Properly functioning brake lights help improve the safety of a vehicle and are legally required in most jurisdictions. Brake lights that do not function correctly can reduce the visibility of the vehicle at night and can also send incorrect signals to other drivers, increasing the chance of a potential accident.
Properly functioning brake lights should activate only when a vehicle's brake pedal is depressed and should be visible to other drivers following behind the vehicle. If a car's brake lights do not light up during braking or are too dim to see, other drivers may not realize that the vehicle is slowing down and may fail to react appropriately. In addition, brake lights double as tail lights in many vehicles, so completely nonfunctional brake lights reduce the visibility of the car substantially when it is dark.
It is also possible for brake lights to be stuck on continuously due to wiring issues or problems with the car's brake light switch. Permanently activated brake lights lead to many of the same safety consequences as brake lights that do not light up at all, as it is impossible for other drivers to tell when the affected vehicle is braking based on the brake lights if they are always illuminated.