BEIJING—Bundled against the cold beneath a highway overpass on a busy Beijing intersection, traffic warden Zhao Delong waved his colored flag in frustration at the new silent killer stalking city streets.

"Those electric bikes just don't listen! The problem is they go too fast. They can't stop like bikes. I saw an accident just over there the other day where someone on an e-bike rushed through the intersection and plowed over someone on a regular bike," Mr. Zhao said as he tried to keep China's newest road hazard in check.

Powerful battery-powered bicycles are crowding out their push-pedal brethren, delivering a jolt to the Bicycle Kingdom.

By some estimates there are 120 million e-bikes on China's roads—up from just 50,000 a decade ago, making it the fastest growing form of transportation in China. Cities at first embraced them as a quieter and cleaner alternative to gasoline-powered scooters.

Officials were caught off guard when that environmentally appealing solution turned out to be deadly on the streets. In 2007, there were 2,469 deaths from electric-bicycle accidents nationwide, up from just 34 in 2001, according to government statistics.