Tuesday, April 19, 2011

Golden Gate Bridge proposed 10 mph speed limit

Jym Dyer sent notice of this gem along the SF Bike mailing list:


SF Examiner story

Here was my response, which I emailed to the Golden Gate Bridge Transportation District:

I have read the consultant’s report and the corresponding proposal to introduce a 10 mph speed limit on the bridge.

The statistics do not justify such an action. Collision rates (per rider) have been decreasing over time, even with an enormous increase in the number of inexperienced cyclists (renting from Blazing Saddles and other agencies) crossing the bridge. And only a minority (39%) of collisions are labeled with “speed as a factor”. This is in spite of the fact that virtually all experienced cyclists ride in excess of 10 mph across the bridge (there are plenty of data available on Strava.com). Checking my own data, 25 kph is a typical speed even when riding alone in a relaxed, noncompetitive fashion.

There isn’t even a case presented that safety is a problem on the bridge. You cite 164 collisions over 10 years on a bridge where “up to 6000 cyclists” used the bridge in a day. If you assume 15 thousand cyclists per week (an average of only 2 thousand per day), that’s roughly 750 thousand cyclists per year, which corresponds to 1.3 million cycling miles per year at one crossing per cyclist (I assume the report counts round-trips as “two cyclists”). Over ten years this corresponds to one collision per 81 thousand miles ridden. This is likely lower than typical urban motor vehicle collision rates.

Even if a speed limit is imposed, 10 mph is too slow, especially on the western path from which pedestrians are forbidden. The report justifies imposing a speed limit on both sides as follows:

“Bicyclists accustomed to riding unimpeded at a relatively quick speed over the Bridge on weekend rides may have difficulty adjusting to sharing the path with slower moving pedestrians. Pedestrians may feel intimidated by large groups of bicyclists passing by.”

With this logic, auto speeds should be limited by the safety concerns of the slowest nearby streets. For example, the motor vehicle limit on the bridge should be no more than 25 mph, as motorists accustomed to going faster may have difficulty adjusting to conditions on the Bridgeway. This line of reasoning is inconsistent with standard motor vehicle policy where the “safest speed” is judged on a road-by-road basis.

If there is a safety issue on the western path, and the data hardly indicate a safety concern given the traffic volume, then that issue is with pedestrians: people standing by their bikes taking photos, often moving without care. Pedestrians are forbidden on the western path, and that implicitly applies to people standing next to their bicycles who are legally pedestrians:

California Vehicle Code 467. (a) A “pedestrian” is a person who is afoot or ….

The bike paths on the bridge are as legitimate transportation infrastructure as the motor vehicle lanes. Regular users of the bridge, such as I, should be able to cross the bridge in a timely manner, subject to the constraints of safety. Yet under general conditions, the maximum safe speed on the bridge is more like 20 mph than 10 mph. Under conditions of exceptional congestion the speed needs to be reduced, as it must be reduced on the roadway when there is heavy traffic. But you do not set roadway limits under the worst-case traffic scenario, and you should not set cycling speeds under worst-case traffic scenarios.

There is a precedent for what constitutes a safe and reasonable speed for cycling infrastructure: Table 1003.1 of the California Highway Design Manual specifies the design speed for Class I bikeways where mopeds are forbidden to be 40 kph. It is clearly recognized that speeds of up to 40 kph are reasonable for bicycle use under appropriate conditions. And those conditions often exist on the bridge between the supports in early mornings and in winter. There is no need for a blanket regulation.

I am thus strongly opposed to this proposed limit.


Eric said...

Very well written! Unfortunately these decisions are often made more on emotion than facts, but it becomes harder to do that when the facts are laid out so well. Thanks for taking the time to send that off.

djconnel said...

Thanks, Eric!

Yokota Fritz said...

Dan, thanks for posting your letter up here.

me said...

Thank you! I posted a link to it on SFBC wall - I think you have very good arguments. Moreover, how can one expect cyclists to know their exact speed? Are speedometers on bikes going to be required?

djconnel said...

Right: so if you enforce a 10 mph limit, people need to go 7-8 mph to keep a bit of a buffer.

On the 5 mph around the pylons: can anyone clean those faster? Seems the pylons are sort of self-enforcing.

They delayed vote indefinitely on it at the meeting today, so no short-term worry, anyway.