The design of a tractor trailer is the result of many things. Federal regulations require certain safety equipment. Fuel efficiency and driver comfort play a role in the design of truck cabs. The basic structure, however, is the result of a standardization method known as containerization.
The idea behind containerization is to create containers of a specific length and width, which can be transferred from cargo ships to trucks more conveniently. The current containerization standards were set decades ago and restrict the size, weight and length of containers. The weight of the containers, combined with Department of Transportation limits on weight per axle is what results in tractor trailers having 18 wheels. If the standards were changed, trucks could have 14 wheels, or even 22. In fact, a group of trucking and shipping companies has been pushing for an increase in the weight limit for trucks that would cause 22 wheels to become the new standard.
The largest trucks are currently limited to 80,000 pounds spread out over five axles. If Congress agrees to the change, the limit would be 91,000 pounds spread over six axles. The push for larger trucks has a number of safety implications. Stopping distance depends on a number of things, including driver reaction time, the quality of the brakes, the quality of the tires and the conditions of the road. Stopping distance is also dependent on the mass of the vehicle and its load. Heavier trucks take longer to stop at a given speed than lighter trucks, all other factors being equal.
In addition to taking longer to stop, heavier trucks have the potential to do more damage in a collision. Heavier trucks apply more force in collisions than lighter trucks. The additional axle and increased cargo capacity also have the potential to increase the number of accidents resulting from unbalanced and improperly secured truck loads.
Source: Yahoo Autos, "Question Of The Day: Why Do Semi Trucks Have 18 Wheels?," by Cameron Aubernon, 4 November 2015