Accident Analysis Based on Fixed and Random-parameters Models

Fixed and random-parameters models were developed in this paper for total crashes (accidents involving material damage, injuries and/or fatalities) occurred within road tunnels. Separate models for unidirectional and bidirectional tunnels are proposed. For unidirectional road tunnels, it was found that the crash frequency is positively associated in a non-linear way with the tunnel length (L) and the annual average daily traffic (AADT). Moreover, the random-parameters model was proved to be statistically superior compared to the fixed-parameters one. This means that the unobserved heterogeneity is not negligible in this case. A model to account for temporal correlations in the data collected from the same tunnel over successive time period, the so-called random-intercept model, was also developed and the trend of accidents over time was captured. However, the model in which both the intercept and the regression parameters are allowed to vary randomly was not found to be statistically superior to the random-intercept model. For bidirectional road tunnels it is confirmed that total crashes are positively associated with L and AADT. However, in the case investigated, in contrast with the results of unidirectional tunnels, the random-parameters model was not to be found to be statistically better than the fixed-parameters one.