Long-haul tractor trailers. We need long stretches of highway where autonomous tractor trailers can carry goods. It'll provide a good business case and limit liability.
Our freight network is great, but trucks are now a vital partner to trains and go more places. If all the trucks in the US stopped and only trains ran, we'd be in a serious world of hurt.
Aren't autonomous cars required to have a human "driver" while on the road? I can definitely see truckers lobbying to have some sort of law of that nature - they sit behind the wheel "in case something goes wrong", but would be allowed to drive longer hours with fewer mandated breaks. Sounds like a win for the profession in general... setting aside that longer drive times mean that at least some will be out of work.
Even if the truck's "supervisor" is paid as much as drivers are today, trucking companies are very hungry to get more miles driven in fewer days. These will let trucks drive for more than 11 hours, thus I believe we'll see these as soon as they're approved.
Do unions still hold significant political sway these days? I can definitely see truckers pulling their weight to keep those laws on the books for commercial freight at least long enough to stay employed 'til retirement.
Assuming the laws are removed at some point, there would still be a huge transition between companies needing truck drivers and not needing truck drivers.
They could just stop hiring new truckers (forgot what it's called when you just let the current staff dwindle, instead of actively firing them) and let the current ones gradually retire. Either way, this is a problem that has been faced before countless times as new technology has appeared.