First, the vehicle will be programmed to stop if it determines there is a serious defect, or if any of its sensors detect contact with another vehicle or fixed object. The vehicle will remain safely stopped on the side of the road until the situation is resolved.
If law enforcement becomes involved, the officer would communicate directly with the carrier about the situation and what is being done or needs to be done (much like officers currently do when encountering a disabled vehicle or at a crash scene where the driver is not present).