Personally a stand out car empty on the streets makes me weary. You cannot do anything with other players PV's so I dont see why people would take a chance in getting in when it could have a bomb. I just move on. Or get out a jerry can and douse it. Line up my car exhaust and away I go. Oops... Blown PV and its not my fault.
Edit: I also agree with senormidget. Have the ability to go more than 10 steps away from a non PV with bomb and not have it dissapear.