If you hear a low whirring sound while you're walking the streets of Japan, you're probably encountering a Prius rather than an alien spacecraft.
Toyota has added a device that makes an artificial sound to the Priuses it sells in Japan. The reason? The hybrid is so quiet at low speed that people fail to realise that it's coming – the US government has found that hybrids are twice as likely to be involved in accidents with pedestrians as conventional cars.
The issue is particularly acute for blind people. The device costs £96 and is installed under the Prius's bonnet, from where it emits a whirring sound of similar volume to a conventional engine.