Forth, our database does not include seatbelt/helmet wearing and mobile phone usage information, though these factors are deemed important, their effects cannot be assessed. Last, the response variable for accident severity is binary, with either fatal/severe injury or not. However, some multiple vehicle accidents involving many fatalities should be considered as “super” severe events, the associated risk factors might be rather unique and in such case an ordinal regression model is needed. In our current data format, these events cannot be distinguished.