The largest blaze in South Florida in three years has burned more than 8,500 acres and threatened to cut off the main road to the Florida Keys. Eight state crews used bulldozers and all-terrain vehicles to make firebreaks while a helicopter and an air tanker dropped water. In some places, flames shot 50 to 100 feet in the air. The blaze began last week, but firefighters got help from higher humidity and slightly reduced winds, said David Crane, a firefighter with the Florida Division of Forestry. Smoke threatened to close U.S. Highway 1, and Forestry Division officials were concerned that if the fire moved 1 1/4 miles and crossed Highway 1, it could threaten Everglades National Park. Hundreds of Keys residents and visitors drove north over the weekend to avoid being trapped south of the fire.