Why Do Trucks have the words "HORN PLEASE" printed to the back side

while driving the driver should be alert . this is a rule and reuired also. the printing reason is just to make an alert to the person comming behind the truck and also for the driver who is driving the vehicle. to err is common . to stop from err they got printed

Because trucks have a limited view from the back. You need to horn so they would know if you are passing them.

There is also an 'OK painted in the middle, or somewhere on the back. This means that if soemone is overtaking, that person should honk first, becasue, unlike most cars wher you can have a clear view behind, just by turning your head, you don't is in a truck. You honk to overatke, and once the truck driver can give way, he will blink the lights, indication that it's OK to overtake.

I guess, Because a truck is a big vehicle and the driver may not be ale to see the cars coming behind? Or the drivers are sleepy?[Just joking!]