We already have self-driving vehicles - buses in Las Vegas. Right now, they have a maximum speed of 15 mph, and serve a loop around the Strip.
18-wheelers are the big market on the interstate. Trucking companies are having trouble filling jobs as older truckers retire, as young men and women don’t want to be away from Facebook/video games/family/friends. Plus many couldn’t regularly pass a drug test, even a pre-employment drug test.