1. select the ambulance chief
2. hold the "SHIFT" or "CTRL" key and move the mouse pointer to an ambulance that needs an emergency doctor. If you can cancel the emergency doctor escort, an icon will appear.
By right-clicking on the vehicle (hold down SHIFT or CTRL), you can now cancel the emergency doctor escort.
3. The ambulance chief will now move to the passenger door and talks to the ambulance crew for about 3 seconds. After that, the emergency doctor icon above the ambulance disappears.
4.Now press "HQ" again. The ambulance will now drive to the hospital without an emergency doctor.