For me it would depend on the road, traffic, and population. I would be much less likely to do so in the city, but much less likely to hesitate on a stretch of highway out in the country.
While I prefer not to let an animal suffer, I am not willing to put myself in legal jeopardy for it. I would, however, consider finishing the animal if I thought there was the possibility it would get back up, and into traffic again.
You are not likely to face this decision. If the animal is injured badly enough it cannot get back up and move, there's no reason to be in a hurry, wait for the police. If it is able to get back up and move, it will likely be up and gone faster than you get out of the vehicle.