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August 2, 2018

Ironman 70.3 Road Detours and Closures – Aug. 4

Significant detour and heavy traffic is expected on SH 119/Diagonal Highway. All vehicle traffic will shift to the Longmont-bound travel lanes to allow for the bike course to use the Boulder-bound lanes.

The bike course will active between the hours of 7 a.m. until 2 p.m. and will use the following roads:
SH 119/Diagonal Highway, N. 63rd St., Niwot Rd., Neva Rd., US 36/N. Foothills Hwy, Nelson Rd., N. 65th St., St. Vrain Rd., SH 66/Ute Highway, N. 75th St./N. 73rd St., and 71st St.

All residents will have access to their homes at all times, however please be aware of the following restrictions:

  • N. 63rd St. will be closed to vehicular traffic from Monarch Rd. to SH 119/Diagonal Highway from 6 a.m. to 4:30 p.m.
  • N. 71st St. will be closed to all traffic from Monarch Rd. to SH 119/Diagonal Highway from 6 a.m. to 2 p.m.
  • Monarch Rd. and Niwot Rd. will be closed to all traffic from SH 119/Diagonal Highway to 71st St. from 6 a.m. to 2 p.m. Please use alternate routes around the course (US 36, Oxford Rd., and Airport Rd).
  • N. 51st St. will be closed from Jay Rd. to Monarch Rd except to local residents between 6 a.m. to 4:30 p.m.

All Ironman spectators must use the shuttle service from Boulder High School after 7 a.m., no exceptions.

Please avoid crossing the course when possible. If you need to get from one side to the other, please use U.S. 36/N. Foothills Hwy, SH 66/Ute Highway, Jay Rd., and Hover Rd. to get around the course.

Avoid using Niwot Rd., Nelson Rd. or St. Vrain Rd. Those roads will be open, but you will experience longer delays than if you use the loop around.

Law enforcement will be at all major intersections to help you turn or go through the race course but you should expect delays if you are crossing the route.

Recreational cyclists should avoid traveling in the same direction as the riders on the bike course and avoid traveling on and across the Diagonal Highway between 55th St. and Oxford Rd. The IBM Connector Trail will be open to maintain cyclist access across the course on the Diagonal Highway/SH 119. Recreational cyclists may use N. 63rd St. from Niwot Rd. to the IBM Connector Trail, but may not cross the Diagonal Highway/SH 119.

Boulder Reservoir will be closed all day on Aug. 3-4.

Please consult our interactive map for more information.

If you have questions or concerns, please email

You can also access more info at the event website: and click on the spectators tab for more info.

Alternatively, you may also contact John Holste, Boulder County Special Events Coordinator at

Detour route map for 70.3 Ironman