No! USDOT Numbers do not have to be on the door. They can be displayed anywhere that is visible as long there is one on both sides of the vehicle. Pickup trucks tend to display their USDOT numbers on the truck doors but heavy haulers (semi trucks) tend to display all of their regulation numbers on their cab side for more space since they usually have a lot more numbers to display. Some box truck haulers even display their USDOT & MC numbers on the box and this is also DOT compliant.