I haven't done this kind of business before but I would think long and hard about doing this kind of business. Are you planning to sell advertising on cars for the businesses that the people driving the cars own? Are you planning to offer the people who drive the cars money in exchange for putting ads on their cards or something else? While it could work, the biggest issue I see is that if you are planning the second option which I suspect is the case, you may have issues with people not wanting to do this. Have you surveyed your local audience to see who would be interested? I will be curious to see what everyone else says.