Simply put...the infrastructure of I-25 north of Denver, and more specifically north of Longmont, is overwhelmed. It is a parking lot formerly known as an interstate highway.
Went to see the Tim McGraw/Faith Hill Soul to Soul Tour at the Pepsi Center Tuesday night. There was also a Colorado Rockies Game and a concert at Red Rocks the same night. I was going nowhere near the speed limit, much less over it. A truck fire south of the Longmont Exit 243 had southbound traffic backed up all the way to the Loveland Exit 257. 14 miles of traffic going 25-30MPH. Freed up a bit beyond the burnt out truck, but slowed again at 120th Ave due to the usual construction going on in that area and south.
This has become the norm here. Any time there's a stall or accident or any sort of event, even a trooper on a traffic stop and giving a ticket, traffic slows down tremendously. Was able to go closer to the speed limit coming BACK from the concert -- at 11pm. Traffic was still heavy and slowed when the highway narrows from 3 to 2 lanes. Commuter life in northern Colorado.