[takk309]

Nice work, I am glad that there is a paragraph talking about exposure. Crash trends based strictly on total number of crashes are easy to predict just based on where there is more traffic. Using crashes per vehicle mile traveled for road segments or crashes per entering vehicle for intersections can help tease out trends. Controlling for severity is also important.

When I do a crash analysis for a city, one of the tasks I do regularly for my job, I generate a crash rate and severity index for each intersection. The severity index is basically a weighted average based on severity, non-injury=1, minor injury=3, and severe injury or fatality=8. The crash rate and severity index are divided to create a Severity Rate. While not perfect or statistically valid, it does help identify trends. Also, I am in a rural state so it is rare that there are enough crashes to make any statistically valid conclusions.

[mikeash]

What’s the basis for the severity weights? I’d expect the weights to be way more spread out, like 1/10/100/1000. It would definitely not be a good trade off to eliminate nine non-injury crashes at the cost of one additional fatality. But I certainly could be missing something about this sort of evaluation.

[dsfyu404ed]

Fatalities are a tiny minority of crashes and aren't really interesting to study because usually you basically wind up studying the behavior of drunk people and people who don't wear seat-belts and if you filter those out there's not much data left making meaningful conclusions hard to draw. Fatal accidents are often just normal accidents with a couple aggravating variables on top (e.g. person rear ends semi-truck instead of normal truck or person gets in minor accident but not wearing seat-belt) so it doesn't make sense to fixate one them. Anything that reduces normal crashes by some amount will also affect fatal crashes.

[cimmanom]

They’re a relatively high percentage of vehicle-pedestrian and vehicle-bicycle collisions, though. Very important for a pedestrian-oriented city like NYC.

[takk309]

Drunk drivers and people that don't wear seat belts are still worth reviewing. While there are rarely engineering solutions to the fatalities that result, it can help inform education programs and initiatives. Amazingly, buckle-up and don't drive drunk advertising can make a difference.

[apendleton]

They absolutely are, but are rare enough that it's difficult to reach statistical significance when talking in the aggregate. That a particular part of town went from one fatality one year to zero fatalities the next year is probably not evidence of the success of any particular safety-related policy intervention, it's just noise. Studying all crashes provides a proxy that hopefully helps decrease the odds that the fatal ones will occur will making it possible to make robust, data-driven claims about success or failure.

[takk309]

On a project I am currently working on, we saw pedestrian fatalities shift from 7 to 13 in consecutive years. it is a nearly 100% increase but like you said, it is just noise. This is in a city with around 100,000 residence. Convincing politicians that it is just noise is a whole different story.

[jamiek88]

I came of age at the end of the campaign making drink driving socially unacceptable in the UK to the point of being a social lepper if you drive drunk, even two or three drinks not falling about drunk - so it was normal to me that you don’t combine a night out with drink and your car.

I then moved to Texas where it was still just a ‘naughty boy’ type social offense until a major campaign that is still underway changed minds and hearts about it.

I’d like to see he stats in 10 years because it has been anecdotally very successful.

[anitil]

It's these sorts of comments that make HN.

[takk309]

Generally it is based on the relative costs of a non-injury vs other injury types. So the weight is equal to cost of fatality divided by cost of non-injury. Keep in mind that these values vary from region to region. We use the 1, 3, 8 grouping because the State uses it and by being consistent we can compare different areas more easily.