Add a 20-30 second hidden timer to players inside of spawns that when it runs out, blocks them from entering any combat vehicle. This would discourage players from camping in spawn all game waiting for vehicles to reappear, and would also give new players more of a chance to enjoy the vehicles since the same camping players aren't going to be hogging them every time they spawn. The blocked players would still be able to operate small transport vehicles like the humvee and jetski, but larger vehicles like tanks and helicopters wouldn't let them in.