"Bicycles Ride Single File" signs: law or suggestion?

We got a great question in our inbox from a Boulder-area cyclist:

"Hi Megan, I appreciated your recent blog post on riding two-abreast in Colorado (http://www.kastrobar.com/blog/2019/12/19/riding-2-abreast-when-and-where-it-is-permitted). it solidified my understanding of how this state law applies to cyclists in practice. 

This week I had an encounter on Four Mile Canyon Dr west of Boulder with an extremely aggressive driver. He was enraged that our group did not single up quickly enough (or more accurately, that we were riding two abreast at all). While out of his vehicle threatening to assault us, he claimed that the presence of yellow "Bicycles Single File in Four Mile Canyon" signs required us to ride single file at all times. My understanding is that these yellow signs are advisory in nature, much like the yellow reduced speed limit signs that precede a corner. They do not trump posted speed limits or state law. Is this an accurate assessment? Or, do these signs overrule the state law on two abreast cycling that you discuss in your blog post?

These signs are common in other Boulder County canyons, so it's a question that is widely applicable to local cyclists. The residents of Four Mile Canyon Dr. however seem to be particularly hostile towards cyclists with regard to two-abreast riding." 

The short answer is: those signs are not laws, they are suggestions.  

I also consulted one of our best contacts in Boulder County law enforcement and he concurred: "The signs came about after a working group comprised of county transportation, fire, law, mountain area citizens, cyclists, etcetera, met to address issues.  I was part of the group that met soon after the 2013 floods.  You may remember Four Mile, Lefthand, Jamestown and St. Vrain canyon roads were severely damaged.  Most have been repaired as of this year.  

Even after the rebuilding there are many sections of our canyons that are narrow and, while legal to ride two abreast, it’s hazardous for the cyclists on the blind curves.  These signs were installed to encourage safe cycling. 

I don’t see any mention that she reported the incident but if she didn’t I would encourage her to do so in the future." (Sidenote: Remember, cyclists, you can call *277 to report menacing drivers to the CSP aggressive driver hotline, or you can call the local authorities to report this kind of conduct!). 

In short: those signs are placed in areas where it is really best and safest to ride single file - the signs were placed with intention and not randomly.  While they are not law, they are suggestive of best practices given the curves, road conditions and so on.