Should warning be removed for sharrows in narrow lanes?


#1

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:
http://mutcd.fhwa.dot.gov/htm/2009/part9/part9c.htm#section9C07
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.


#2

Thanks for the comment, @navd88. We do have rationales for most of these decisions and they may evolve over time. That being said, because laws are different in different jurisdictions, and context creates different requirements, Streetmix guidance represents opinions of our team or other planners that we happen to agree with, even if they’re not the letter of the law :slight_smile: We also have these documented but they’re in an internal Google doc, so this reminds me that we should open source this as well.

Basically, most (speaking only for the USA) federal-level guidance don’t specify hard minimums. But our sources seem to indicate the following:

  • Preferred width of 14-ft (4.2m) - Sources: AASHTO Green Book 2011, page 100; Complete Streets Chicago 3.2.1 “Cross Section Assemblage” (2013)
  • Preferred width of 13-ft - NYC DOT’s Street Design Manual (Second Edition, 2013, page 59) officially does not require a minimum amount of space, but does say “A wide (13-foot) travel lane is preferred.”
  • Inferred width of 12-13 ft (3.6-3.9m) - Source: AASHTO Green Book 2011, page 316, describing a situation where an outside drive lane is slightly wider than an inside drive lane only to allow for bicycles, although not referring to a shared bike lane specifically.

The meta-issue behind this is that sharrows might not be a preferable strategy for bike infrastructure at all. So we have defined this narrow range of allowed sharrow width so that you can do something with awkward leftover spaces that are too narrow for a separate bike lane but too wide for a normal car lane. (We set the maximum on a car lane to 12-ft, even though plenty of guidance documents encourage or prefer lane widths much wider than that, so we’re adopting a very specific position on what we think a car lane ought to be.) Beyond that 12-ft limit, you might as well put in a shared lane marking. And then we cap our shared lanes at 14-ft - even though no jurisdiction specifies a maximum allowed width for sharrows, beyond that width, you may as well design a bike-only lane by a drive lane.

This isn’t to say we wouldn’t ever reconsider this setting. If it makes more sense to show a shared lane even in the normal range of 10-12ft, we’ll consider making that switch.


#3

I’ve just made our Google doc for rationale publicly viewable and commentable: https://docs.google.com/document/d/1GLZGnSOqXdORc-13UwV766jgYpVppXMxC_naL1tor3o/edit?usp=sharing

I’ll also link to this from our documentation site here: https://streetmix.readme.io/docs/street-segments


#4

I agree with OP. Sharrows on wide lanes like 14ft mentioned above go to the right side of the lane, but in narrower lanes, like a 10 foot lane they go in the center.

I also agree in general, Sharrows are not a preferable strategy for bike infrastructure (sharrows are hardly worth the cost of the paint or plastic).