Andy from Arvada writes, “What's driving you crazy? Why does CDOT inconsistently sign concurrent routes? For example, US-6 and US-85 are concurrent with portions of I-76, I-70, and I-25. With the exception of one forgotten reassurance sign on I-70, you would never know that. Same thing with I-70 and US-6. They are mostly concurrent through the mountains, but US-6 just seems to appear, then disappear. But, at the same time, I-25 is co-signed with US-24 in Colorado Springs and US-160 in Southern Colorado. I-70 is also co-signed with CO-9 from Frisco to Silverthorne. I've driven through many states and never seen a state disregard concurrent routes like Colorado does. In my opinion, it is far more confusing to have these routes appear and disappear than to simply label a road with both.”
Concurrent routes are roadways that share two or more separate route numbers. When highway numbering first started this was not a problem as each road had their own number. It became more common to see concurrent routing once construction of the interstate highway system began to use highways already in place. In most cases, it was easier to improve existing US highways up to the new interstate standards than to start a new roadway from the bare ground.
Many decades ago, route concurrences along portions of I-70, I-25 and I-76 were regularly posted in Colorado. It was also during that time when the interstates gradually supplanted the former role of US highways in long distance travel for Americans. Over time, as the interstate highway system became the dominant long-distance roadway network, the usefulness of concurrent route information declined.
Although interesting from a historical perspective, the full route of U.S. 6 in Colorado is likely traveled by only a very small number of people today. CDOT tells me putting up signage with every concurrent route on every roadway that shares signage would not improve traveler guidance sufficiently to offset the downsides of cost and sign clutter.
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A few states have retained this practice. Wyoming, for example, but most have gradually stopped signing these overlaps. In most states, they base the priority of roadway name on the priority of the route. Interstate and tolled interstate highways first. Followed by U.S. Highways, State Highways, Loops/beltway/spurs, all the way down to Ranch Roads.
CDOT tells me their practice in Colorado is to sign the significant route first as they overlap with secondary routes, like those you've mentioned, as they are useful in traveler guidance. Another example is U.S. 87, which technically follows I-25 in its entirety in Colorado. CDOT said Highway 87 is no longer signed due to extremely low demand for trips using this connection between the highway’s longer independent segments in Texas, New Mexico, and Montana.
So like many aspects of the old time road trip, the old way of fewer concurrent highway markings is having to yield to the new way of less cost and clutter.
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