This is not legal Advice!
Hey,
I would presume they are simular if not the same as a Taxi driver, thats if you charge a fee.
If you do charge a fee you should look more in to the rules a taxi driver has to adhere to.
You also say "potential DUI" if you get in to a verbal contract with someone who may be potentially drunk later, then get them to pay before they are under the influence. A bit like AAA, pay first and then you have the joy of calling someone to your rescue.
Try you local chamber of commerce they should be a wealth of information for you.
Good luck
Mark.