HOT lanes, that is. Carpool lanes that drivers without the qualifying number of passengers can buy their way into are one of the hottest trends in transportation. The [California]state Legislature, Congress and Bay Area transportation officials all have embraced them as a creative way to finance highway or transit expansion and cut congestion. “The last 18 months have seen an explosion of proposals and interest,” said Robert Poole, a transportation researcher and founder of the Reason Foundation, which advocates toll lanes. “Virtually every major metropolitan area in the country is talking about them.” The voluntary toll lanes could become a common sight throughout the Bay Area in the next decade. Construction of the region’s first toll lane — on the Sunol Grade on southbound Interstate 680 between Pleasanton and Milpitas – – was approved by the Legislature and Gov. Arnold Schwarzenegger in August. It’s now under design, and tentatively scheduled to open in 2009 or 2010. The Metropolitan Transportation Commission, the Bay Area’s transportation planning and financing agency, has grander plans. Its transportation plan for the next 25 years, released last month, suggests that the Bay Area complete its network of carpool lanes on every major freeway and pay for the work by making all of them combined carpool-toll lanes.