Boulder is 40km northwest of Denver, directly accessible from Denver International Airport (DEN). The airport is 80km away — about an hour's drive in light traffic, longer during rush hour. The AB (Flatiron Flyer) bus service connects DIA to downtown Boulder; the ride takes about 75 minutes and costs significantly less than a taxi or rideshare.
By car, Boulder is reached from Denver via US-36 (the Boulder Turnpike), which ends on the east side of town on 28th Street. From the west, US-36 runs through the mountains from Estes Park. The canyon roads — Boulder Canyon (Highway 119) and Left Hand Canyon — are beautiful drives if you're coming from the mountains, but they are narrow and can be slow.
Boulder has no train station. Amtrak's California Zephyr stops at Denver Union Station but not Boulder.