It's a scary proposition to realize that someone is following you. Whether you know this person and have reason to be uneasy or you don't know this person and a total stranger is following you, there are precautions you need to take to make sure that you stay safe.
Keep a charged up cell phone---and a car charger--with you at all times. Have 911 on speed dial (call the cell phone carrier if you don't know how to do this) so you only have to hit one button to be in touch with the police. As soon as you think someone is following you, get on the phone. Tell the police you believe that someone is following you. Tell them your name and then exactly where you are. Describe your car; make, model, color and license plate. Describe the car behind you as best you can. Give them the color, make and model or as much info as possible. Tell the police why you think you're being followed.
Always keep all of your doors locked. Someone can gain access to a car that's stopped at a red light or stop sign in a few seconds.
Always keep your windows rolled up. You can keep them opened a crack for a breeze but never roll them down far enough to put your hand in.
Stay on well traveled roads, if at all possible. People looking to hurt people in their cars are looking for people who are the most vulnerable. Your vulnerability increases with your degree of aloneness. One or two cars on the highway aren't a huge threat, but it's a great deal better than no cars. Likewise, a busy street will be one of the safest places to drive if you want to avoid someone stalking victims in cars.
Never stop to let a car that's following you pass. Pull over to another lane to let someone pass, but don't come to a stop. If there's only one lane of traffic, don't pull over at all. If the person wants to pass and there's no traffic, they can go around you if they want to pass that badly.
Make noise. Even if you're alone on a dark road, a constantly beeping horn will alert people. You never know when a police officer is within earshot. Blow your horn and keep blowing it if you feel afraid.
Grab another driver's attention. Beep the horn and make a hand signal like you're holding the phone. You should have a sign in your car that says "Call Police." Either buy one or write one with large black letters. Keep it right under or beside the driver's seat where you can reach it while driving. Pull the sign out and hold it up if you get near any other cars and you can't use a phone yourself.
Drive to a police station. The guy behind you is not likely to stop there. If he or she does, don't get out of your car until there are policemen outside your door to escort you. Another choice is to go to a fire station, hospital, or an all night superstore. Don't go anywhere where there aren't a lot of people. Keep driving until you come to something well lit and populated. Never get out of your car unless you know that you are safe.
Don't assume things are safe because they look that way. Don't assume someone is safe because they're well dressed, female, young or old or because there's a child in the car. Criminals often go way out of their way to appear as ordinary and trustworthy as possible. Many, many crime victims thought someone was safe because of the way they looked or because someone was with them. Crooks even use young kids to do their dirty work. (Home invaders are famous for this one - using a child to knock on a door selling something so they can get you to open up and then rush you. See the eHow article on how to prevent a home invasion.) Don't assume anything. If someone is following you assume that it's dangerous and err on the side of caution.