Protecting a PVC downspout under a driveway?
I'd be surprised if the pipe was crushed by a typical car. Very heavy pickups or SUVs, maybe. PVC is surprisingly tough. 

However, you could scrape away the gravel to the level of the pipe to a width of 8-10 inches and lay some treated two-by lumber or steel plate over the pipe, resting on the adjacent gravel. You could drive some large pole barn nails through the edges of the lumber (after drilling appropriately-sized holes) to keep it in position if your gravel tends to be loose. Cover again with gravel and be happy.