Stay in your car and keep the doors locked. If someone stops, ask them to go telephone the police or a service station for you. Do not exit the vehicle for anyone that you do not know. Roll down the window enough to talk to whoever stops to help. Always carry emergency change for cab or bus fare, or for a telephone call.
Cellular phones are a good investment for security and safety.
And if you see another motorist in trouble, signal that you will get help and then go to a telephone and call the police. Plan your route in advance, particularly on long or unfamiliar trips. Have enough gas and money to get there and back. Drive with all car doors locked and keep your windows rolled up. And never pick up hitchhikers.