I used to have one in Australia, sourced from a retired policeman.

My friend had recently purchased an ex police car that he picked up at auction, and we thought we'd have a laugh one night. There was a mountain in the southern part of town which had a great twisty road to the lookout where hundreds of car nuts would gather on Thurs nights.

We got dressed like undercovers - shirts, ties, slacks, etc. and went up there and as we arrived at the top a guy in his Holden was doing a huge donut show. :-) Out came the light on the dash and the whole crowd ran like hell. He pulled over and got his licence out, just standing by the car.

I wandered over to him and asked him what "the hell" he was doing. He crapped himself and started making excuses. I warned him and sent him home, after threatening to take his licence off him.

Funniest prank ever. Still laugh about it now.