[Politically Correct]

I have recently seen signs along a highway where I live that says drug search ahead. I personally have not been on the highway when they are occurring. Has anyone else seen this type of activity where you live and under what law can they randomly stop a car and have a dog walk around it and sniff for drugs?

The hwy patrol pull this one every so often on I55 in the middle of nowhere between St Louis and Cape Girardeau.

There is no "Drug Search" on the main hwy but at the bottom of an exit right after the sign there is.

The drug mules panic and get off the road to go around the supposed search and right into the arms of the hwy patrol.

It's surprisingly effective.
The locals who happen to actually use that exit are known by the hwy patrolman who lives there are just waved thru.

[Travis McGee]

Along with what you said about the real trap at the next exit, such checkpoints also rely on video cameras with powerful zoom lenses, often located on a highway overpass ahead of the bogus “drug search” sign. They are observing cars through the front windshield for sudden activity, including tossing anything out of a window.