If someone begins chasing you in your car when you're driving down the road you should first call the 911 and let the police know about it so that they can try and catch the person chasing you.
Remain calm and keep driving and if possible drive to a place where lots of people are such as Walmart or go to a police station, or hospital if one is nearby.
Let the 911 dispatcher know the description of the vehicle that is chasing you and if you have to speed up to get away from the other car but do so safely and watch your surroundings.
If possible try to pull off to the side of the road and see if the person goes on past you but if not continue on down the road and do not get out of your car.
I've had a few people chase me out of road rage and usually they would just go on their way when they knew they were not getting a reaction but I still called the cops so that they could catch them because you never know exactly what they are gonna do or want to do.