What is the purpose of having side reflectors on trailers or semi-trailers?

The purpose of having side reflectors on trailers or semi-trailers is primarily to enhance visibility. These reflectors play a crucial role in ensuring that the vehicle is seen by other road users, particularly in low-light conditions or during nighttime. By reflecting light from other vehicles, streetlights, or any external light sources, side reflectors significantly contribute to road safety. They help make the dimensions and position of the trailer clear to drivers of other vehicles, thus reducing the risk of accidents.

The other options do not accurately represent the function of side reflectors. For instance, trailers have separate indicators for weight limits and licensing information, which are typically displayed in different formats, and they do not indicate a vehicle's speed. Therefore, the enhancement of visibility through the use of side reflectors is a vital feature for maintaining safety on the road.