Currently, Streetmix displays a warning message if you put sharrows (aka shared lane markings) in a lane that is less than 12 feet in width. However, this appears to be in contradiction with guidance contained within the Manual of Uniform Traffic Control Devices (MUTCD), which is issued by the Federal Highway Administration to serve as final word on the usage of signals, signs, and pavement markings on American roadways. This guidance can be found here:
Specifically, the second use of shared lane markings that the MUTCD permits is to “Assist bicyclists with lateral positioning in lanes that are too narrow for a motor vehicle and a bicycle to travel side by side within the same traffic lane.” The MUTCD also makes clear that the shared lane marking is equivalent to and interchangeable with the R4-11 “Bicycles May Use Full Lane” sign.
I looked up guidance for the use of shared lane markings in both the NACTO Urban Bikeway Design Guide and the AASHTO Guide for the Development of Bicycle Facilities, the two most widely consulted guidebooks in designing bikeways in the U.S. Both guidebooks repeat the guidance listed in the MUTCD that situations that may warrant shared lane markings include bikeways in lanes that are too narrow for a bicycle and motor vehicle to travel side by side within the same lane.
I would like to know what guidance informed the decision to tell Streetmix to issue a warning for sharrows in lanes narrower than 12 feet in width.