01152091

Component

https://doi.org/10.3141/2148-05

http://scholar.google.com...egum&publication_year=2010

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.

01164464

10-3572

English

pp 38-46

2010

Transportation Research Record: Journal of the Transportation Research Board

9780309142793

Print

Figures (7) ; References (18) ; Tables (3)

Dispersion (Statistics); Highway safety; Mathematical prediction; Rural highways; Safety factors; Safety management; Two lane highways

Empirical Bayes method; Negative binomial regression; Safety performance functions; Segment length

Highways; Safety and Human Factors; I82: Accidents and Transport Infrastructure

TRIS, TRB, ATRI

Jan 25 2010 11:49AM

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