Considering the distances they cover, it's no surprise that trucks go through serious wear and tear. That's the reason behind some of their more eye-catching design features, like the spikes on their wheels and their extra sets of tires. The plastic arrows on the wheels of trucks and buses aren't as noticeable, but they serve an important purpose as well.
According to Jalopnik, the brightly colored bits of plastic on wheel lugs are called "loose wheel nut indicators." As the name suggests, they're supposed to alert the driver when the wheel's nuts are coming loose.
Lug nuts help fasten a vehicle's tires to their axles, which means they're just as vital as the more sophisticated machinery beneath the hood. Lugs become less secure over time and need to be refastened. In large trucks that spend a lot of time on the road, wheel maintenance is a constant concern.
Loose wheel nut indicators are a cheap and simple way to spot loose nuts before they become a problem. When they're first attached to the wheel nut, the plastic arrows point in a specific direction. Maybe they're lined up end-to-end to form a circle, or paired together so the points of every two arrows are touching (as pictured above). However they're positioned, the indicators make a pattern that's easy to recognize. That way, when a wheel nut starts to come loose, the driver can spot the arrow that's askew.
The shiny spikes on semi-truck wheels also attach to the lug nuts, though they serve a different function. Instead of indicating when a wheel nut is loose, the dramatic ornaments protect them from dirt, rain, and other elements that could damage them and cause them to loosen.