Why can a tractor-trailer appear to be travelling at a slower speed than it actually is?

A tractor-trailer can give the impression of traveling at a slower speed than it actually is primarily because of its large size. When drivers observe a large vehicle, such as a tractor-trailer, their perception can be distorted due to the vehicle's length and height. Larger vehicles can appear to move more slowly in relation to their surroundings, making it challenging for other drivers to gauge their actual speed accurately.

The other options do not provide a valid explanation. While a tractor-trailer might sometimes travel below the speed limit or have issues that require frequent braking, these factors are not the reason for the perception of speed. Additionally, engine noise is not a reliable indicator of speed perception, as many other aspects, such as visual cues from the road and surrounding vehicles, are the primary contributors to how speed is perceived.