Open AccessMultivariate poisson-lognormal model for modeling related factors in crash frequency by severity
Author(s) -
Mehdi Tazhibi,
Iraj Kazemi,
Somaye Momenyan,
Hossein Haghshenas
Publication year - 2013
Publication title -
international journal of environmental health engineering
Language(s) - English
Resource type - Journals
ISSN - 2277-9183
DOI - 10.4103/2277-9183.114193
Subject(s) - univariate , statistics , crash , poisson distribution , log normal distribution , count data , multivariate statistics , poisson regression , bayesian probability , mathematics , dispersion (optics) , econometrics , computer science , demography , population , programming language , physics , sociology , optics
Aims: Traditionally, roadway safety analyses have used univariate distributions to model crash data for each level of severity separately. This paper uses the multivariate Poisson lognormal (MVPLN) models to estimate the expected crash frequency by two levels of severity and then compares those estimates with the univariate Poisson-lognormal (UVPLN) and the univariate Poisson (UVP) models. Materials and Methods: The parameters estimation is done by Bayesian method for crash data at two levels of severity at the intersection of Isfahan city for 6 months. Results: The results showed that there was over-dispersion issue in data. The UVP model is not able to overcome this problem while the MVPLN model can account for over-dispersion. Also, the estimates of the extra Poisson variation parameters in the MVPLN model were smaller than the UVPLN model that causes improvement in the precision of the MNPLN model. Hence, the MVPLN model is better fitted to the data set. Also, results showed effect of the total Average annual daily traffic (AADT) on the property damage only crash was significant in the all of models but effect of the total left turn AADT on the injuries and fatalities crash was significant just in the UVP model. Hence, holding all other factors fixed more property damage only crashes were expected on more the total AADT. For example, under MVPLN model an increase of 1000 vehicles in (average) the total AADT was predicted to result in 31% more property damage only crash. Conclusion: Hence, reduction of total AADT was predicted to be highly cost-effective, in terms of the crash cost reductions over the long run