Undoubtedly, if Mayor Michael R. Bloomberg carries through on a proposal to bring congestion pricing to Manhattan, as reported by Diane Cardwell and Charles V. Bagli today -- he will be taking on one of the most politically combustible issues in the five boroughs. (In his radio remarks this morning, he left little doubt that he plans to forge ahead.)Attentive transportation observers know that congestion pricing has a long and unsuccessful history in New York City.An Aug. 13, 1987, article by David W. Dunlap stated: "A group of prominent business leaders yesterday attacked a proposal to charge motorists $10 to enter the Manhattan business district, an idea that Mayor Koch endorsed last week as a way of reducing air pollution."In February 2002, early in his first term, Mayor Bloomberg drew scorn when, during the fiscal crisis that followed the 9/11 attack, he floated the notion of charging tolls on the East River bridges controlled by the city. The idea -- which would have raised an estimated $2.4 billion -- went nowhere. By July 2003, it had been quietly withdrawn.The latest chapter in the congestion-pricing conversation began in November 2005, when aides to the mayor, fresh off a tremendous reelection victory, again floated the notion of adopting congestion pricing to spur economic development, relieve pollution and make the busiest parts of Manhattan more accessible to pedestrians and cyclists.Later that month, this reporter explored the topic of congestion pricing, which has most noticeably been adopted in London. Just as quickly, the mayor backed away again; Edward Skyler, a top aide, insisted that the idea "is not on the mayor's second-term agenda."Under the London congestion charging scheme, drivers are charged a daylong flat fee of