Bay Area commuters are being encouraged to trade four wheels in for two as part of Bike to Work Day on Thursday, with events scheduled throughout the region to celebrate the benefits of commuting by bicycle. Hundreds of thousands of Bay Area bicyclists are expected to participate Thursday, according to Andrew Casteel, executive director of the Bay Area Bicycle Coalition, which is helping to organize events regionwide. Casteel said it is estimated that about 3 percent of Bay Area commuters already bike to work, and that number usually spikes on Bike to Work Days, typically held in May, which is National Bike Month.