Geometric Design Practices for Resurfacing, Restoration, and Rehabilitation (2011)

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93 APPENDIX F E Highway Safety Manual and Crash Reduction Factors Many of the criteria used for design of new roads and roads being rehabilitated or reconstructed, including the 13 critical design elements discussed in this report, have been developed, in part, based on safety relationships. With an understanding of how crashes change with changes in lane width, for example, under varying conditions of speed, alignment, traffic volume, etc., design standards and guidelines can be developed. TRB Special Report 214 devoted a full chapter discussing the relationships of safety and geometric design. This was also the subject of an NCHRP study that was reported as NCHRP Report 374: Effects of Highway Design Standards on Highway Safety. In recent years, research on safety relationships have focused not just on developing prediction models; that is, equation relationships, but also on developing crash reduction factors (CRFs) and crash modification factors (CMFs). For example, NCHRP Report 633: Impact of Shoulder Width and Median Width on Safety yielded CMFs for shoulder width and median width for four-lane roads with 12-ft lanes. The results of safety relationship research such as this has been included in the newly published Highway Safety Manual and the Crash Modification Clearinghouse provides an on-line database of all current crash reduction factors. These two sources are summarized in this appendix as they apply to 3R projects. HIGHWAY SAFETY MANUAL The Highway Safety Manual has been recently completed and is published by AASHTO. The purpose of the HSM is to provide analytical tools and techniques for quantifying the potential effects on crashes as a result of decisions made in planning, design, operations, and maintenance. As stated in the HSM, it can be used to: • Identify sites with the most potential for crash frequency or severity reduction; • Identify factors contributing to crashes and associated potential countermeasures to address these issues; • Conduct economic appraisals of improvements and prioritize projects; • Evaluate the crash reduction benefits of implemented treatments; • Calculate the effect of various design alternatives on crash frequency and severity; • Estimate potential crash frequency and severity on highway networks; and

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95 CMFs being developed through research by states or other agencies are being amassed in a national database that can be accessed from a web-based clearinghouse. CRASH MODIFICATION CLEARINGHOUSE Crash Modification Factors can also be found at the CMF clearinghouse, which is a web-based database of CMFs along with supporting information. It is sponsored by the FHWA and maintained by the Highway Safety Research Center at the University of North Carolina. It can be accessed at http://www.cmfclearinghouse.org/. It contains all the CMFs in the HSM and continually updates that list as new CMFs are developed. CMFs are provided for the following categories: • Access Management • Advanced Technology and ITS • Alignment • Highway Lighting • Interchange Design • Interchange Geometry • Intersection Traffic Control • On-street Parking • Pedestrians and Bicyclists • Railroad Grade Crossings • Roadside • Roadway • Roadway Delineation • Roadway Signs and Traffic Control • Shoulder Treatments • Speed Management • Work Zones Upon entering the clearinghouse, the user has the ability to search for specific countermeasures within the above groupings. The information that is provided includes— • The CMF expressed as a decimal. • The CRF expressed as a percentage. • A star quality rating—The star quality rating indicates the quality or confidence in the results of the study producing the CMF. It is based on a scale (1 to 5), where 5 indicates the highest or most reliable rating.

96 • The crash type to which the factors apply. • The crash severity to which the factors apply. • The roadway type to which the factors apply. • The area type to which the factors apply. • The reference from which the factor was obtained. The reference can be viewed as well. An example of one listing from “Shoulder Treatment” is shown here. Countermeasure: Continuous milled-in shoulder rumble strips CMF CRF (%) Quality Crash Type Crash Severity Roadway Type Area Type Reference 0.21 [B] 79 Run off road All Principal Arterial Other Freeways and Expressways Not specified Perrillo, K., 1998 0.9 10 Run off road All Principal Arterial Other Rural Carrasco et al., 2004 0.78 22 Run off road Serious injury, Minor injury Principal Arterial Other Rural Carrasco et al., 2004 0.84 [B] 16 All All Principal Arterial Other Rural Carrasco et al., 2004 0.83 [I] 17 All Serious injury, Minor injury Principal Arterial Other Rural Carrasco et al., 2004