Your car builds up static electricity from travelling the roads. Once you step out of the car you ground yourself and there fore the energy which has been built up int he body of the car is transfered through you to the ground, effectively once you step out of the car you become a conductor, like a steaple lightning rod. It is very unlickly that this will have any adverse effects on you as the charge built up in the car is quite minimal. You might try attaching a static charge stick to the rear of the car, this drags along the road as you are driving and automatically transfers the charge being built up to the ground. So when you get out of the car there is no charge left for you to conduct.