Colfax Avenue is the main street that runs east–west through the Denver metropolitan area in Colorado. As U.S. Highway 40, it was one of two principal highways serving Denver before the Interstate Highway System was constructed. In the local street system, it lies 15 blocks north of the zero meridian, and would thus otherwise be known as 15th Avenue. The street was named for the 19th-century politician Schuyler Colfax. At just under 50 miles in length, it is known as the "longest continuous commercial street in America."

U.S. Route 40 (US 40) is a part of the U.S. Highway System that travels from Silver Summit, Utah, to Atlantic City, New Jersey. In the U.S. state of Colorado, US 40 is a major east–west route. It crosses the Rocky Mountains, passing over the Continental Divide at Berthoud Pass before descending to the Front Range. It then traverses through the Denver Metro Area, then exits by following Interstate 70 (I-70) and US 287. It is concurrent with US 287 for about 145 miles to Kit Carson. US 40 exits into Kansas east of Arapahoe in Cheyenne. At a length of almost 500 miles, US 40 is the longest numbered route in the state.