Silence,

I came across this article today;

Because hybrids make virtually no noise at slower speeds when they run solely on electric power, blind people say they pose a hazard to those who rely on their ears to determine whether it's safe to cross the street or walk through a parking lot.

I can vouch for this. One time while walking to my campsite at Kohler-Andrae at night, I had a similar incident with a hybrid with only its parking lights on. Maybe they should put bells on the bumpers.