Wednesday, March 18, 2009


Let's continue on the topic of street marking.

Wanting to know more about sharrows, I went to Wikipedia and read about shared lane marking. (I had to read about a district in Sheffield, England first.) I was a little unclear on the topic, I suppose, as I had thought that sharrows were rather vague in their application. Nothing could be further from the truth, as there appears to be a small if well defined body of traffic standards that apply to shared lane markings.

If you've (like me) not been paying close attention to the sharrows issue, here's a brief definition: Sharrows are "bicycle use" road markings that are installed where complete bike lanes cannot be installed for various reasons including:
  • Not enough cyclists to justify bike lanes;
  • Too expensive to install bike lanes;
  • Use of bike lanes would require loss of parking; and/or
  • Use of bike lanes would require road widening;
The city of San Francisco did a study on sharrows in 2004 that they have published in PDF form. The study's stated goals for sharrows is the following:
  • Improve positioning of both cyclists and motorists on streets without bike lanes;
  • Reduce aggressive motorist behavior;
  • Prevent wrong-way bicycling;
  • Prevent bicycling on sidewalks;
San Francisco additionally studied two different forms of sharrow marking, the "bike in house" design and the "chevron" design. Their study logged 140 hours of videotaping of before-and-after activity on six different streets in SF, three of which were two-lane and three of which were four-lane. Even though this study is not exactly "new news", I must say that I'm impressed with the size, thoroughness and rigor of this study. San Francisco has got a bunch of wonks that know what they're doing, statistically.

So, what happened? Well, the city of San Francisco (CSF) videotaped and analyzed a lot of traffic behavior. In 6 street locations, and before painting the streets, they taped cyclists' positions on the street, motorists' locations, and clearances afforded cyclists by motorists. (They did this for 1100 cyclists.) Then they painted the sharrows with the center of the figure 11'-0" out from the curb. And they ran the video study again. The results of the study are as summarized in the graphic at right.

I don't know about you, but I think these are pretty significant results. Cyclists are clearly less crowded towards parked cars, and motorists are clearly making better affordances for them.

One of the interesting things that the CSF found was that while the "chevron" design eliminated wrong-way cycling where it was used, the "bike-in-house" design appeared to have no effect in this area. For this reason (and the slightly better affordances noted above), the CSF approved the "chevron" design for use on CSF streets.

Because the markings were applied to the streets with no preceding public education program (the CSF evidently felt this might have skewed the results,) cyclists and motorists were then surveyed as to their perception and understanding of the markings. I won't go into the results of this survey in detail, (you can peruse the PDF if you like, as it's all there,) but will hit on some highlights:
  • Most cyclists felt that the markings indicated a bike route;
  • 60% of the cyclists felt an increased sense of safety;
  • 33% of the cyclists felt that the markings caused them to "take the lane" more;
  • 35% of the cyclists felt that the markings improved motorists' behavior;
After reading and reflecting on this excellent study, I feel that I have a much more positive feeling about sharrows than I had before. First and foremost, they are effective; they work to improve cyclist/ motorist interactions. Secondly, they have a "skills improvement" component, that of helping the cyclist attain proper position on the road. Finally, their ambiguity (which had put me off a little before I read the study) may prove to be one of their strong points: while a cyclist may feel he is "unprotected" if he ventures outside a fully-marked bike lane, no such boundary exists or is even implied with a sharrow, and this is probably a good thing.

1 comment:

Anonymous said...

Great post. Thanks. I am a traffic engineer for a city in California and we have just begun to install sharrows. So far we have noticed an increase in acceptability by motorists when bicycles take the lane where the sharrows are located. We plan on installing more.