This reminded me of the day I almost started running away from undercover cops. They stopped their car next to me, stepped out and looked at me in a way I knew they were trouble. It was also in a part of town known for more crime. I started walking faster and crossed a corner and almost started running but then I heard "Police! Stop!" from behind and I indeed stopped. They wanted to search my bag and I didn't resist because I didn't want any trouble, had nothing to hide and was still trusting authority. I should have at least asked for their ID though.

But how could a civilian recognize a fake ID from a real one anyway if they have never seen one before?

Also, anyone can shout "Police! Stop!".