[photosinensis]

The key there is that every car that passed through that part of town during that hour or so did the same.

You can set up checkpoints as per Federal law (though your state may vary--some states have provisions against it, in which case your recourse is to state, not Federal law). However, these checkpoints must be indiscriminate: they have to stop everybody that passes them while they're active.

A cop cannot just pull a single car over to see what's going on. He has to have reason.