Sending a truck down an interstate for 20 hours has to be the lowest hanging fruit for self driving vehicles. When was the last time an entire category of jobs became obsolete fast enough that laid off employees blamed the technology? When long haul truckers are out of work, how will municipal bus driver unions react?
Containerized shipping. Interestingly, it's in the world of logistics too. Longshoremen/dockworkers/stevedores used to be among the most common occupations in the US at the turn of the century, but now the industry employs a tiny fraction. A small group of a few hundred people can effectively run an enormous port like Oakland's.
Yes, the employees blamed the technology, but so did local governments. Places like Baltimore relied pretty heavily on the docks to supply their population with jobs; when those disappeared quickly many cities (especially on the eastern US coast) had a really tough time economically.