HEALTHY DEVELOPMENT MEASUREMENT TOOL

Indicator ST.3.c Vehicle-bicycle injury collisions

Descriptive Title: Vehicle-bicycle injury collisions

Geographic Unit of Analysis: Census tract and intersection

Download PDF map.

Why Is This An Indicator Of Health and Sustainability?

Ensuring real and perceived safety for bicycle riders is necessary to encourage bicycle travel, a lower-cost active travel mode that does not contribute to air pollution or greenhouse gas emissions and has physical and mental health benefits.  In the United States, cyclists experience a disproportionate share of traffic injury and fatality risk compared to vehicle occupants on per-trip basis -- with estimated per-trip fatalities over two times that of motor vehicles (21.0 vs. 9.2 per 100 million person trips).^a Despite higher risk of injury per trip, cycling is a relatively low‐cost, physically active form of transportation that is growing in popularity in the U.S. in recent years ‐ with an overall 64% increase in bike commuters in the U.S. from 1990‐2009 and with trends particularly notable in urban areas including San Francisco, Portland, Minneapolis, Washington DC, Chicago, and New York City.^b From 2006 to 2011, the San Francisco Municipal Transportation Agency’s annual citywide bicycle count found that counts of observed cyclists increased by 71%.^c

Changes in the roadway environment can address both real and perceived safety hazards for bicyclists - including adding bicycle lanes, reducing vehicle speeds (traffic calming), and introducing additional traffic engineering measures at intersections (e.g., signs, turning restrictions, left-turn bicycle lanes, dashed intersection bicycle lanes) to reduce conflicts between bikes and cars. Other interventions to reduce deaths and serious injuries include use of bicycle helmets and increased driver and bicyclist education.^d,e 

Interpretation and Geographic Equity Analysis

Vehicle-bicycle injury collisions are shown in proportional blue symbols on the map (above).  Patterns of injury are also reflected in the number of collisions in each census tract (also depicted on the map). 

The location of vehicle-bicycle injury collisions are primarily on arterials in the northeast portion of San Francisco, in areas with or near more heavily trafficked roadways (including freeways and highways) with faster speeds where there are also higher cyclist volumes.  A high concentration of the collisions are located along or near Market Street (a major cyclist corridor), and Downtown, in the Financial District, South of Market, Mission, and Western Addition neighborhoods. This is also reflected in the number of collisions in each census tract (red on map). 

Methods

The tabular SWITRS (see data sources) collision-level data of vehicle-bicycle injury collisions were geocoded to the nearest intersection by the San Francisco Department of Public Health in 2012.  The collisions were simplified to the count of collisions per intersection, shown in the proportional blue dots in the map (above). The count of collisions were summed for each census tract and planning neighborhood based on spatial location using ESRI’s ArcMap.

Limitations

Collisions were geocoded to the nearest intersection based on data on primary and secondary streets as reported in SWITRS (see data sources).  Census tracts and planning neighborhoods are delineated by streets, typically major arterials, which tend to have higher rates of vehicle collisions.  This becomes a limitation when collisions counts that fall on the border of two areas need to be assigned to census tracts or planning neighborhoods.  For this reason, we have included both intersection level and census tract level collision counts on the map.

SWITRS data (see data sources) includes collisions on a public roadway that are reported to the California Highway Patrol.  However, some collisions go unreported for a number of reasons.  A recent analysis by the San Francisco Injury Center of patients seen by the San Francisco General Hospital (SFGH) for trauma sustained while cycling found that approximately 25% of those patients were injured in a “cyclist only” crash that did not involve contact with a motor vehicle.  These “cyclist-only” crashes were notably under-reported in the police-reported SWITRS data historically relied upon by the City to monitor collisions over time.  Specifically, 22% of SFGH patient records did not match with a SWITRS record; of those SFGH records, 58.5% were cyclists injured in a crash with a motor vehicle and 41.5% were of cyclist-only injuries.  Notably, 42% of cyclist injuries reported by SWITRS did not match with a SFGH record; of those SWITRS records, 91% were injured in a crash with a motor vehicle and 9% were cyclist-only injuries.^f

In recent years in San Francisco, bicycle infrastructure has and is expected to continue to improve citywide due to programmed improvements.  The aggregate data presented here does not capture the impacts of those time and site-specific improvements which would require more refined evaluation.  For more information please see: http://www.sfmta.com/cms/bhome/homebikes.htm.

Data Source

Raw collision data obtained from the California Highway Patrol, Statewide Integrated Traffic Records System (SWITRS) for 2006 to 2010. More information can be found at: http://www.chp.ca.gov/switrs/. Geocoded by the San Francisco Department of Public Health (2012).

Map and table prepared 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. 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.2012.pdf

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