New York City drivers, anguished by bike lanes and pedestrian plazas, have long insisted that the Bloomberg administration was slowing them down.

On Tuesday, in a rare moment of harmony, the city agreed.

Mayor Michael R. Bloomberg announced a broad expansion of the city’s so-called neighborhood slow-zone program, which will lower the speed limit to 20 miles per hour from 30 in designated areas across as many as 13 neighborhoods.

Mr. Bloomberg, speaking at a news conference in Corona, Queens, cited the city’s recent safety gains, which, he said, have driven annual traffic fatalities to their lowest rates since the age of horse-drawn carriages. He called the program the latest tool in “our assault on the No. 1 traffic killer — speeding.”

The city’s first and only neighborhood slow zone was instituted in some areas of the Claremont section of the Bronx in November. Janette Sadik-Khan, the city’s transportation commissioner, said speeding in the zone had fallen about 10 percent.