Starting this week, when the wipers on the car go swish, swish, swish, the car's headlights better go shine, shine, shine, or else.

A new Pennsylvania law, effective tomorrow, requires motorists to turn on their headlights whenever they run the windshield wipers, even intermittently, due to rain, snow, sleet, mist, or other inclement weather. Violators can be pulled over and fined $25.

It seems like a no-brainer: Can't see. Turn on lights.

Yet "sometimes it takes a law to prompt people to do something that makes them safer," Catherine Rossi, spokeswoman for AAA Mid-Atlantic, said.

The law's language is a little foggy, however. It does not explicitly require full headlamps, so daytime running lights - not to be confused with parking lights - are acceptable, Trooper Lynette Quinn, public-information coordinator for the state police, said.

In 2005, more than one-fifth of the 133,000 crashes on Pennsylvania roads occurred during poor weather, according to the most recent data available from the state Department of Transportation.

Pennsylvania joins New Jersey and at least 14 other states with similar legislation, according to the AAA national office.

Last week, many drivers were still in the dark but not without opinion.

"I guess it's good for visibility," said Daniel Brown, 33, of Philadelphia.

Paul Dunn, 44, of West Chester, also was unaware. "It's starting to sound like a police state around here," he said. "It's another reason to pull you over."

Contact staff writer Lini S. Kadaba at 610-701-7624 or lkadaba@phillynews.com.