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What's Driving You Crazy? Why did the bridge configurations for I-25 and Highways 402 and 56 change?

CDOT has to have some sort of engineering explanation, I’d love to hear it.
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Posted at 5:35 AM, Aug 30, 2022
and last updated 2022-08-30 10:45:31-04

Bob from Windsor writes, “What’s driving you crazy? When CDOT rebuilt the I-25 and Highway 402 interchange in Larimer County, prior to reconstruction, I-25 passed over Highway 402. Now, Highway 402 passes over I-25. This seems to work and vastly improved the traffic issues at the interchange. A few miles south in Weld County, CDOT is reconstructing the interchange at I-25 and Highway 56. Prior to reconstruction, Highway 56 passed over I-25. The new design has I-25 passing over Highway 56. Highway 34 still passes over I-25, as does Prospect Road. Highway 66, 119, 60 and 52 all stayed the same when they were rebuilt. My question is why did the configuration at Highways 402 and 56 flip-flop? I’m sure CDOT has some engineering explanation. I’d love to hear it.”

As a general answer to the question, Bob, the Colorado Department of Transportation believes these changes make each interchange flow better than before reconstruction. Jared Fiel, the CDOT Region 4 communication manager, said the improvement plans for I-25 north and these interchanges have been in some phase of design for around 20 years.

“Each intersection, bridge and segment of roadway has been analyzed for what would be the best way to accommodate future growth, traffic innovations, topography and a slew of other engineering things that people way smarter than me understand,” Fiel said.

Fiel said that’s the long answer for saying that every bridge and every interchange on this project has been looked at on an individual basis. CDOT also said they are doing things a little bit differently on the I-25 North Express Lanes project between Mead and Fort Collins than they have with past projects, including working on the entire corridor and at multiple interchanges all at once.

What's Driving You Crazy? Why did the bridge configurations for I-25 and Highways 402 and 56 change?

With specific respect to Colorado Highway 402, the decision to flip-flop the bridge configuration made sense for multiple reasons.

“There is an awareness and understanding that northern Colorado is growing... fast. At some point, CO 402 will potentially need to be widened. We flipped the 402 interchange because it is vastly cheaper and less impactful to eventually widen 402 over I-25 than it would be to widen 402 if it was under I-25. In this particular case, because we are reconstructing both 402 and I-25 in the same project, building an entirely new interchange, there was no downside to flipping from a cost, scope and schedule point of view. In other instances, we may be just reconstructing a bridge over I-25. I'm thinking Harmony or 392, etc., which would incur significant additional cost if we were to flip those as we would need to design/construct portions of the interstate and ramps in addition to the bridge. We also see some operational improvements at 402. Traffic, particularly trucks, exit the interstate going slightly uphill, which allows gravity to help slow them. Likewise, traffic, particularly trucks entering I-25, heads slightly downhill which allows gravity to help them accelerate,” Fiel said.

MORE: Read more traffic issues driving people crazy

Fiel said there was good reason to flip the Colorado Highway 56 bridge configuration, too.

“The topography worked vastly better with the flip configuration. The CO 56 area was the most dangerous section on north I-25 because of the infamous curve in I-25 and other factors, so we wanted to make sure the safety issues were addressed. The flip helped the vertical alignment for both mainline I-25 users and CO 56 users, so people didn't have to go uphill followed by having to go downhill. The configuration also helped construction phasing because the CO 56 bridges were able to be built 'offline' of traffic. The horizontal alignment of the roadways were able to be fixed as well. Those were the two big advantages. Also, we designed the I-25 bridges to accommodate future widening on CO 56,” Fiel said.

Only time will tell how well the flipping of the interchanges will allow for all movements to flow better over or under I-25. There is still quite a lot of work to finish as this project moves into its last year plus of construction. The project is scheduled to be done in 2023 with the final tolling work done in 2024. More information about the project can be found from the North I-25 Project Page.

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