part of this project will allow ITD to assess the potential safety benefits of shoulder widening projects. In addition to all crashes, models for single-vehicle and multiple-vehicle crashes were also developed. The CMFs presented in this study follow the general trends of prior knowledge and research. The results of the analysis showed that there is no significant difference between 12 ft lanes and 11 ft lanes in terms of safety for all types of crashes. The CMF for highways with 11 ft lanes was 1.02 indicating a marginal 2 percent increase in all crashes in comparison to highways with standard 12 ft lanes. The CMFs for highways with very small shoulders (less than 1 ft) were 1.16, 1.17, and 1.15 for all crashes, single-vehicle crashes, and multiple-vehicle crashes, respectively. This corresponds to an average increase in crashes of 16 percent when compared to highways with a 3-ft shoulder width. For highway sections with a shoulder width of 8 ft or more, the CMFs were 0.87, 0.90, and 0.83 for all crashes, single-vehicle crashes, and multiple-vehicle crashes, respectively, indicating an average reduction in crashes of approximately 13 percent when compared to highways with a 3-ft shoulder width. Idaho's crash data was also used to investigate the characteristics of pedestrian and bicycle crashes on two-lane rural highways. The results show that roadway sections with a right paved shoulder width of 4 ft to 6 ft had the lowest number of pedestrian and bicycle crashes. The probability for a pedestrian/bicycle crash increases significantly for roadway sections with shoulder widths less than 3 ft. The likelihood of a crash also increases for roadway sections with shoulder widths of 8 ft or more.

Study Citation: Abdel-Rahim, A. and J. Sonnen. "Safety Effects of Lane Width and Shoulder Width on Two-Lane Rural State Highways in Idaho." Report No. FHWA-ID-12-200. Idaho Transportation Department, Boise, ID. (July 2012).

Study Report: Download the Study Report Document


CMFs Associated With This Study

Category: Roadway

Countermeasure: Reduce lane width from 12 ft to 10 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.05-53 StarsAllAllNot specifiedRural
1.07-73 StarsSingle vehicleAllNot specifiedRural
1.04-43 StarsMultiple vehicleAllNot specifiedRural
1.04-43 StarsAllAllNot specifiedRural
1.06-63 StarsSingle vehicleAllNot specifiedRural
1.04-43 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Reduce lane width from 12 ft to 11 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.02-23 StarsAllAllNot specifiedRural
1.02-23 StarsSingle vehicleAllNot specifiedRural
1.03-33 StarsMultiple vehicleAllNot specifiedRural
1.01-13 StarsAllAllNot specifiedRural
1.02-23 StarsSingle vehicleAllNot specifiedRural
1.02-23 StarsMultiple vehicleAllNot specifiedRural

Category:Shoulder treatments

Countermeasure: Reduce paved shoulder from 3 ft to 0 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.16-163 StarsAllAllNot specifiedRural
1.17-173 StarsSingle vehicleAllNot specifiedRural
1.15-153 StarsMultiple vehicleAllNot specifiedRural
1.13-133 StarsAllAllNot specifiedRural
1.14-143 StarsSingle vehicleAllNot specifiedRural
1.11-113 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Reduce paved shoulder from 3 ft to 1 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.13-133 StarsAllAllNot specifiedRural
1.07-73 StarsSingle vehicleAllNot specifiedRural
1.18-183 StarsMultiple vehicleAllNot specifiedRural
1.11-113 StarsAllAllNot specifiedRural
1.09-93 StarsSingle vehicleAllNot specifiedRural
1.13-133 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Reduce paved shoulder from 3 ft to 2 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.03-33 StarsAllAllNot specifiedRural
1.02-23 StarsSingle vehicleAllNot specifiedRural
1.03-33 StarsMultiple vehicleAllNot specifiedRural
1.03-33 StarsAllAllNot specifiedRural
1.03-33 StarsSingle vehicleAllNot specifiedRural
1.02-23 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Widen paved shoulder from 3 ft to 4 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9733 StarsAllAllNot specifiedRural
0.9553 StarsSingle vehicleAllNot specifiedRural
0.9733 StarsMultiple vehicleAllNot specifiedRural
0.9823 StarsAllAllNot specifiedRural
0.9463 StarsSingle vehicleAllNot specifiedRural
0.9733 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Widen paved shoulder from 3 ft to 5 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9553 StarsAllAllNot specifiedRural
0.9373 StarsSingle vehicleAllNot specifiedRural
0.9553 StarsMultiple vehicleAllNot specifiedRural
0.9463 StarsAllAllNot specifiedRural
0.9463 StarsSingle vehicleAllNot specifiedRural
0.9373 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Widen paved shoulder from 3 ft to 6 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9373 StarsAllAllNot specifiedRural
0.9193 StarsSingle vehicleAllNot specifiedRural
0.9463 StarsMultiple vehicleAllNot specifiedRural
0.9373 StarsAllAllNot specifiedRural
0.9103 StarsSingle vehicleAllNot specifiedRural
0.9463 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Widen paved shoulder from 3 ft to 7 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9193 StarsAllAllNot specifiedRural
0.9283 StarsSingle vehicleAllNot specifiedRural
0.89113 StarsMultiple vehicleAllNot specifiedRural
0.9103 StarsAllAllNot specifiedRural
0.9283 StarsSingle vehicleAllNot specifiedRural
0.88123 StarsMultiple vehicleAllNot specifiedRural

Countermeasure: Widen paved shoulder from 3 ft to 8 ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.87133 StarsAllAllNot specifiedRural
0.9103 StarsSingle vehicleAllNot specifiedRural
0.83173 StarsMultiple vehicleAllNot specifiedRural
0.88123 StarsAllAllNot specifiedRural
0.9193 StarsSingle vehicleAllNot specifiedRural
0.84163 StarsMultiple vehicleAllNot specifiedRural