I would have a great difficulty backing the trailer to the dock through a complicated grid of obstacles. It would take me several tries, and a few scratches on the vehicle.
However I can program the path for the robot driver in such a way that it can drive in reverse at 10 mph without ever stopping at any of those obstacles. The path of a complex vehicle may be hard for a human to predict; however it is a trivial task for a computer. Not many humans can continuously steer and observe all the volume of the vehicle. Not all humans have eyes on eyestalks to see from the end of the trailer. Not all humans have enough eyes to see both sides of all vehicles (tractor and trailer) at all times. But a robot can do all that, easily.
A well designed robot driver will be safer and more reliable than any human. As other people already pointed out, the computer does not get tired, drunk, or angry. Its errors will be limited to lack of perception - and given that it "sees" with radars, GPS, and in visible and infrared light, with more than two eyes - it will be exceptionally well aware of the surroundings. Fears about robot drivers are like fears of autopilots. But the Shuttle flew to the orbit and back on autopilot - in part because humans are /not capable/ of controlling the vehicle with enough accuracy in space and time.
I hear the problem, though. Yes, truckers - and cab drivers, and many other commercial drivers - will be restricted in employment. They will be doing the last mile, and they will be doing everything else that humans are good at. (Packing and securing of the cargo may be somewhat automated, as it is now with shipping containers.) But even in social sense, there should be no need for a human to sit in a chair for hours, watching the endless hundreds of miles of a freeway.
The problem is caused by the lack of a transition plan from an early, primitive society to an advanced, technological society. We lost hundreds of occupations already - hunters and gatherers are replaced by ranchers and farmers who alone do the work that a whole tribe couldn't do. Now we have access to lots of foods; would we be better off if we could eat only what our wives are able to collect on a given day? As costs of transportation get reduced, the free market will bring the costs of products down as well - trucks are a very expensive mode of transport.
There is yet another interesting aspect. Human-driven trucks cannot be electrically powered because human's time is too expensive. However robot-driven trucks can afford time to charge; this will increase the trip time, but some goods are not sensitive to that, as long as the delivery time is predictable. This will reduce our dependency on oil. (If gas/diesel fuel supplies stop today, half of all Americans will be dead from starvation 2-3 weeks later.)
What should unemployable truckers do? Futurists tell us that they should learn a new occupation. Say, they could become roboticists. We know, of course, that this won't work for most, and it won't be enough anyway. As the society becomes more automated, the mandatory workload on an average human has to decrease, ultimately going to zero when robots do everything. The society, however, does not have a mechanism to do that. Significant changes in methods of production historically resulted in large scale unrest within the society. I am sure Obama is thinking about this problem day and night :-)
With so many idle, where are the taxes going to come from? Who is going to pay those?
I really do not see a significant further reduction in workforce without starving millions. The government can't keep creating "money", taking it out of one pocket and putting it in the other, less the overhead for that. Someone has to create wealth, and if not the people at the bottom of the economic food chain, where does the wealth come from?
If you tax the ones who will just add the tax to the price of goods, to feed the multitude, you create a spiral of increasing prices to cover increasing taxes to pay for feeding the increasing idle multitude, which means higher taxes, etc. A vicious cycle.
Nope, there will always be a need for someone to feed the machine.