Now after ten years Toyota has realized the the Prius’ utter silence can be actually dangerous! They are offering an onboard device for the Prius and other hybrid electric models, designed to alert pedestrians and others audibly to the presence of a quiet vehicle. Apparently this device automatically emits a synthesized sound of an electric motor when the Prius is operating as an electric vehicle at speeds up to approximately 25 km/h. Maybe at next stage they allow you to download your own noises onto this device!
Toyota also plans other versions of the device for use in gasoline-electric hybrids, plug-in hybrids, electric vehicles as well as fuel-cell hybrid vehicles planned for launch.