Recently, the Carrboro Transportation Advisory Board brought a proposal for a “Slow Zone” in downtown Carrboro to the Board of Aldermen. A Slow Zone is an area in which traffic calming measures are deployed and speed limits are lowered to 20 mph to reduce the number of bike and pedestrian crashes.
Does this really matter for pedestrian safety? I decided to look into the data for Carrboro, and here’s what I found.
The Highway Safety Research Center in Chapel Hill has a queryable database for all sorts of crash data, including pedestrian crashes. I ran a query for Crash Severity and Estimated Driver Speed where that data was reported for Carrboro from 1997 to 2012, the full dataset.
The database counted the following number of pedestrian crashes by severity:
- Killed: 2 deaths
- Disabling Injury: 20 injuries
- Evident Injury: 33 injuries
- Possible Injury: 37 injuries
- No Injury: 3 non-injuries
Considering that when nobody is injured, a report is much less likely to be filed in the first place, I threw the “No Injury” category out due to the small number of responses. While the number of pedestrians killed was also small, the gravity of the consequence led me to leave this category in the next step of analysis.
I then took the midpoint of the ranges of each speed reported for the various types of crashes. This means that for crashes coded “Zero to 5 mph,” I put the speed down as 2.5 mph. For “16 to 20 mph,” I put down 18 mph. You get the idea.
Then, using the number of crashes of each type, I created a weighted average speed in Excel for every type of injury that pedestrians experienced from cars reported in Carrboro from 1997 to 2012. The bottom line:
- For those Killed, the average speed of the car involved was 38.0 mph
- For those experiencing Disabling Injuries, the average speed of the car involved was 28.2 mph
- For those experiencing Evident Injuries, the average speed of the car involved was 17.1 mph
- For those experiencing Possible Injuries, the average speed of the car involved was 11.2 mph
There’s no mystery here- the slower the speed of the car involved in the crash, the less harm to people on Carrboro streets.
I’ve been following this discussion online as it pertains to New York City, where incoming Mayor Bill DeBlasio has realized that more people die due to traffic than crime in NYC, and the goal of #VisionZero – no traffic deaths in New York City, has become one of their key policy priorities for quality of life in New York. The graphic accompanying this post is based on NYC data, which shows that New Yorkers are NINE TIMES more likely to die when struck at 30 mph than at 20 mph.
Clearly there’s a link between vehicle speed and pedestrian crash severity in Carrboro as well.
I encourage everyone to check out the TAB’s discussion of the Slow Zone(PDF) before the Aldermen, and if interested, to view the data behind this post at the link below.