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 this Sunday, Sept. 27, 2026 in Boulder County and the City of Boulder.
All community members will have access to their homes but may encounter delays and should follow recommended detours. 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.
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.
Sunday Sept. 27 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. 27.
US 36 Through Traffic
- Northbound US 36 traffic from Denver and points south should use CO 157/Foothills Pkwy north to Valmont Road, then west to 28th Street, then north on 28th Street from 6:30-11 a.m.
- Southbound US 36 traffic from Lyons/North Boulder should use 28th Street to Iris Ave, east to CO 157/Foothills Pkwy, then south to US 36 from 6:30-11 a.m.
SH-119/Diagonal Highway Access From Boulder to Gunbarrel, Niwot, and Longmont
- Iris Avenue & CO 119/Diagonal Highway will be closed eastbound at 47th Street until 10 a.m. Northbound CO 119/Diagonal Highway must be accessed via Valmont Road 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 Road will be closed to all traffic between 47th St to CO 119/Diagonal Highway from 7 a.m.-1 p.m.; residents may access via one way loop: beginning at Niwot Jay Road and 55th Street, continuing south on 55th/51st Streets to Jay Road, then west to 28th Street and Jay Road.
-
Jay Road will also be closed to eastbound traffic between 57th Street and 63rd Street from 7-10 a.m.
-
Eastbound Independence Jay Road and northbound N. 57th Street will be closed from 7-10 a.m. Residents may access via one way westbound/southbound loop starting at N 57th Street and Jay Road.
-
Southbound N. 61st/63rd Street will be closed to traffic from Jay Road. to Valmont Road from 7-10 a.m.
-
N. 51st St/N 55th Street will be closed to all traffic between Jay Road to Niwot Jay Road from 7:15 a.m.-1 p.m.; residents may access via one way loop: beginning at Niwot Jay Road and 55th Street, continuing south on 55th/51st Streets to Jay Road, then west to 28th Street and Jay Road.
-
Monarch Road will be closed to all traffic from N 55th St to N. 71st St from 7:30 a.m.-noon.
-
Niwot Road will be restricted to alternating one-way traffic between N. 55th Street and N 73rd Street from 7:45 a.m.-noon.
-
N. 63rd Street will be closed to all traffic between CO 119/Diagonal Highway and Niwot Road from 7 a.m.-noon. Residents may access via 63rd Street and Niwot Road.
-
Westbound Valmont Jay Road will be closed between Butte Mill Road and 55th Street from 7-9:30 a.m.
-
Access to the Valhalla neighborhood will be maintained via a one-way loop, beginning at Niwot Road and 55th Street, continuing south on 55th/51st Streets to Jay Road, then west to 28th Street and Jay Road.
City of Boulder Closures
- Pearl Street/Pearl Parkway will be closed to all traffic:
- Between 15th Street (east end of Pearl Street Mall) and 20th Street from 55th Street from 1 a.m.-10:30 a.m.
- Between 20th Street and Folsom Street from 6:30-8:30 a.m.
- Between Folsom Street and 55th Street from 6:30-10 a.m.
- Spruce Street will be closed to all traffic from Folsom Street to 14th Street from 1 a.m.-4 p.m.
- 55th Street will be closed to all traffic from Pearl Parkway/Valmont Road to Flatiron Parkway from 6:30-10 a.m.
- 47th Street will be closed to all traffic between Independence Road and Jay. Road from 7 a.m.-1 p.m.
- 26th Street will be closed from Topaz Drive to Iris Avenue from 7:30 a.m.-2 p.m.
- Folsom Street will be closed from Iris Avenue to Pearl Street from 7:30 a.m.-2 p.m.
- 14th Street will be closed to all traffic between Spruce Street and Walnut Street from 12 a.m. Friday morning through 6 p.m. on Sunday evening.
- The Fourmile Canyon Creek trail will remain open, but will have heavy runner traffic from 7:30 a.m.-1 p.m.
- Boulder Reservoir will be closed until 1 p.m.
Questions Or Concerns?
If you have questions or concerns, please visit the Boulderthon event website, call or text Boulderthon’s Community Coordinator at 720-378-8653, or email community@boulderthon.org.
You may also email John Holste, Boulder County Special Events Senior Planner, at jholste@bouldercounty.gov, or Justin Greenstein, City of Boulder Special Events Coordinator, at greensteinj@bouldercolorado.gov.
Boulderthon All-In-One Course Map, Sunday, Sept. 27, 2026
Boulderthon Impact Zone Map, Sunday, Sept. 27, 2026
Boulderthon Downtown Boulder Access Map, Sunday, Sept. 27, 2026
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.