1. Drive your car in front of a self-driving rig
1. assortment of HD cameras record your every move.
2. Bring your car to a stop, thus forcing the rig to stop
2. An operator at a centralized dispatch/monitoring center is notified, as the truck's video feed pops up on their console.
3. Help yourself to whatever goodies the truck is hauling
3. As you step out of the vehicle, the operator presses the button that will connect them with law enforcement in the region in which you are located. They coordinate with local law enforcement to give them a detailed description of your person and vehicle. The video can be passed off to authorities who can process it for biometrics, and based on gait analysis and other data identify the perpetrator, who likely already has a record — and substantial data on file. High-resolution imagery of the vehicle will also assist in positive identification.
4. Could be, but likely, you're going to get boned.
5. You can already do this with a truck with a human driver. They're not just going to run you off the road. If you've got two or three other people who can all get out of the vehicle with shotguns at the same time, the driver is extremely unlikely to resist, or even to call the police. These vehicles will actually be more secure than manned trucks, not less.
6. ...and actually, these trucks will be manned; self-driving trucks without drivers are still well into the future. Likely, those won't even have cabs, and perhaps the trailers will also be redesigned to be harder to get into.