Development of Safety Performance Functions for Multilane Highway and Freeway Segments Maintained by the Virginia Department of Transportation

Report No: 14-R14

Published in 2014

About the report:

The Virginia Department of Transportation (VDOT) adopted the software Safety Analyst™ as its highway safety management tool in 2009. One of the requirements for implementation of Safety Analyst is to have appropriate safety performance functions (SPFs) reflecting Virginia conditions. The purpose of this study was to develop such SPFs for multilane highway and freeway segments that could replace Safety Analyst’s default SPFs. Five years (2004-2008) of data collected from 20,235 multilane highway segments and 2,905 directional freeway segments in Virginia were used in the development of the SPFs. Statewide SPFs were developed for 4 subtypes of multilane highway segments and 10 subtypes of freeway segments.
VDOT district-group SPFs were developed for 4 subtypes of multilane highway segments.

The default SPFs in Safety Analyst were found to be different than the developed Virginia SPFs with respect to their curve shapes, and, as a result, adjusting the default SPFs to Virginia conditions by calibration factors resulted in inaccurate crash predictions at low and high volumes of annual average daily traffic. Thus, the Virginia-specific statewide SPFs developed in this study should be used when implementing Safety Analyst in Virginia.

Although the shapes of the multilane highway segment SPFs were found to vary across VDOT districts, incorporating variations through the creation of new subtypes was found to be inappropriate for the current version of Safety Analyst. As a consequence, district-group SPFs for the multilane highway segments cannot be implemented in Safety Analyst. However, all SPFs developed in this study, including district-group SPFs, can be implemented without the use of Safety Analyst. Therefore, use of the statewide SPFs developed in this study is recommended when Safety Analyst can be used and use of the statewide or district-group SPFs developed in this study is recommended when implementation of Safety Analyst is not feasible.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


Other Authors

Young-Jun Kweon, Ph.D., P.E., In-Kyu Lim

Last updated: November 13, 2023

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