As Mayor Michael Bloomberg prepares to do battle with the City Council and Albany to rid midtown of excessive car traffic, environmental advocates say the mayor can start by cleaning his own house.Twenty-seven percent of New Yorkers who work in the public sector drive into the city, totaling more than 47,000 cars, according to a recent study. By contrast, only 14% of private sector workers commute by car.And many of those government workers have an extra incentive -- free parking thanks to placards they receive from the city, state and federal governments that allow them to park in designated areas.About 150,000 public sector workers have such placards, according to Transportation Alternatives, an advocacy group."If public sector employees drove at the same rate as their private sector counterparts, 19,000 less cars would be on the street," said the group's Matthew Roth. "We can bring that number down dramatically overnight and we don't have to get the state legislature approval, we don't need technological improvements, we just need government workers to tighten their belts."Studies have shown that many placard holders abuse the privilege."I think the parking abuse going on now is the worst I've ever seen," said Sam Schwartz, former New York City traffic commissioner under Mayor Ed Koch. "Government workers think they have an inalienable right in this city to park. The mayor's credibility with congestion pricing is at risk unless the city gets its own act in order."The mayor and the Department of Transportation commissioned a study last fall to determine the exact number of cars parked illegally in lower Manhattan."Since 2001, the administration has worked with DOT to reduce the number of city issued placards by approximately 25 percent," mayoral spokesman John Gallagher said. "The mayor will continue to encourage New Yorkers to follow his example and take public transportation whenever possible."Experts, though, maintain that curbing placard abuse -- or even better, reducing the number given out -- would have an equal and immediate effect on the number of cars driving into Manhattan as congestion pricing."One of the main factors on whether people decide to drive into the city for work is whether or not they can park for free," said Bruce Schaller, principal of Schaller Consulting, a transportation research firm. "Clearly, having a placard in Manhattan provides an incentive to drive and that's why you see the government workers drive at the twice the rate of private sector workers."Being able to park anywhere, for free, leads many to drive who would otherwise take public transportation the outer boroughs, according to Kathryn Wilde of the Partnership for New York City."It creates not just a parking privilege but a driving privilege," she said.Environmentalists say such privileges widen the gap between government officials and regular New Yorkers."Such a high percentage of city workers are driving to work and they are out of touch with how average New Yorkers travel," Roth said. "We have a transit oriented city but the people running the city have this suburban mindset."