Its simple math: 1+1=2. Or isn’t it? When a person has a locker which needs to be secured, he employs a lock which is opened with a key, or a combination. Why is it called a combination lock? Because there is more than one number needed to open the device. When multiple objects are together, the singular is lost and the plural is found. This is no different in truck world, but for some reason the difference between a single vehicle and a combination of vehicles eludes both police officers and truckers alike.
First, for purposes of this conversation, the word “vehicle” is very generic. A vehicle is not defined as to whether or not the vehicle is used for commercial purposes. It’s not defined by the requirement for registration. It’s not defined by needing a certain class or endorsement of a driver’s license. It’s defined by whether or not it moves on its own power.
Now stretch and free your mind. Take a single vehicle (represented by the integer “1”) and add it to another single vehicle (also represented by the integer “1”). The sum total of 1+1=2. When this equation is deployed in the vehicle world, there is no longer a single vehicle, but a combination of vehicles. It could be a truck and a trailer, or conversely a trailer and a truck. Either way it is two vehicles.
Interestingly, folk superstars Peter, Paul & Mary had something to say about this in their blockbuster 1969 single, There is Love: “A truck will leave its mother, and a trailer leave its home, and they shall travel on to where the two shall be as one.”
Since the confusion is now clear, here are some practical truck/trailer configuration examples, from easy to hard:
A pickup truck towing a tag trailer
This is a combination of vehicles. The pickup truck (1) is towing a trailer (1). Therefore, sum total of vehicles = 2.
A semi-tractor towing a semi-trailer
This situation is very similar to #1 above, but there is a twist. What confuses some people is the fact a portion of the load of the semi-trailer (1) is being carried upon the semi-tractor (1). Regardless of the weight distribution, the sum total of 1+1 in this configuration equals 2. This is a combination of vehicles.
An all-terrain crane towing a dolly
Now things are starting to get interesting. Imagine a massive all-terrain crane with a total of nine axles. Due to bridge load ratings on the crumbling infrastructure of Illinois, these 175,000 pound (or more) behemoths cannot safely operate on the six axles native to the power unit portion of the crane.
To compensate, the carrier will spin the boom backwards and put a three-axle dolly under the boom to spread the weight of load. So there is a power unit (1) and a dolly (1), bringing the sum total of vehicles to 2. This is a combination of vehicles.
“Whoa” says the naysayer. Since the boom is connected to the power unit and now the dolly, this is a single vehicle. Incorrect. These are two separate vehicles. Isn’t this just the reverse of a semi-tractor, semi-trailer combination? In the case of the semi’s, a portion of the weight of the trailer is being carried on the tractor. In the case of the crane, a portion of the weight of the power unit is being carried on the dolly. Combination.
An articulated bus
These are the accordion buses commonly operated in big cities. The trailer portion of the bus is coupled by a stinger under the rear of the power unit. Then they add this funky accordion curtain to allow passengers to move freely between both vehicles. Do the math: bus (1) + trailer (1) = 2. Very good!
For the legalists though, it must be made clear an articulated bus, while undeniably a combination of vehicles, is considered a single vehicle in the Federal Motor Carrier Safety Regulations for the purpose of driver’s licensing. This means an operator only has to have a Class-B CDL with a passenger endorsement.
Never let your schooling interfere with your education.