Travelers in the area may encounter delays and should follow recommended detours

Boulder County, Colo. - The Boulderthon Marathon, Half Marathon, 10K, and 5K races will be held on Sunday, Sept. 28, in Boulder County and the City of Boulder.

Travelers in the area of the races may encounter delays and should follow recommended detours. All community members located on or near the races will have access to their homes. View Boulderthon course maps.

If you are traveling through the area during the event please pay close attention to the Boulder County Sheriff, City of Boulder Police, and traffic marshals to ensure safety for runners and travelers alike. In general, it’s best to travel around the course rather than trying to cross it.

View information on community and traffic impacts on the Boulderthon website.

Sunday Sept. 28 Road and Trail Impacts

The Marathon, Half Marathon, 10K, and 5K courses will be active between 7 a.m. and 4 p.m. on Sunday Sep. 28.

SH-119/Diagonal Highway Access From Boulder to Gunbarrel, Niwot, and Longmont

• Iris Ave. & CO 119/Diagonal Highway will be closed eastbound at 47th St. until 10 a.m. Northbound CO 119/Diagonal Highway must be accessed via Valmont Rd. Southbound CO 119/Diagonal Highway into Boulder and through traffic on CO 157/Foothills Hwy. in both directions will remain open throughout the race.

County Closures

• Jay Rd will be closed to all traffic between 47th St to CO 119/Diagonal Highway from 7 a.m. to 1:45 p.m.; residents may access via pilot car starting at 47th St. and Jay Rd.
• N 51st St will be closed to all traffic between Jay Rd to Boulder Reservoir from 7:15 a.m. to 1 p.m.; residents may access via pilot car starting at 47th St. and Jay Rd.
• N 55th St will be closed to all traffic between Boulder Reservoir to Niwot Rd from 7:30 a.m. to 1 p.m.
• Monarch Rd will be closed to all westbound traffic from N 55th St to N 71st St from 7:30 a.m. to noon.
• N 71st St will be closed to all southbound traffic between Monarch Rd and Niwot Rd from 7:30 a.m. to noon.
• Niwot Rd will be closed to all eastbound traffic between N 55th St and N 73rd St from 7:30 a.m. to noon.
• Access to the Valhalla neighborhood will be maintained via a pilot car starting from either 47th St. and Jay Rd. or Kelso Rd./Valhalla Dr.

City of Boulder Closures

• Pearl St./Pearl Pkwy. will be closed to all traffic between 15th Street (east end of Pearl St. Mall) and 55th St. from 12 a.m. midnight to 10 a.m.
• Spruce St. will be closed to all traffic from Folsom St. to 14th St. from 12 a.m. midnight to 4 p.m.
• 55th St. will be closed to all traffic from Pearl Pkwy./Valmont Rd. to Flatiron Pkwy. from 6:30 a.m. to 10 a.m.
• 47th St. will be closed to all traffic from Independence Rd. to Pearl St./Pearl Pkwy. from 7 a.m. to 10 a.m. and between Independence Rd. and Jay. Rd. from 7 a.m. to 1:45 p.m.
• 26th St. will be closed from Topaz Dr. to Iris Ave. from 7:30 a.m. to 2 p.m.
• Folsom St. will be closed from Iris Ave. to Pearl St. from 7:30 a.m. to 2 p.m.
• 14th St. will be closed to all traffic between Spruce St. and Walnut St. from 12 a.m. Friday morning through 6 p.m. on Sunday evening.
• The Fourmile Canyon Creek will remain open, but will have heavy runner traffic from 7:30 a.m. to 1 p.m.
• Boulder Reservoir will be closed until 1 p.m.

Additional Questions Or Concerns?

If you have questions or concerns, please visit the Boulderthon event website, or call or text Boulderthon’s Community Coordinator at 970-610-0556, or email community@boulderthon.org.

You may also contact John Holste, Boulder County Special Events Supervisor at jholste@bouldercounty.gov or Justin Greenstein, City of Boulder Special Events Manager at greensteinj@bouldercolorado.gov.

Sunday, Sept. 28, 2025 Boulderthon - All-In-One Course Map

Sunday, Sept. 28, 2025 Boulderthon - Impact Zone Map

Sunday, Sept. 28, 2025 Boulderthon - Downtown Boulder Access and No-Parking Zone Map

Boulder County wants to ensure that everyone has equal access to our programs, activities, and services. To request an Americans with Disability Act (ADA) accommodation, please email ada@bouldercounty.gov, or call 303-441-1386. If you need help in another language, please email cppfrontdesk@bouldercounty.gov or call 303-441-3930.