If there is drag racing going on, arrest the ringleaders and disperse the crowd under threat of arrest. If they are not engaged in commerce, tell them to move on, and only then, arrest those who loiter, for at that point, it is more obvious as to who is loitering.
Don't cops know how to say "Okay, break it up!" anymore? Where is common sense?
It died on September 11, 2001, along with 3000 other innocent victims.