As an EMT (I work on an ambulance part-time) I always carry a trauma kit in my trunk.

I have seen a fair share of accidents happen or was driving when I get to the scene of one where no help had yet arrived. More than once I feel that by my stopping and taking medical control of the situation may have saved a life or at least helped lesson severe injury.

The very first thing I do before getting out to help is call 911 on my cell phone. If I am in distance to dispatch, I call them on my radio, but it's really no difference. Help doesn't come quicker if you have a direct radio connection or a cell phone.