Heavy traffic

A fellow was following a truck in heavy traffic. Every block or so, when
they were stopped at a stop light, the driver of the truck would jump out
of the cab with a big stick and bang on the side of the cargo bay. He'd
then jump back into the cab in time to drive away when the signal changed.
The first fellow observed this for several miles, until he could stand it
no longer. The next time the truck driver jumped out with the stick, the
first fellow jumped out and ran up to him. "I'm sorry to bother you," he
said, over the din of the banging, "but I am very curious; could you tell
me what you are doing?" Without breaking rhythm, the truck driver replied,
"Sure, Mac. Ya see, this here's a six-ton truck but I've got eight tons of
canaries aboard, so I've gotta keep two ton of them flying all the time so
I don't break an axle".