Here is a photo I had to take when I came to a stop behind a truck recently.
You have to be completely stopped, and the car immediately behind the truck to be able to read it. So this marketing tactic will work on exactly ONE person.
What's worse, even if you happen to be the lucky one that has the chance to read it, you can't even tell whose truck it is!
Again, why bother?
If you have the opportunity (i.e. if you have a truck), post your brand name and your tag line. And by the way, your tag line should be somewhat descriptive or at least intriguing.
All it takes is some common sense and some attention to detail. It just upsets me a bit to see companies wasting marketing opportunities like this. Don't waste. Pay attention to how you position yourself in everything that you do.