Construction finally began on I-70 in Glenwood Canyon in 1980, after more than twenty years of studying natural obstacles and of legal battles with environmentalists. Colorado Department of Transportation engineers and the architects, Joseph Passonneau of Washington, D.C., and Edgardo Contini of Los Angeles, reduced the environmental impacts, protected vegetation, and made this a beautiful ribbon of highway whose concrete is colored to match the craggy cliffs. The four-lane freeway flows through the canyon, sometimes rimming the river, sometimes floating over the treetops. An army of workers sculpted and stained newly blasted rock to match the patina of the unmolested canyon walls. Cantilevered, cliff-hugging lanes, over 6 miles in 40 bridges and viaducts, twin 4,000-foot-long tunnels, and an interwoven bike path complement the river's work. Some 140,000 native trees and shrubs were planted to enhance this showpiece freeway. The project, which cost $500 million, was the final, 12.5-mile segment of the 43,000-mile, $115-billion interstate highway system mandated by Congress in 1956.