MrM and I love our Prius - it's a great car to drive, as well as making us feel less guilty than if we were still driving a petrol-guzzling Commodore - but there are drawbacks.
And the big one is needing to be super careful around pedestrians when we're driving slowly, because they will just step out in front of us without looking, assuming (presumably) that because they can't hear an engine, there is no car. Pedestrian crossings are okay - I expect people to step out there; it's all the people milling out in all directions from a tram stop or train station that scare me.
And clearly we're not the only ones to notice this issue, although the idea of replacing actual engine noise with speakers that play a recorded sound just seems silly.