The use of electric-assisted bikes and scooters has exploded into the zeitgeist, the streets and (mostly illegally) the sidewalks. But these unprotected vehicles can run at automobile speeds, posing a risk of injury that's greater than for those pedaling nonpowered, slower two-wheelers. Riders, bicycle-makers, drivers, pedestrians and lawmakers need to have their heads on swivels, as some have been too complacent about the dangers.