With a federal deadline looming, advocates of Mayor Michael Bloomberg's congestion pricing plan are willing to negotiate virtually every detail -- a tactic to win over state lawmakers that, at the same time, makes it difficult to evaluate whether the plan can achieve the mayor's transportation and environmental goals.Originally, the mayor's plan called for charging motorists $8 to enter and leave Manhattan below 86th Street during certain hours on weekdays, a proposal that city-generated computer modeling shows would result in 112,000 fewer vehicle trips while raising more than $380 million for transit projects in the first year.Those estimates don't include any information on traffic or revenue generated by motorists leaving Manhattan, or indicate how the numbers could change if discounts are offered for hybrid cars or if the boundaries of the congestion-pricing zone are changed.It also is unclear whether Manhattanites would be charged for driving out of the zone; the original proposal calls for that, but the proposed legislation does not. City officials say that detail, among others, is negotiable."The city has always been open to negotiation in regards to congestion pricing," mayoral spokesman John Gallagher said earlier this week.The city has applied for $500 million in federal money to pay for start-up costs and new bus routes, but needs state approval to enact congestion pricing. Federal transportation officials have said the state must act by Monday or risk losing eligibility for the grant."Give us the basic authorization," said Andy Darrell, regional director of the nonprofit Environmental Defense Fund. "There will be plenty of opportunity to work out the details."Those specifics are critical to the program's overall success, experts say."From the standpoint of a visionary kind of goal, I think the mayor's plan is fine, but that's where the details come into play," said John Falcocchio, an engineer and director of the Urban Intelligent Transportation Systems Center at Polytechnic University in Brooklyn.He backs congestion pricing but testified at a state legislative hearing in June that the city would have to charge $12 or $15 to appreciably reduce gridlock. "Congestion pricing can be a very powerful tool you have to put your ducks in a row so you can get them to go where you want," Falcocchio said.Supporters contend that the goals of Bloomberg's plan are so compelling -- reducing traffic and pollution to accommodate expected population growth and pay for better mass transit -- that it should be an easy sell."We know the basics, we know a $7 or $8 fee applied to vehicles coming into the central business district will reduce vehicle traffic," said Kathryn Wylde, president and chief executive of the Partnership for New York City, a nonprofit group of top business leaders."They have to have faith in the mayor. He's got two-and-a-half years to make it work," Wylde said, referring to the end of Bloomberg's term. "Have faith that he will not allow himself to be embarrassed, that he will make it happen in a way that works for all New Yorkers."Combined traffic on every bridge and tunnel into Manhattan was almost 2 million vehicles in 2005, the latest year for which city Department of Transportation figures are available. Factoring in the traffic drop after the Sept. 11 terrorist attack, overall traffic into Manhattan has steadily increased since 2002."I think what you're seeing reflects the fact that the crossings are at or near the saturation point," said Paul Steely White, executive director of Transportation Alternatives, an advocacy group that backs Bloomberg's plan.Opponents, however, say congestion pricing should be a last resort and want the city to try other measures first, such as carpooling restrictions or stepped-up traffic enforcement."It's not a congestion problem, it's a traffic-flow problem," said Corey Bearak, executive vice president of the Queens Civic Congress, which represents several community groups that oppose congestion pricing. "You can make traffic flow more intelligently without charging people."