The impact of self driving trucks is much scarier than the impact of what is being put out there today.
In today's environment, companies are out there that can design an automated widget. However, each time they sell it, there is some level of custom engineering that goes into actually implementing that widget. This, along with capital, constrains how fast companies can implement automation, and usually gets offset by some level of job creation in other sectors/functions at a similar pace.
For example, there are dozens of companies that can design robots to build cars. However, they can't just sell the exact same thing to Toyota and Ford with no work - they need to design the robot to build Toyotas vs Fords, fit in a Toyota factory vs a Ford factory, etc. This takes time from engineers to make happen. Same for software - automated software can be purchased, but needs to be molded to the company it is being sold to.
However, once the self driving truck puzzle is solved (technically and financially) and infrastructure implemented, an 18 wheeler is an 18 wheeler is an 18 wheeler. There are a restively limited set of variations on trailers that need to be pulled - the industry has a lot of standardization. The only thing slowing companies down from executing this automation is how fast they can get capital to spend.
There are 3.5 million truck drivers in the US, and their jobs disappear in what will feel like overnight, and even though it will create jobs in manufacturing and maintaining these trucks, the truck drivers themselves likely aren't set up to move into these roles as quickly as their jobs will disappear. That's <2% of the US work force.
I don't know what impact that will have, but it will be a shock.