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Differences in the Performance of Safety Performance Functions Estimated for Total Crash Count and for Crash Count by Crash Type

Jonsson, Thomas LU ; Lyon, Craig; Ivan, John N.; Washington, Simon P.; van Schalkwyk, Ida and Lord, Dominique (2009) In Transportation Research Record p.115-123
Abstract
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... (More)
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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Transportation Research Record
issue
2102
pages
115 - 123
publisher
Transportation Research Board, Washington DC, USA
external identifiers
  • wos:000272002300015
  • scopus:76149146958
ISSN
0361-1981
DOI
10.3141/2102-15
language
English
LU publication?
yes
id
8e0cb317-bca9-4e2c-b61d-71840c7365a5 (old id 1517909)
date added to LUP
2010-01-13 11:42:22
date last changed
2017-04-09 04:00:24
@article{8e0cb317-bca9-4e2c-b61d-71840c7365a5,
  abstract     = {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.},
  author       = {Jonsson, Thomas and Lyon, Craig and Ivan, John N. and Washington, Simon P. and van Schalkwyk, Ida and Lord, Dominique},
  issn         = {0361-1981},
  language     = {eng},
  number       = {2102},
  pages        = {115--123},
  publisher    = {Transportation Research Board, Washington DC, USA},
  series       = {Transportation Research Record},
  title        = {Differences in the Performance of Safety Performance Functions Estimated for Total Crash Count and for Crash Count by Crash Type},
  url          = {http://dx.doi.org/10.3141/2102-15},
  year         = {2009},
}