My uncle has been a slug for most of his career, grabbing a ride from Springfield to DC. I never really thought of it as hitchhiking, but you’re right — it totally is!
We have something similar in the Bay Area (we call it “casual carpool”) for East Bayers crossing the Bay Bridge into SF to get the carpool rate for crossing. The carpool lane used to be free, so it was the same concept as the slug lines/HOV-3 into DC. But it’s not anymore, so I’m sure if the people who are carpooling throw cash in for the crossing or not.
So, I guess hitchhiking still exists, but not necessarily for the free-wheeling backpackers of the world?