[flickr]photo:8148572201[/flickr]
Clark Street buffered bike lane just north of Diversey Avenue. Photo by Adam Herstein.
CDOT responded today to my inquiry asking why Clark Street between Diversey Avenue and Addison Avenue received a buffered bike lane and not a protected bike lane. Bikeways planner Mike Amsden writes:
Clark Street (Diversey to Addison) was striped as a buffer protected bike lane, and not a parking protected bike lane, because it is a 51′ roadway throughout the project limits. As you might know, the minimum roadway width for installing barrier protected bike lanes on roadways with one travel lane and one parking lane in each direction is 52′, and the preferred minimum width is 58′. A 52′ roadway allows for a 5′ bike lane, a 3′ buffer zone, an 8′ parking lane and a 10′ travel lane in each direction. However, even when a roadway is 52′, other roadway characteristics and operational considerations must be assessed before installing parking protected bike lanes. These characteristics include the amount of bus, truck and bicycle traffic, loading and delivery needs of local businesses, emergency vehicle access and maintenance requirements.
These standards come from the NACTO* Urban Bikeway Design Guide. If you have not read this guide I recommend doing so as it will likely answer many other questions you may have. The guide can be found here. http://nacto.org/cities-for-cycling/design-guide/
To answer your final question, if we were to recommend a parking protected bike lane on a 51′ roadway, all curbside uses (parking, standing, loading, valet, etc.) would have to be eliminated on one side of the roadway for the entire stretch in order to do so.
1 foot is all it takes, it seems. You can see the relevant NACTO standard in this image. Notice items 4, 5, and 6, that describe the minimum widths for the bike lane, buffer area, and parking lane, respectively.
Update: Why can’t certain lanes be narrower?
Someone asked in the comments this question, which Amsden anticipated: “We can’t go with a 7′ parking lane because then you’d have a 7′ parking lane next to a 10′ travel lane. People wouldn’t be able to get out of their cars, buses/trucks wouldn’t be able to maneuver, emergency vehicle access would be restricted, etc. That’s why even a 52′ roadway (with a 8′ parking lane next to a 10′ travel lane) is considered really tight.”
* National Association of City Transportation Officials, the yin to American Association of State and Highway Transportation Officials yang (AASHTO).