It seems like the first and most obvious step for the trucking industry is to replace trucks on the long haul only. For example, one driver might drive the truck to the highway onramp and send it on its way, then the truck drives itself for hours and hours to where it is at an offramp by another driver who takes it to its final destination.
Self-driving will certainly reduce the work available for truckers, but it will be a really long time before it eliminates them. Tractor trailers are not only difficult to maneuver, but often require very difficult maneuvers to park where they can be unloaded or unhitched. One way to look at it is that, in the near future, the computers will just be handling the boring part of the drive.
And automation does promise to reduce accidents significantly, and it can seriously reduce fuel use (and pollution) by allowing lines of trucks to coordinate their movements tightly, staying close to each other's slip streams. And self-driving trucks will certainly be more patient with each other--as in less likely to block traffic with a +2mph pass of another truck while going uphill--because they won't require such stringent timelines, which will make the roads a better place for everyone.