Police in Larimer County are going to crack down on cyclists riding two abreast. A recent article in the Coloradoan talks about the Larimer County Sheriff's plans.
Here is a portion of the Colorado Statue that details riding single file:
Colorado Statutes : 42-4-1412
(6)(a) Persons operating bicycles on roadways shall ride single file; except that riding no more than two abreast is permitted in the following circumstances:
(i)When riding two abreast will not impede the normal and reasonable movement of traffic; or
(ii)When riding on paths or parts of roadways set aside for the exclusive use of bicycles.
(b) Persons riding two abreast shall ride within a single lane.
Know the laws when riding your bicycle. For more information about bicycle laws in Colorado goto Bicycle Colorado.