A city proposal to put Park Slope’s Ninth Street on a “road diet” — narrowing the busy thoroughfare to two lanes of traffic from four — was approved last night during a board meeting of Community Board 6.

Starting in July, the street will be converted to just one driving lane in each direction, and two five-foot bike lanes, one in each direction, will be added to the traffic mix. The redesign of the street came at the community’s request, after a series of accidents underscored the problems with pedestrian safety and reckless driving along the main corridor.

“The problem here is not the volume of cars, so much as their speed,” the chairman of Community Board 6, Craig Hammerman, said in an interview. Ninth Street is as wide as Atlantic Avenue in Brooklyn, but carries less than half of its traffic load, according to the transportation department.

In 2005, a sedan careened into Dizzy’s Diner as it maneuvered a sharp right turn off of Ninth Street. Two boys were killed a year before by a truck that turned into a crosswalk on Third Avenue from Ninth Street.

Opponents of the redesign say that cutting down on street space for cars will only aggravate the neighborhood’s parking woes, as residents would not have room to park and unload their vehicles. Transportation department officials said they would seek to modify curb regulations to allow more space for residents to load and unload their cars without double parking.