Do not get a pay as you go phone for emergency only use. Most carriers will have a basic service for around $20 a month that would be better as an emergency plan. Most prepaid services have expiring minutes, unless you add more minutes before the due date, then your unused minutes will carry over. Even if you do not use the phone, but just keep adding minutes before yours expire, you'll be spending $20 or more to keep it going. If that isn't enough to dissuade you consider this, what happens if you car dies in the middle of the night, on a lonely stretch of highway, and you run out of minutes while waiting to talk to an actual person at AAA? You my friend, will be up the proverbial creek without a paddle.
I know T-Mobile and Nextel (now Sprint) have $20 plans. Cingular and Verizon used to have them, but don't have them on the website. The $20 plans are never advertised (any carrier), and you usaually have to ask a manager about them, but they do exist.