What type of modification is typically illegal for vehicles?

Tinting windows beyond legal limits is illegal because each state has specific regulations regarding how dark vehicle windows can be tinted. These regulations exist primarily for safety reasons; excessively tinted windows can impede a driver's visibility, making it difficult to see other vehicles, pedestrians, or obstacles. Furthermore, overly dark windows can make it harder for law enforcement to see into a vehicle during a traffic stop, potentially creating safety risks and complications.

While adding fog lights, upgrading a sound system, or changing tires can all enhance a vehicle's functionality or aesthetics, they typically do not violate vehicle modification regulations as long as they adhere to safety standards and do not obstruct the driver's visibility. Thus, exceeding the legal limits on window tinting stands out as a modification that can directly impact safety and is, therefore, considered illegal.