DENVER — The Denver Colfax Marathon races will cause some detours as well as traffic and transportation impacts this weekend. 

The 20th annual event will feature eight different races over two days on Saturday and Sunday. It’s Denver’s largest running event, with more than 28,000 runners expected to participate across four race lengths: full marathon, half marathon, 10 miler, and 5K. 

Several of those races will take place starting at 6 a.m. Sunday. The route starts in Denver’s City Park and extends through Lakewood. 

Roads will close to traffic on a rolling basis between 5 a.m. and 1 p.m. Sunday depending on which part of the city they’re in, according to the event’s website.

If you’re traveling north and south:

• I-25 will be open
• On the east Colorado Boulevard will be open (except southbound from 29th to 17th), and all roads east of Colorado Boulevard.
• To the west Kipling will be open, and all roads west of Kipling.

If you’re traveling east or west:

• In general stay south of Colfax (14th and lower) and north of 29th (30th and higher).

Roads will be closing and reopening on a rolling basis once the last runner is past the area. 

The Regional Transportation District (RTD) said some of its bus and train services will experience detours from 5:30 a.m. Sunday until about noon, especially RTD light rail and bus routes that run along and across Colfax Avenue.

• D and H lines will not serve Colfax at Auraria Station through the Downtown Loop stations on Stout and California streets.
• D and H lines will continue to serve 10th-Osage Station, and will reroute to serve Auraria West Station, Empower Field at Mile High Station, Ball Arena-Elitch Gardens Station and Union Station.
• The L Line will not be in service during the event, and customers are encouraged to use bus Route 43 as an alternative.
• Customers should expect delays on D, E, H and W lines during the event due to increased rail traffic at Union Station.

RTD said a Colfax 5K race on Saturday will have no impacts to rail service.

• May 16 only: routes 20 and 24 will have a minor detour around the event, 9 a.m.-noon during the Colfax 5K race.
• May 17 only: the following bus routes will be temporarily detoured in many areas, and will be allowed to cross the marathon route when possible: 0, 1, 6, 8, 9, 10, 12, 15, 15L, 16, 19, 20, 24, 28, 30, 38, 40, 43, 44, 48, 51, 52, 76, 83L, ART, FREE.

Customers using routes affected by the marathon should expect delays and detours.

On Sunday from 6 a.m. to 2 p.m., RTD is accepting Denver Colfax Marathon 2026 race bibs as valid fare payment on the following rail lines and bus routes: 

• Rail lines: W Line 
• Bus routes: 15, 15L, 16