Open Thread: How Would You Use City Traffic Crash Data?

While recent improvements have made streets safer, from '95 to '09 more pedestrians were struck in Midtown than in any other district. Image: TA

On Monday Transportation Alternatives released a report tallying pedestrian-involved crashes in each of the city’s community board districts, based on numbers from the state Department of Motor Vehicles, between 1995 and 2009. Not surprisingly, the data reveal that the most collisions occurred in Midtown Manhattan, where high-density auto and foot traffic led to 8,604 crashes in District 5 alone.

The TA study, along with the relaunch of CrashStat and the (however convoluted) release of crash data from NYPD, have raised the profile of the city’s traffic violence epidemic. This is undoubtedly a positive development, and one that will hopefully continue to generate headlines as stats become more accessible. But as noted by Streetsblog readers, raw data accumulated over such an extended period of time can be misleading, and could potentially be used to undercut future efforts to improve safety.

So we ask you: How would you put to use the influx of city- and state-generated crash data? What would your criteria be for employing data to guide tangible street safety measures?

Share your ideas, from the pragmatic to the fantastical, in the comments.

  • At the very least, I’d control for the following:

    1. Population, both residential and daytime.
    2. On-street car traffic counts (i.e. excluding expressways).
    3. Some measure of foot traffic: population alone works as a first-order term, but some additional numbers are required to make core and suburban numbers comparable.

    The importance of using all three controls is that to figure out how safe an area is for pedestrians, we need to look at both the numerator (crashes) and the denominator (ped activity times car traffic). There are fewer car-ped crashes per capita today than there were in 1920, not because streets are any safer, but because fewer pedestrians go on the streets. More recently, we have the Prospect Park NIMBYs who think that there are fewer bike crashes on PPW than on 9th Avenue for any reason other than that there are fewer cyclists on PPW.

  • moe

    There is certainly value in calculating relative risk exposure, but do not discount the need for intersections with the absolute number of crashes to get more aggressive safety improvements. Contrary to what sb commenter ‘ shemp ‘ said previously, looking at multi year averages is not ‘moronic’. In most cases, streets that were dangerous in 1995 are still most dangerous in 2011 despite the dot’s best efforts. When faced with this data, dot says ‘streets are safer than ever’. Tell that to a mother who lost a child because dot is still trading lives for vehicular throughput.

  • Joe R.

    I would try to find correlations between the number of accidents versus the type of traffic controls at the intersection, and also lane width, number of lanes, etc.  I suspect intersections with traffic signals are the most dangerous by far, in both quantity and severity of accidents.  The data could lead DOT to start doing things which actually reduce accidents, instead of pandering to ill-informed community boards pushing for traffic signals or stop signs which only make things worse.

  • Prof Curly

    Moe, it is standard practice to use a “multi-year average” of three years of crash data to assess a location — not 15 years as TA did with their report. Using statistics averaged over such a long time frame obscures both positive and negative trends. In this case, the trend on city streets is positive. Pedestrian crashes have declined, in some places a lot. By averaging over 15 years, TA produced a crash number that in most places will be higher than the most recent few years. This maybe good for scare headlines, but it discounts the pedestrian safety improvements that TA itself has been working for.

  • Boris

    Alon, how do you know there are fewer pedestrians today than in the 1920’s?

    And why multiply peds and cars in the denominator?

  • Boris, photos of cities from the era show children playing on streets, statistical databases show low car ownership (hence, much larger pedestrian volumes), department stores offered free deliveries on the theory that customers didn’t have cars (and were located in walkable downtowns), and so on.

    The reason to multiply cars and pedestrians is that all else being equal, we should expect the probability that a car will hit a pedestrian to be proportional to the amount of car traffic times the amount of pedestrian traffic. Of course, this suggests an obvious way to increase pedestrian safety – reduce the number of cars – but the point here is to figure out whether road engineering solutions like left turn lanes really increase safety or just make traffic planners feel better about themselves.

  • Anonymous

    If I were a crack statistician, I would combine NYPD’s data re enforcement with Crashstats numbers to see whether the most lawless precincts are also the most dangerous.


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