Vehicle density is itself a cause of increased accidents. If you normalized by it you would lose accuracy. To make it more clear, 10 cars on the same road would

Vehicle density is itself a cause of increased accidents. If you normalized by it you would lose accuracy. To make it more clear, 10 cars on the same road would increase the chance of an accident non-linearly. It would be more than 10x more likely to have an accident occur there simply because there is more chaos.