Car noise is generated by the engine and friction of the tyres as they move across the road surface as well as voluntary sources of noise a car might make, such as a music centre or its horn. Car noise has decreased over time due to inventions like the muffler/silencer attached to the exhaust pipe and better engine construction. For those inside the car, better padding can mean virtually no noise (for the worse in some instances, because engine noise is a good indicator of speed). However, the current level of car noise is destined to stay, at least until we invent new technology (such as hydrogen-powered cars). Car noise along with visual cues assists pedestrians awarness of motor vehicle location.