Shut down a party that's randomly ejecting teenage drunk drivers, and you've done a Good Thing. (That's the goal. Now, let's achieve it without trampling rights.)
Several times I've seen teen parties at a house, and set up down the block to run some traffic enforcement. It's a good way to catch DUI By Minors near the source. If you spot a car pulling out of a party, and observe a traffic infraction (say, inoperative tail light), and pull them over to give them a warning (or a fix-it ticket), and it's occupied by minors under the influence, you've got pretty good reason to believe that there's underage drinking going on in at the party. Get two cars in that situation, and you've got pretty good probable cause.
Now, what if you don't find any drugs or alcohol on the occupants, but they don't have drivers' licenses or they're suspended? Or what if the the drivers don't have insurance? Or what if they have little kids in the front seat unbuckled?Suppose the stop was for 12 mph over (kind of a gray area) in a residential neighborhood, and the driver has had 5 speeding tickets in the last year? Do you just let it go, because you initially made the stop looking for DUIs? No, most reasonable cops will go right ahead and write tickets for such offenses.
But by the time 3 such stops have been made, it looks like the cops are just a bunch of reactionary jerks who are retaliating for not being allowed to enter the property.
Just so you know-- I've had many a time where I set up a block away from such a party where I KNEW (but couldn't yet prove) kids were drinking, and carefully observed cars exiting... and couldn't make a stop. No reasonable suspicion means I've got nothing. No offense in view means no P.C. I've later heard about the ones that got away, and realized that I followed 'em for a couple of blocks. They managed to wriggle through undetected. Plenty more do than don't. Ah well. We'll get 'em next time. Glad if they made it home safe.