FELIX SALMON points out a study showing that hybrid vehicles are more likely to get into accidents with cyclists and pedestrians than are traditional vehicles. It seems that 1.2% of surveyed hybrids experienced a low-speed crash with a pedestrian, to 0.6% for traditional vehicles, while 0.6% of hybrids hit a cyclist, compared to 0.3% for traditional vehicles. Mr Salmon writes:

The reason, of course, is that the hybrids are so quiet: bikers and pedestrians use car noises to help them work out which cars are moving and which aren’t. That’s why hybrid manufacturers are now talking about adding vroomtones. Sounds like a good idea!

Perhaps so, though I don't really think that pedestrians, which tend to move across traffic rather than with it, are really using sound to gauge movement all that much. As much city noise as is present in places where pedestrians tend to walk, it's very difficult to pick out the sound of individual automobiles at any distance.

To me a more likely explanatory factor is that hybrid ownership is correlated with higher rates of pedestrianism and cycling. Ownership of a hybrid continues to be as much about buyer beliefs and priorities (and status) as about economic decisions, and one would expect a green car to be a popular choice in places where other green behaviours—like walking or cycling—are common. Of course, it might also be the case that locations which prize green choices are more likely to raise petrol taxes, and it might be the case that places with higher petrol taxes are going to have higher rates of pedestrianism and cycling (both as a reponse to the tax and because petrol tax increases will be easier to pass politically where cycling and walking to work are feasible substitutes for driving).

At any rate, I'd be very surprised to learn that hybrids aren't commonly found around walkers and cyclists, which should, of course, increase the rate at which collisions between the two occur.