Police impersonation is a serious crime; it is not just a practical joke. Police impersonators have been implicated in many crimes including rape and murder. Just think about it.
Also ask any of the following victims of a police impersonator how they would feel about a person who thinks that it is a great joke to dress up as a police officer and go stop innocent victims in public (short list).
http://www.wusa9.com/news/article/204856/158/Victim-Of-Police-Impersonator-Speaks-Out
http://thefan.nbc12.com/news/news/83557-police-looking-victims-police-impersonator
http://www.youtube.com/watch?v=ASkjevbBRlI
http://blogs.findlaw.com/blotter/2012/05/fake-cop-murdering-miss-motorists.html
http://abclocal.go.com/kgo/story?section=news/local/east_bay&id=8790272