Revisiting Variability of Dispersion Parameter of Safety Performance for Two-Lane Rural Roads

Safety performance functions (SPFs) are commonly calibrated with negative binomial regression in which a dispersion parameter that represents extra-Poisson variation is estimated. The primary use of this parameter is in empirical Bayes estimation for safety management applications such as treatment evaluation and network screening. It stands to reason that the importance of precise estimation of the dispersion parameter should be established. Recent research has suggested that the dispersion parameter is not constant but actually varies from site to site, depending on site characteristics such as segment length. In revisiting the dispersion parameter issue in this empirical investigation, previous research on this issue is reinforced by filling a number of knowledge gaps. First, light is cast on the dispersion parameter variation for SPFs for two-lane rural roads, an important entity type for which there is little or no knowledge in this regard. In this study, more precise model forms are investigated to represent the variation with respect to the key variable, segment length. This investigation confirms that the dispersion parameter is inversely related to segment length but reveals that it is not inversely proportional to segment length, as suggested in other research. Second, it was found that the dispersion parameter is smaller and the variation less pronounced with better-specified models. Finally, additional evidence is provided to suggest that dispersion parameter variation matters more for shorter segment lengths.

• Record URL:
  https://doi.org/10.3141/2148-05
• Summary URL:
  http://scholar.google.com/scholar_lookup?title=Revisiting+Variability+of+Dispersion+Parameter+of+Safety+Performance+for+Two-Lane+Rural+Roads&author=S.+Cafiso&author=G.+Di+Silvestro&author=B.+Persaud&author=M.+Begum&publication_year=2010
• Availability:
  • Find a library where document is available. Order URL: http://www.trb.org/Main/Blurbs/Highway_Safety_Data_Analysis_and_Evaluation_2010_V_163895.aspx
• Authors:
  • Cafiso, Salvatore
  • Di Silvestro, Giacomo
  • Persaud, Bhagwant
  • Begum, Morjina Ara
• Publication Date: 2010

Language

• English

Media Info

• Media Type: Print
• Features: Figures; References; Tables;
• Pagination: pp 38-46
• Monograph Title: Highway Safety Data, Analysis, and Evaluation 2010, Volume 2
• Serial:
  • Transportation Research Record: Journal of the Transportation Research Board
  • Issue Number: 2148
  • Publisher: Transportation Research Board
  • ISSN: 0361-1981

Subject/Index Terms

• TRT Terms: Dispersion (Statistics); Highway safety; Mathematical prediction; Rural highways; Safety factors; Safety management; Two lane highways
• Uncontrolled Terms: Empirical Bayes method; Negative binomial regression; Safety performance functions; Segment length
• Subject Areas: Highways; Safety and Human Factors; I82: Accidents and Transport Infrastructure;

Filing Info

• Accession Number: 01152091
• Record Type: Publication
• ISBN: 9780309142793
• Report/Paper Numbers: 10-3572
• Files: TRIS, TRB, ATRI
• Created Date: Mar 16 2010 6:12AM