The bad guy is fleeing the scene of the crime in a vehicle, and our hero officer is without his or her car at the moment. No problem, he just hops in front of the next car that comes down the street, flags it to a stop, flashes his badge and commandeers the vehicle. The occupants will either gladly surrender their car, or only offer token resistance, maybe a confused "Hey! What's going on?" as they are unceremoniously dumped in the street. See also Hero Stole My Bike, Follow That Car!. If the hijacker doesn't actually have the authority to hijack the car, it's a Bavarian Fire Drill.