Imagine driving on a city street where the speed limit is 25 mph. You accelerate and are going 30 mph.

Not for long. An alarm suddenly goes off, the car slows down and the vehicle is back moving at the speed limit. That's intelligent speed assistance (ISA) at work.

"When it starts to go above the speed limit in any zone, the engine computer will no longer send signals to the accelerator, so you can press the accelerator, but the car won't accelerate," Keith Kerman, the deputy commissioner of fleet management and the chief fleet officer for the city's Department of Citywide Administrative Services (DCAS), which runs the city's fleet of vehicles used by more than 50 agencies, said.

DCAS is testing ISA in 50 of the vehicles used by a number of city agencies. It is all controlled by GPS and the DCAS Fleet Office of Real Time Tracking. The city says the pilot program has been successful in preventing speeding and reducing instances of unsafe driving.

"Ninety-nine percent of the 130,000 miles we have done so far has been within the speed limit, and that 1% is really just initial acceleration beyond the speed limit, and then the car is bringing you right back to the speed limit," Kerman said.

The vehicles feature an override button that can temporarily disable the system for 15 seconds, and DCAS can program the vehicles to allow travel slightly above or below the speed limit, depending on the situation.

"We can set a more aggressive speed limit in certain locations, so around school zones, we can lower the speed limit further that is set by the Department of Transportation so that our vehicles would go even more slowly around school zones," Kerman said.

DCAS is expanding the program to 25 more vehicles by end of February and is working on a larger plan for expansion. The department is applying for federal funding for its entire light- and medium-duty fleet.