Friday, January 17, 2014

"20 is plenty" in San Francisco?

20 is plenty

San Francisco Bike Coalition and San Francisco announced a new program, Zero Vision, for a nominal goal of zero tolerance for pedestrian deaths. The idea is to shift the perception that pedestrian deaths are an acceptable cost of doing business, that crossing the street is an activity like skydiving or smoking where the risk of death is implicitly accepted.

But it's mostly hot air. Looking at the article, the term "speed" isn't to be found at all. The #1 risk factor for pedestrians is vehicle speed. Going from 20 mph to 40 mph increases the risk of pedestian fatalities by approximately 17 times.

Perhaps it's perceived promoting slower vehicle speeds is politically unrealistic. I don't give a crap. If you're serious about safety, then it's important safety be given a priority.

And what's the cost of lower speed? Consider an example of a driver crossing the city at either 35 mph or 25 mph maximum speed. San Francisco is approximately a 7 by 7 mile square. So take 7 miles as a typical long trip. Assume the driver is at speed for approximately 2/3 of this, the rest of the distance the driver is acceleration-limited, deceleration limited, or congestion-limited. The driver will additionally be stopped at traffic signals for much of the trip, but that's a fixed time cost.

The travel times at speed are then 8 minutes for 35 mph versus 11.2 minutes for 25 mph. So the slower speed results in 3.2 minutes of additional time to cross the city.

If that was all there was to it, then that wouldn't be so bad. But there's compensating advantages. For example, see this discussion of a 20 mph speed limit in London.

The key point is when vehicles go slower, everything goes smoother. Consider gas flow. When air flows over an object, if it moving slowly, you get laminar flow, which is smooth with relatively little resistance. However, when the flow becomes extremely rapid, you get turbulent flow, with more resistance.

Vehicles aren't gas molecules, but they often behave with no greater intelligence. The principle holds.

So Ed Lee should step up and take a bold position here. If he's serious about safety, follow London's example and reduce vehicle speeds in the city. 20 mph would be especially bold, but I'll accept 25 mph. The simplistic model is it adds only 3.5 minutes to a cross-city trip. In reality it likely adds less. And the side effect is traffic flows more smoothly, which means MUNI can stick closer to schedule, which means bicyclists are better integrated with vehicular traffic, which means pedestrians can trust their survival chances more. All of this means less need for people to drive, and less traffic means less congestion, and less congestion just moves things along even faster.

Unfortunately, after I wrote the bulk of this blog post on Wednesday, this article was posted on Streetsblog. There's nothing bold coming from Ed Lee. Once again, San Francisco will need to look elsewhere for leadership, even within the US and not just the world. I am not surprised. I was opposed to Ed Lee in the 2011 mayor's election. There's just nothing there.

There was an effort to pass a 20 mph speed limit in New York. The bill introduced by David Greenfield eventually sputtered. But these things don't happen quickly. At least New York had the dialogue. In San Francisco, addressing vehicle speed is barely, if at all, on the government table.

1 comment:

ladyfleur said...

One more advantage of slower speeds: not only is the chance of death lower, but the chance of the driver having time to stop is dramatically different.

A driver who brakes hard when seeing something in the roadway 100 feet ahead will be going 38 mph when the car hits it. A driver going 20 mph will stop in time and won't hit it. People don't realize the dangers behind speeding.