Randy Overholt has witnessed several accidents on the corner near his Porter Ranch home, including one that ended with a truck careening through the shrubs of his front yard.

So it seemed to defy logic when city workers last year replaced the 25 m.p.h. speed limit signs on his street--Vanalden Avenue between Wilbur Avenue and Devonshire Street--with 30 m.p.h. signs.

"Some people are literally doing freeway speeds," he said, only moments after shouting at a car squealing past his house. "I yell at them because they go so fast that they can't stop for a child."

The increase on Overholt's street is the result of a 1972 state law that says that in order for police to use radar guns to catch speeders, posted speed limits must reflect prevailing traffic speeds.

Since 1991, this has led the city of Los Angeles to increase limits on at least 55 streets while lowering the limit on only five, according to a review of city records.

Such increases have perturbed Councilman Hal Bernson, who last year persuaded the City Council to ask state lawmakers to change the law so that speeds can be set 15 m.p.h. below prevailing speeds.

In his motion, Bernson said the speeds traveled on surface streets are rising faster because commuters are increasingly using streets as alternatives to freeways and major thoroughfares. The state has not yet acted on that request.

Although the increases usually only raise the limit 5 m.p.h.--most commonly from 25 to 30 m.p.h.--it is a maddening policy for residents already upset about motorists speeding.

"This place is like a raceway," said Pier Spaccia, who has lived across the street from Overholt, in an otherwise quiet residential area, for two years. The fear of speeders has prevented Spaccia from letting his five children play in the street or even on the sidewalk.

Spaccia didn't realize the speed limit had been increased, but said it didn't make sense to him. "Whenever you raise the limit, they are going to go 10 m.p.h. over the limit," he said.

Police and city traffic engineers say it is a sensible law that isn't always easy to defend, especially to those demanding that something be done about speeders. The increases in speed, they say, are the reasonable cost of using radar.

"It's a real difficult thing to explain to the citizen groups," said James H. Sherman, the city traffic engineer who approves such increases.