Boulder sits at the base of the Flatirons, where the foothills rise straight out of the plains. US 36, the Boulder Turnpike, connects the city southeast to Denver and becomes 28th Street through town. SR 119 runs northeast along the Diagonal Highway toward Longmont and west up Boulder Canyon toward Nederland. SR 93, Broadway, heads south toward Golden, SR 157, the Foothills Parkway, runs along the east side, and SR 7 follows Arapahoe Avenue east toward Lafayette.
28th Street, Broadway and Arapahoe Avenue carry most of the city's traffic, and Baseline Road crosses the south side near the university. US 36 backs up at rush hour with commuters headed toward Denver and the suburbs, and the Diagonal is busy with traffic to Longmont and Gunbarrel. Canyon roads such as Boulder Canyon Drive and Flagstaff Road are steep and twisting with few pullouts, and in winter they can turn treacherous within minutes of the first flakes.
RTD's Flatiron Flyer bus rapid transit runs along US 36 to Denver, and local buses and an extensive network of bike paths cover much of the city, so plenty of people ride or bike to work. Most households still keep a car for trips into the mountains and across the metro. Boulder Municipal Airport serves private aviation, and air travelers drive to Denver International, a long trip around the northern suburbs. The city's open space and growth limits keep development compact, so most neighborhoods sit close together and a mechanic can usually move between them quickly, with the canyon homes as the exception.