Study Details

Study Title: Developing Crash Modification Factors for Interchange Influence Areas on Urban Four-Lane Freeways Using Multivariate Adaptive Regression Splines

Authors: Haleem, Gan, and Lu

Publication Date:JAN, 2012

Abstract: Crash modification factors (CMFs) are used to measure the safety impacts from changes in specific geometric characteristics. Their development has gained much interest following the adoption of CMFs by the recently released Highway Safety Manual (HSM) and SafetyAnalyst tool. This paper aims at developing CMFs for interchange influence areas on urban four-lane freeways in the state of Florida. The need to model freeway interchange areas apart from their basic mainlines has been recognized by SafetyAnalyst. Different methods for developing CMFs can be found in the literature. This paper introduces a promising data mining method known as multivariate adaptive regression splines (MARS) for CMF development. MARS is characterized by its capability to accommodate the nonlinearity in crash predictors, which can be a mixture of continuous and categorical variables. In addition, MARS allows the impact of more than one geometric variable to be simultaneously considered when estimating CMFs. Separate CMFs were developed for inside and outside shoulder widths within the interchange influence areas. The results showed that MARS produced CMFs with higher confidence than those using the negative binomial model. The influential variables identified included the inside shoulder width, outside shoulder width, traffic volume, shoulder type, and speed limit. It was also found that inside shoulders less than 4 ft and 4 to 6 ft experienced safety improvement in relative to the 4-ft width.

Study Citation: Haleem, K., A. Gan, and J. Lu, "Developing Crash Modification Factors for Interchange Influence Areas on Urban Four-Lane Freeways Using Multivariate Adaptive Regression Splines." TRB 91st Annual Meeting Compendium of Papers, Washington, D.C, (2012).

Study Report: Download the Study Report Document

Comments: Study 276 (TRB Paper) and Study 351 (AAP Paper) are from the same authors and present different results. The differences in the CMFs between the two studies could be due to the different time periods analyzed (Study 276 used 2005 – 2008 FL data; Study 351 uses 2007 – 2010 FL data), size of the dataset (the dataset used in Study 276 has 12,078 total crashes – both calibration and prediction dataset, combined; the dataset used in Study 351 has 58,394 total crashes – both calibration and prediction dataset, combined) and base conditions for inside and outside shoulder widths (Study 276 used 4 ft inside and 10 ft outside shoulder width as the base condition; Study 351 used 10 ft inside and 10 ft outside shoulder width as the base condition). Both studies show a decrease in crashes, in either direction of the base condition. The difference in the base condition coupled with unknown disaggregate sample sizes makes it harder to interpret these results. It is expected that the crashes would decrease with an increase in shoulder width; however, the authors state that reducing the shoulder width from the base condition would lead to most drivers being unable to park alongside the narrow width and could easily change one lane (along a two-lane per direction freeway) to park on the right (outside) shoulder width. This reduces the risk of fatal crashes with vehicles traveling in the higher-speed leftmost lane due to stopping on the left (inside) shoulder. FHWA encourages customers to review the full study to determine the applicability of the CMFs.


CMFs Associated With This Study

Category: Shoulder treatments

Countermeasure: Decrease inside paved shoulder width from 4ft to 2ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.72827.23 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Decrease inside paved shoulder width from 4ft to 2ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.66833.23 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Decrease inside paved shoulder width from 4ft to 2ft (outside paved shoulder width = 8ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.74925.13 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Decrease outside paved shoulder width from 10ft to 4ft (inside paved shoulder width = 4ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.41758.33 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Decrease outside paved shoulder width from 10ft to 8ft (inside paved shoulder width = 4ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.81918.13 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 10ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.66533.53 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 10ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9772.33 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 10ft (outside paved shoulder width = 8ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.77622.43 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 11ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.173-17.33 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 11ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.294-29.43 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 12ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9920.83 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 12ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.9465.43 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 12ft (outside paved shoulder width = 8ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.043-4.33 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 5ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.57342.73 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 6ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.47252.83 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 6ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.56143.93 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 6ft (outside paved shoulder width = 8ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.75724.33 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 8ft

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.78223 StarsAllK,A,B,CPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 8ft (outside paved shoulder width = 10ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.224-22.43 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase inside paved shoulder width from 4ft to 8ft (outside paved shoulder width = 8ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.63936.13 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase outside paved shoulder width from 10ft to 11ft (inside paved shoulder width = 4ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.169-16.93 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban

Countermeasure: Increase outside paved shoulder width from 10ft to 12ft (inside paved shoulder width = 4ft)

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.79320.73 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysUrban