Whether you're broken down because of a flat tire or an accident, you should keep the following safety tips in mind from the Insurance Information Institute and Consumer Reports:

Turn on your car's hazard lights and slowly reduce your speed
Pull your car over to the side of the road or shoulder onto level ground as soon as it is safe to do so.
Keep your car's hazard lights on and put out your emergency reflector triangles, if you have them, to help alert other drivers.
Don't attempt to repair your vehicle on a busy highway.
If you're able to safely walk to another location, leave a note on the windshield of your car with your phone number, lock your car and call for assistance once you're in a safe location.
If you cannot walk to a safe location, stay in your car with the doors locked and your seat belt fastened. Call for assistance and wait for them to arrive.

Roadside assistance may not be as expensive as you might think but be sure you understand exactly what is included in your plan to ensure you have the right coverage. You may not need roadside assistance often, but you'll be glad you invested in it if a problem arises.