CO 119 Diagonal Highway


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On May 2, 2015, CDOT began a $12 million project along CO 119 (Diagonal Highway), just west of Longmont in Boulder County, from mile points 44.995 to 54.42, to improve the ride-ability and traffic operations for motorists.

In addition to resurfacing work, guardrails and bridge rails will be replaced, and minor bridge repairs will be performed and signs replaced along the construction zone. Motorists can expect the project to be completed by October.

During the project and construction, crews will work to minimize any traffic impacts. Lanes shifts and lane closures may occur and will be directed by flaggers at both ends of the work zone. All construction requiring lane closures will occur from 9 a.m. to 7 p.m. in the southwest direction, and from 7 a.m. to 3 p.m. in the northeast direction. Work not requiring lane closures will occur from 7 a.m. to 7 p.m. in both directions.

Damage Overview

This section of CO 119 is a major expressway providing access between Boulder and Longmont.  Several creeks and drainages flow through the area and pass under the highway.  High flows during the flood event caused varying erosion at the bridge structures, damage to drainage culverts, and damaged trail facilities near the highway.  Some channels were degraded and new flow paths were created by the floodwaters causing a buildup of sediment and debris.

Emergency (Temporary) Repairs

Minor work was done mostly by local agencies including sediment removal and repairs to the adjacent railroad grade (by the railroad line owner).


Permanent (Long-Term) Repairs

Permanent repairs will include re-stabilizing channel embankments, re-vegetation, culvert inlet/outlet protection, and repairs on localized erosion around bridge abutments and piers with additional erosion protection.

Estimated Timeline

  • Est. Design Start: Fall 2014
  • Phase I – Structure repairs and protection to be completed by summer 2015

Estimated Budget

  • $1 - $4 million


The permanent repair phase will allow for the betterment of drainage facilities and erosion protection areas throughout the corridor that can handle the high flows from large storm events and protect facilities from potential damage in the future.