These trucks are NHTSA Level 3, so the driver must be ready to take over the control if there's an unexpected problem.
From the article :
NHTSA Level 3 is all well and good, but it dictates that the driver always has to be able to take over—if the software bugs out, or there aren't any road markings, for example. With NHTSA Level 3, liability still lies with the driver. Stepping up to Level 4, where the driver is "not expected to be available for control at any time during the trip," liability becomes a much thornier issue.
NHTSA - U.S. Department of Transportation Releases Policy on Automated Vehicle Development
Limited Self-Driving Automation (Level 3): Vehicles at this level of automation enable the driver to cede full control of all safety-critical functions under certain traffic or environmental conditions and in those conditions to rely heavily on the vehicle to monitor for changes in those conditions requiring transition back to driver control. The driver is expected to be available for occasional control, but with sufficiently comfortable transition time. The Google car is an example of limited self-driving automation.
Full Self-Driving Automation (Level 4): The vehicle is designed to perform all safety-critical driving functions and monitor roadway conditions for an entire trip. Such a design anticipates that the driver will provide destination or navigation input, but is not expected to be available for control at any time during the trip. This includes both occupied and unoccupied vehicles.