Indicator ST.3.f Posted speed limits of 20 mph or less

ST.3.f Posted speed limits of 20 mph or less

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Data Source

Speed limit sign data provided by the San Francisco Department of Public Works and current through August 2009.  Geocoding and mapping conducted by City and County of San Francisco, Department of Public Health, Environmental Health Section using ArcGIS software.

Map data is presented at the level of the census tract. The map also includes planning neighborhood names, in the vicinity of their corresponding census tracts.

Table data is presented by planning neighborhood. Planning neighborhoods are larger geographic areas then census tracts. SF DPH used ArcGIS software and a 'centroids within' methodology to convert census tracts to geographic mean center points. We then assigned census tracts to planning neighborhoods based on the spatial location of those geographic mean center points and calculated the planning neighborhood totals for the table.

Detailed information regarding census data, geographic units of analysis, their definitions, and their boundaries can be found in the HDMT at the following links:

http://www.thehdmt.org/etc/Geographic_Units_of_Analysis.September_2009.pdf

http://www.thehdmt.org/data_map_methods.php

Explanation and Limitations

This indicator reflects data on posted speed limit signs of 20 miles per hour (mph) or less on streets in San Francisco maintained by the City and County of San Francisco with signs maintained by the San Francisco Department of Public Works.  It therefore does not include posted speed limits on roads maintained by the state (Caltrans).  Posted speed limits greater than 20 mph are also not included in the above data. 

The data includes speed limits at or less than 20 mph in special conditions (e.g., with buses present or near senior facilities).

Notably, not all streets in San Francisco have posted speed limits.  The facto speed limit on San Francisco streets is 25 mph; alleys narrower than 25 feet have a de facto speed limit of 15 mph.

This indicator reflects posted - as opposed to actual - speeds.  There is likely variability between posted speed limits and actual vehicle speeds throughout San Francisco - impacted by factors including street design features (land width, traffic calming), traffic volumes, and local law enforcement practices.

Why is this a Community Health Indicator?

Traffic speed contributes both to the frequency of traffic collisions as well as the seriousness of injury sustained in vehicle collisions.a Lower speeds give drivers time to react and reduce the severity of an impact. With regards to collision prevention on residential streets, one study found that an increase in average speed from 20 to 30 mph increased the risk of pedestrian injury over seven-fold.b The likelihood of a pedestrian surviving being struck by a vehicle moving at 20 mph is approximately 95%; the likelihood of suviving decreases to 55% at 30mph.  As speeds increase to 40 mph, the likelihood of a pedestrian surviving a motor vehicle collision further decreases to only 15%.c

  1. Ewing R, Frank L, Kreutzer R. Understanding the Relationship between Public Health and the Built Environment: A Report to the LEED-ND Core Committee. 2006.
  2. Peterson P. Child Pedestrian Injuries on Residential Streets: Implications for Traffic Engineering. ITE Journal on the Web. 2000.
  3. U K Department of Transport. Killing Speed and Saving Lives. London 1997.