When you were a child, did you ever stand by the side of the road waiting for a Semi truck to come by? And when it did you would raise your arm and make a motion like you were pulling down, you would do this motion repeatedly until the truck driver would reward you with 2 or 3 blasts of that insanely loud horn. You were ecstatic for hours after! That is a truck train horn.
Believe it or not, this horn was not put in the Semi’s to entertain us! They were designed for safety, and that is still their main purpose. These horns are also called ‘air horns’ because of the fact that they are operated pneumatically (by pressurized air). They produce 135 decibels (dba). They were originally used on trains (and still are). When Semi’s came into being the train horn kit was engineered into the Semi for safety and alerts.
Semis are big – huge – the average trailer is 58 feet long and that does not include the cab, which varies in length. A full load for the 58’ trailer is 84,000 pounds. This is just the load, not including the weight of the vehicle itself. Semi’s do not maneuver like a car. They cannot whip over to another lane or stop on a dime. Do NOT pull out in front of a Semi and expect him to change lanes 
for you, or slow down – he CAN’T. He will blast the loud truck train horn at you. Remember, your truck or car is no match for these monsters. Note the picture to the right, the car is demolished – the semi probably doesn’t have a scratch.
If you are driving on a windy road and you hear a truck train horn, it more than likely means that a Semi is coming around the curve partially or fully in your lane, and he is trying to warn you. If you are in this situation, pull to the right as far as you can to let him get around the curve safely.
You are driving in the mountains going down hill. You hear a truck train horn and see a Semi in your rearview mirror, GET OUT OF THE WAY – his breaks have failed and he cannot stop.
The truck train horns’ main job is to keep us safe. But I still appreciate the blast of the horn when kids ‘signal’ for it!