Differences in the performance of safety performance functions estimated for total crash count and for crash count by crash type

Jonsson, Thomas, Lyon, Craig, Ivan, John, Washington, Simon, van Schalkwyk, Ida, & Lord, Dominique (2009) Differences in the performance of safety performance functions estimated for total crash count and for crash count by crash type. Transportation Research Record, 2102(2009), pp. 115-123.

View at publisher (open access)


In recent years the development and use of crash prediction models for roadway safety analyses have received substantial attention. These models, also known as safety performance functions (SPFs), relate the expected crash frequency of roadway elements (intersections, road segments, on-ramps) to traffic volumes and other geometric and operational characteristics. A commonly practiced approach for applying intersection SPFs is to assume that crash types occur in fixed proportions (e.g., rear-end crashes make up 20% of crashes, angle crashes 35%, and so forth) and then apply these fixed proportions to crash totals to estimate crash frequencies by type. As demonstrated in this paper, such a practice makes questionable assumptions and results in considerable error in estimating crash proportions. Through the use of rudimentary SPFs based solely on the annual average daily traffic (AADT) of major and minor roads, the homogeneity-in-proportions assumption is shown not to hold across AADT, because crash proportions vary as a function of both major and minor road AADT. For example, with minor road AADT of 400 vehicles per day, the proportion of intersecting-direction crashes decreases from about 50% with 2,000 major road AADT to about 15% with 82,000 AADT. Same-direction crashes increase from about 15% to 55% for the same comparison. The homogeneity-in-proportions assumption should be abandoned, and crash type models should be used to predict crash frequency by crash type. SPFs that use additional geometric variables would only exacerbate the problem quantified here. Comparison of models for different crash types using additional geometric variables remains the subject of future research.

Impact and interest:

9 citations in Scopus
Search Google Scholar™
6 citations in Web of Science®

Citation counts are sourced monthly from Scopus and Web of Science® citation databases.

These databases contain citations from different subsets of available publications and different time periods and thus the citation count from each is usually different. Some works are not in either database and no count is displayed. Scopus includes citations from articles published in 1996 onwards, and Web of Science® generally from 1980 onwards.

Citations counts from the Google Scholar™ indexing service can be viewed at the linked Google Scholar™ search.

ID Code: 38180
Item Type: Journal Article
Refereed: Yes
DOI: 10.3141/2102-15
ISSN: 0361-1981
Subjects: Australian and New Zealand Standard Research Classification > ENGINEERING (090000) > CIVIL ENGINEERING (090500)
Australian and New Zealand Standard Research Classification > BUILT ENVIRONMENT AND DESIGN (120000) > URBAN AND REGIONAL PLANNING (120500)
Divisions: Current > Research Centres > Centre for Accident Research & Road Safety - Qld (CARRS-Q)
Current > QUT Faculties and Divisions > Faculty of Health
Current > Schools > School of Psychology & Counselling
Copyright Owner: Copyright 2009 Transportation Research Board of the National Academies
Deposited On: 26 Oct 2010 05:26
Last Modified: 29 Feb 2012 14:18

Export: EndNote | Dublin Core | BibTeX

Repository Staff Only: item control page