Vince,
Of course, you wouldn't be seen as often as a delivery truck. And, it is advertising, so there may not be a direct correlation to sales. But, if you are seen often enough, it will make an impression in people's minds.
Then, when people need a plumber and look in the phone book, they might choose to call you because your name is familiar.
I had advertising on a vehicle. One time, someone chased me away from a traffic light. Scared me to death. Turned out, all they wanted was to know exactly where to find my business. So, I think people do notice. The amount of time you'll know whether you get a lead from it is probably low. Even if you ask, the person would probably say the phone book, since that was the place they looked to get your phone number.
I wouldn't expect too many people chasing you down the street!
Kathleen