NEW TEENAGE drivers must get more experience behind the wheel and have fewer pals in the car, thanks to the beefed-up safety demands of "Lacey's Law," which took effect yesterday.

Named after Lacey Gallagher, the Little Flower High School senior who died on her prom night in 2007 when the SUV in which she was riding crashed, the new law:

* Limits an under-18 new driver with a junior license to one under-18 passenger who is not a family member, unless a parent or guardian is present. After the first six months of driving without a violation or an accident, up to three passengers are allowed.

* Increases supervised, behind-the-wheel driver training from 50 to 65 hours - including 10 hours at night and five in bad weather.

* Makes driving without a seat belt a primary offense, which empowers police to pull the under-18 driver over for that violation alone.

State Rep. Katharine Watson, R-Bucks County, who fought for years to get this law passed, told the Daily News yesterday that it was a longer, tougher battle than she had expected.

"There is a lengthy cultural heritage in large portions of Pennsylvania that says something might be a great idea, but the government doesn't legislate how I raise my children," Watson said. "When I did the booster-seat law, I got hate mail asking, 'What are you - the state's official nanny?' "

But when statistical studies from 40 states showed that passing Lacey's Law should dramatically decrease fatal accidents involving teenage drivers, even the less-government legislators supported it.

Studies showed that more than half the teenagers who die in auto accidents were not wearing seat belts.

According to the American Automobile Association, a 16-year-old's chances of dying in a crash go up 39 percent with one teenage passenger, 86 percent with two, and 182 percent with three or more.

"Too many passengers, too many electronics, too many distractions - very dangerous," said Mark Ross, director of A Confident Driving School, in Malvern, which has been teaching teenagers to drive for 35 years.

"By restricting the number of teenagers in the car," he said, "this law will save lives. It's a good law."