In estimating safety performance, the most common probabilistic structures of the popular statistical models used by transportation safety analysts for modeling motor vehicle crashes are the traditional Poisson and Poissongamma (or negative binomial) distributions. Because crash data often exhibit overdispersion, Poissongamma models are usually the preferred model. The dispersion parameter of Poissongamma models had been assumed to be fixed, but recent research in highway safety has shown that the parameter can potentially be dependent on the covariates, especially for flow-only models. Given that the dispersion parameter is a key variable for computing confidence intervals, there is reason to believe that a varying dispersion parameter could affect the computation of confidence intervals compared with confidence intervals produced from Poissongamma models with a fixed dispersion parameter.

This study evaluates whether the varying dispersion parameter affects the computation of the confidence intervals for the gamma mean (m) and predicted response (y) on sites that have not been used for estimating the predictive model. To accomplish that objective, predictive models with fixed and varying dispersion parameters were estimated by using data collected in California at 537 three-leg rural unsignalized intersections.

The study shows that models developed with a varying dispersion parameter greatly influence the confidence intervals of the gamma mean and predictive response. More specifically, models with a varying dispersion parameter usually produce smaller confidence intervals, and hence more precise estimates, than models with a fixed dispersion parameter, both for the gamma mean and for the predicted response. Therefore, it is recommended to develop models with a varying dispersion whenever possible, especially if they are used for screening purposes.

Geedipally, S.R. and D. Lord. Effects of Varying Dispersion Parameter of Poisson-Gamma Models on Estimation of Confidence Intervals of Crash Prediction Models. In Transportation Research Record: Journal of the Transportation Research Board, No. 2061, Transportation Research Board, National Research Council, Washington, D.C., 2008, pp. 46-54.