Study Details

Study Title: Estimation of Safety Effectiveness of Changes in Shoulder Width using Case-Control and Cohort Methods

Authors: Gross and Jovanis

Publication Date: 2007

Abstract: This paper explores the use of observational data to estimate safety effectiveness for changes in shoulder width. Common statistical models are applied to estimate safety effectiveness of shoulder width; however, two unique approaches, the case-control and cohort method, are used in the design stage. Case-control and cohort designs are common methods in epidemiological studies where they are used to develop relationships between risk factors and disease. In highway safety the case-control and cohort designs show potential for relating geometric and traffic characteristics to crashes. Case-control and cohort methods produce estimates of the odds ratio and relative risk, respectively. Odds ratios and relative risks indicate the expected percent change in the probability of an outcome given a unit change in the risk factor. In the highway safety application, the odds ratio and relative risk may provide a reasonable estimate of the crash modification factor (CMF) for the risk factor in question. Geometric, traffic and crash data were obtained for two-lane, rural, undivided highway segments in Pennsylvania. Conditional logistic regression models were developed to estimate the odds ratio for shoulder width while Cox Proportional Hazard models were developed to estimate relative risk. Confounding variables were identified in the preliminary analysis and adjustments were made for confounders during the design and final analysis stages. Estimates from the two methods indicate that crashes decrease as shoulder width increases. Results are compared to the CMF presented in the Highway Safety Manual and confidence intervals are included to illustrate the certainty of the estimates.

Study Citation: Gross, F. and Jovanis, P. P., "Estimation of Safety Effectiveness of Changes in Shoulder Width using Case-Control and Cohort Methods." 2007 TRB 86th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#07-2762, Washington, D.C., (2007)


CMFs Associated With This Study

Category: Shoulder treatments

Countermeasure: Reduce shoulder width (6 ft to 0 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.12-123 StarsAllAllNot specifiedRural

Countermeasure: Reduce shoulder width (6 ft to 1 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.17-173 StarsAllAllNot specifiedRural

Countermeasure: Reduce shoulder width (6 ft to 2 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.11-113 StarsAllAllNot specifiedRural

Countermeasure: Reduce shoulder width (6 ft to 4 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.06-63 StarsAllAllNot specifiedRural

Countermeasure: Reduce shoulder width (6 ft to 5 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.02-23 StarsAllAllNot specifiedRural

Countermeasure: Widen shoulder (from 6 to 7 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.01-13 StarsAllAllNot specifiedRural

Countermeasure: Widen shoulder (from 6 to 8 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9643 StarsAllAllNot specifiedRural

Countermeasure: Widen shoulder (from 6 to 9 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.79213 StarsAllAllNot specifiedRural

Countermeasure: Widen shoulder (from 6 to >9 ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.82183 StarsAllAllNot specifiedRural