I have heard that if you indicate there is liquid on the ground, you go to the top of the list. I have a friend who was rear-ended, waited 45 minutes for police response, called back to inquire. Dispatcher asked if there were any injuries, fire, or liquids on the ground and my friend said yeah there's a little bit of liquid (may have been oil or A/C condensation, etc), police were there 30 seconds later.