Daily Mail: Evidently, the Toyota Prius and other hybrid electric cars are too quiet, particularly at low speeds when only the electric motor is running. So Toyota has decided to produce an “on-board approaching vehicle audible system"âmdash;initially in Japan only, where the Prius is the country’s best-selling car. The device will emit a humming sound from a speaker mounted under the hood, and the sound will rise and fall in pitch as the car speeds up and slows down. According to the Daily Mail‘s Colin Fernandez, a US study by the National Highway Traffic Safety Administration last year found hybrids were twice as likely to be involved in a pedestrian crash as conventional cars when reversing and parking.