I read this story at cleveland.com
Traffic cameras in East Cleveland nabbed U.S. Postal Service Vehicles breaking the law seven times. The city sent a notice to the Post Office with a picture of the infraction and a demand for payment.
The Postal Service's attorney sent a letter in reply to remind the city about the U.S. Constitution. The letter said:
"As you are probably aware, the Postal Service enjoys federal immunity from state and local regulation. The Postal Service requires its employees to obey all traffic laws and rules while operating Postal Service vehicles. However, the state and/or local ordinances imposing penalties and fines cannot be enforced as against the Postal Service."
The East Cleveland Mayor had the comeback of the year: "I was unaware that the Post Office doesn't have to stop at red lights or obey the speed limit, but since they are, I wish I'd get my mail faster."