It's not like driving south of 60th Street in Manhattan has ever been speedy. But over the past few years, it's gotten 9 percent slower. In 2010, the average vehicle speed in Manhattan was 9.35 MPH. In 2014, it dropped to 8.51 MPH. (See the city's data below.) The MTA, which has GPS data from its buses, is reporting a 5 percent decline in rush hour bus speeds.

Meanwhile, car ownership has remained relatively flat, fewer drivers are using the Port Authority's bridges and tunnels to get into the city, and transit usage is booming.

So why the slowdown?

"Uber and the proliferation of for-hire vehicles might be a cause," said city transportation commissioner Polly Trottenberg, who pointed out that 25,000 new black and livery car drivers — many of them employed by Uber — have entered the market since 2011. The city's Taxi and Limousine Commission issues 2,000 new for-hire vehicle licenses each month. And 72 percent of those cars' trips originate in Manhattan.

"There are other things that cause congestion," Trottenberg said, including economic activity and construction. But under new legislation proposed by the City Council, the TLC would temporarily curb the growth of for-hire vehicle companies while at the same time studying the root cause of the congestion.

A similar effort is happening in London, which is also seeking to limit Uber to combat rising traffic.

Uber spokesman Matt Wing condemned the legislation, saying that it benefits medallion owners at the expense of New Yorkers. "Three months ago, the taxi industry put forward a proposal to protect the status quo, and limit competition and innovation," he said. "Today, the de Blasio administration and City Councilmembers revived a nearly identical proposal. Unfortunately, this would reverse improvements made by Uber and others to our transportation system and most notably, stand between New Yorkers looking for work and their opportunity to make a better living.”

But Bhairavi Desai, the head of the New York Taxi Workers Alliance, said she "strongly supports" the city's move to curb growth of for-hire vehicles. "For months, we have seen drivers in all sectors lose income because of the saturation of vehicles," she said. "You now have essentially close to 40,000 taxis and black cars all competing in Manhattan during the same limited prime hours.  No one can move around."