I would say if you bury it to the correct depth, it shouldn't be a problem. Poly pipe is not going to break when driven over, we did a sprinkler system a couple years back and when they remodeled the house recently, all of the trucks, mixers, and dozers drove over it with no problem. PVC shouldn't break either, but it is more likely than poly. Driving over it may cause stress fractures in the PVC however.
Tony Posey
Ridge Run Landscapes