Vehicles are usually under human control.
Example 1. An airliner starts at rest on the runway and then is accelerated until it achieves lift-off. To achieve this, the engines are run almost flat out. Once the aircraft reaches cruising altitude, the pilot will reduce the amount of fuel going into the engines, and fly at a steady speed.
Example 2. A car on the motor way is not run flat out. Instead, the driver regulates the speed by means of the accelerator, so that when the car is going too fast, say over 70 miles per hour, the driver can reduce the amount of fuel going into the engine, and thus slow down the car. Likewise the driver can speed up the car by allowing more fuel to go into the engine. Regulating speed thus involves the continual attention of the driver; so can we automate this?