Although, if I understand the way the Prius works, the engine kicks on at a certain speed. So hopefully any mishap would be at a low speed.

It does create a very interesting situation - particularly in a neighborhood. Although I love my Civic, I kind of admire the Prius going up and down the street without a sound. Toyota should run a commercial where a mischievous Prius owner is "sneaking" up on people and scaring them. You can see the scene outside a Starbucks where someone enjoying a coffee has a Prius cruising up behind them - honks the horn and gives them a start. I think it would be a funny commerical....

 

Past discussions on the Prius I asserted with a gentle acceleration, it could get up to 40mph before the gas engine engaged. Note this is a second-hand source.

 

As someone who used to ride a bicycle all over many busy roads, I can tell you the engine noise even of regular cars is completely drowned out by the sound of the wind. The air turbulence caused by fast-moving vehicles is very easy to detect, and it makes no difference how loud their engines are.

The only thing that matters is the speed of the car. A regular gasoline car at very low speed is very difficult to hear on a bike when you're moving due to the sound of your own wheels and air turbulence.

 

My Grandmom used to live out on a Farm-to-Market road. A quarter of a mile away, I could hear the air wake of a car or truck behind the hill, passing the country store and finally passing past her house. Fond memories....