South Dakota state troopers sit on this transition and will stop out-of-state vehicles for any reason they can (I was pulled for going 67 in a 65 as I slowed down). Prepare to spend 15 minutes being interrogated about your reason for being there and pressured repeatedly to allow the officer to search your vehicle. The officer I spent some quality time with was pleasant enough when I refused, and said that he generally likes to let people off with a warning if they’re less than five over and honest with him, but that my license plate was now entered into a state database, and I could expect to be pulled over again any time I ventured into South Dakota by officers who "might not be as understanding" as he was of my refusal to allow a search. Not coincidentally, I’m sure, within an exit of parting ways with this officer, a blacked-out Suburban with blue lights in the grill entered the freeway and spent a few miles riding my tail until I’d had enough and exited for fuel, at which point they also exited the freeway to turn around and head back eastward.

The officer who questioned me said very clearly that he had "colleagues" on that stretch of road who did not appreciate being refused a search, and if a driver said no, those colleagues would typically find reasons to write "two or three" tickets for drivers who said no. Might not be a speed trap in the classic sense, but it’s strong-arm intimidation of the sort that one would expect to see in Georgia or the Carolinas. It’s ok; I didn’t want to go back to South Dakota anyway. Mount Rushmore’s underwhelming in person.