Driving You Crazy: Why does I-25 go from 4 lanes to 3 lanes at the County Line exit then back to 4 lanes?

LONE TREE, Colo. — Dave from Castle Rock writes, “What's driving you crazy? Why does I 25 northbound go from 4 lanes down to 3 lanes at the County Line exit, then back to 4 lanes a quarter of a mile later? It looks wide enough, can’t they just re-stripe it for a continuous 4 lanes?”

What looks like an easy fix to you and me is never the case when it comes to traffic engineering and federal interstate guidelines. When you drive under C-470, it is easy to imagine how you would get behind the controls of a bulldozer and start carving out a continuous lane from the County Line exit to the merge at C-470. But when you look at the area more closely, you will see several obstacles are in the way.

The first is an elevation difference between the exit lane and a potential fourth lane that would have to be leveled. There is a hard to see drainage area about half way to the C-470 merge that would have to be accounted for. Then you have underground powerlines, a power box and a super tall streetlight that would have to be moved. There is a pole that houses a traffic speed and volume device that would need to be relocated. And a major adjustment would need to be made to the left lane of the ramp from C-470 to northbound I-25 to accommodate the new northbound I-25 lane.

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Colorado Department of Transportation (CDOT) engineers agree, telling me, “Modifying this section to create a continuous fourth lane would require a major, complex redesign of the approach sign bridges, downstream entering gore areas, and the exits to County Line Road.”

Another part of this puzzle would be carving out the room needed to accommodate a new right shoulder. While the highway looks wide enough to just re-stripe using the existing paved shoulder, the extra pavement next to the right lane is actually a federally mandated emergency breakdown area and it and needs to be preserved. CDOT engineers told me taking away those vital shoulders to squeeze in a fourth lane would create a major safety hazard.

Another unforeseen constraint here is the bridge from C-470 that goes over the current County Line exit. It is too close, I’m told, to easily make an accommodation for an additional travel lane while adding room for a new shoulder and maintaining the same distance for a downstream merge.

One of the last issues to deal with would be at the end of the new proposed lane. A new added fourth lane on I-25 would eliminate how the left most lane from C-470 to northbound I-25 merges. Right now, C-470 drivers have a dedicated lane that keeps traffic flowing off the ramp but a new merge lane with I-25 drivers could be problematic. The new merge would naturally slow drivers on both the new northbound I=25 lane and the left C-470 ramp lane as they navigate a new short merge. We all know how challenging Denver drivers have with considerate merging so at peak volume, this merge would fail resulting in a more severe choke point that is considerably more dangerous than the current continuous lane configuration.

I don’t mean to sound like a Debbie Downer but bottom line — to completely alter and partially rebuild the surrounding highway infrastructure would require significant investment and major reconstruction that CDOT has no interest in doing not to mention the lack of funding to complete.

Denver7 Traffic Expert Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook, Twitter or Instagram or listen to his award winning Driving You Crazy podcast on any podcast app including iTunes, iHeartRadio, Spotify, Podbean, or YouTube.