In order to examine the relative effect of the explanatory variables included in the model, the type III analyses
were performed for different correlation structures and for each variable as shown in Table 1. For example, the
type III chi-square value for the variable Precipitation is the difference between the generalized score statistic forthe model with all the variables included and the generalized score statistic for the model with the variable
Precipitation excluded. The hypothesis tested in this case is the significance of the variable Precipitation given
that all the other variables are in the model (i.e., it tests the additional contribution of Precipitation). The small pvalue
for Precipitation indicates that the effect of this variable is highly significant. Based on the p-value of type
III analysis, the effects of causative factor are assessed for disability incidents and crash incidents respectively as
followed.