What’s the Right Metric for Transportation Safety?

This week, Walker wrote an excellent post comparing crash statistics and the structural responses to incidents by mode in the wake of the Amtrak derailment.

It’s a great read with great questions, and we are reminded once again how safe we’ve made rail and air travel relative to other modes. But one thing sticks out to me – how we frame safety through statistics. Like any data, statistics can be misleading in how they’re presented. Sometimes it’s intentional (like denigrating light rail transit because it’s unsafe for pedestrians). Other times, misleading data might be more benign, simply out of convenience of available datasets. So, how should we talk about transportation safety in the fairest, most honest way possible?

Incidents Per Passenger Mile or Per Passenger Trip?

Ultimately, what really matters is the big picture: how many people die or are injured in transportation-related incidents per inhabitant. It tells us how our land-use/transportation combination is doing at keeping us safe. In that regard, we know we’re doing very poorly (even if fatality rates vary greatly by state, road classification, etc):

It’s more common for DOTs to report safety as per mile (or km) driven. This is fine when comparing between countries, and it gives an idea of how good or bad our drivers and roads are by normalizing total distance traveled.

However, when comparing different modes to one another, it’s common to see another layer added: per passenger mile. The logic is that some modes carry more passengers per vehicles than others, and so it’s a fair comparison to normalize for the bus carrying 40 people that hits a bicyclist vs a car carrying 1 driver who does the same. Those 40 people are each going somewhere, traveling some miles, so the risk rate (to both the occupant and non-occupants, ie pedestrians and cyclists) should be equally compared.

But a per-passenger mile metric fails the sniff test despite its broad-usage. Look at the general formula and see if you can tell why:

WrongFormula

Cheat sheet answer: increasing the denominator automatically makes the safety rating better without reducing the number of fatalities. Imagine you’re only comparing non-occupant safety (pedestrians, cyclists, etc) between modes as the linked O’Toole post above does.

Three Ways of Becoming Data

I’ll give a visual example of why passenger-miles of the vehicle striking the non-occupant favor motor vehicles over transit. Let’s say I’m a pedestrian walking along Lake Street in the LynLake area of Minneapolis. A person making a 1 mile trip in their car hits me halfway through its journey and I die:

1_Mile_Trip_Crash

Now, let’s rewind this (very sad) story, and a different person making a 2 mile trip by car hits me 1.5 miles into their journey:

2_Mile_Trip_Crash

Once again, let’s think bigger, and a person drives into LynLake from Richfield, hitting me 5.5 miles into their 6 mile desired journey:

6_Mile_Trip_Crash

Here’s the thing. As a non-occupant, it doesn’t matter how far that car traveled or intended to travel. All that matters is that I’m dead. Another statistic. All that matters is that driver walked out their front door with the intention of making the trip from A to B. They made a modal choice. They could have biked, or walked, taken transit, or driven. Yes, some modes would have taken more time, but we’re not talking about accessibility vs mobility right now, we’re talking about safety.

Big Picture Data

Now, multiply this by the millions of trips taken every day. Some are short, some are long, some are on buses, some on light rail, many in cars. Those modes have different numbers of passengers in each vehicle. But what matters is that each vehicle has a number of people making individual trips.

We should evaluate fatality or injury rates based on passenger-trips instead of passenger miles. Otherwise we could just convince everyone to drive 100 miles (compared to the average 9.7 mile car trip length in 2009) for every trip, hold pedestrian fatalities steady, and look like we’re all of a sudden ten times safer than we currently are. It’s misleading in favor of automobiles. Put another way, it doesn’t matter if my grocery store is 1 block by foot or 4 miles by car, but only how many times out of 100 I die or am injured making that trip.

Using numbers taken from a mixed-bag of sources*, here is what that looks like:

Fatality Rates by Mode Trip vs Mile

Thoughts:

  • Occupant death rates for transit of all modes is clearly much lower than driving, whether taken as per-mile or per-trip.
  • Car drivers kill far fewer non-occupants per mile driven or trip taken. No doubt about it.
  • LRT is especially dangerous for pedestrians, no matter how you slice it.
  • The total safety gaps between cars and transit widen (or, narrow for LRT) when passenger-trips are used as the metric.
  • Random stat: 2/3 of heavy rail and 1/3 of LRT non-occupant deaths are suicides – a lot of people intentionally walk or jump in front of trains (there’s no hard data to support it in those reports, but my gut feel is suicides make up a very small share of non-occupant car crashes).
  • Intercity rail (ex. Amtrak) and air travel are excluded here, but fatality rates so low they don’t warrant a comparison.

A similar trend is observed when comparing injuries by mode:

Injury Rates by Mode Trip vs Mile

Cars look roughly the same as every other mode per passenger mile, but are clearly more dangerous than transit. The ratio of occupant to non-occupant injuries for transit is much closer to passenger cars than deaths. The most likely factor is that trains and buses are heavy and difficult to stop quickly and therefore kill at higher rates than a car-on-pedestrian or bike crash.

Takeaway

Our land-use and transportation priorities advocate for mobility over accessibility, and as a society we seem to be willing to let drivers and vehicle occupants take their lives into their own hands to achieve longer distances traveled. This is clearly a losing strategy from a safety perspective – vehicle occupant death and injury rates per trip are higher in cars than any transit mode, by a fairly wide margin too.

However, pedestrians are more likely to die per passenger trip or mile by a light rail vehicle than the vast number of cars on the road. Urbanists need to be aware of this – we shouldn’t accept the high number of pedestrian fatalities from car crashes, nor should we for transit. Transit deserves more grade separation in heavily-trafficked pedestrian areas and better station design to help reduce total fatalities.

But from a metrics standpoint, we should start moving toward a per-trip method rather than per-passenger mile. This informs our land-use decisions from a safety standpoint much better than per-passenger mile.

*Sources:

2011 Transit Passenger Miles, Fatalities, & Injuries by Mode taken from NTD data
2011 Occupant Fatalities/Injuries, Non-Occupant Fatalities/Injuries by Vehicle Involved, Miles Traveled by Vehicle Type
Vehicle Occupancy & Trips Taken for Passenger-Trips and Passenger Mile Calculations

This post is adapted from my personal blog, the Fremont Avenue Experience


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7 Responses to What’s the Right Metric for Transportation Safety?

  1. Wayne May 21, 2015 at 10:36 am #

    I’m a bit confused here, it seems like you’re taking the denominator of the number of trips by mode and comparing to the number of non-occupants killed in the numerator, right? But what about if you look at pedestrian or bike deaths via how likely you are to be killed on any trip by some other mode–so for every 100 walking trips I take how many times will I be killed by a car or a bus or a train? I’m assuming I’m far more likely to be killed, as a pedestrian, by a car than by a bus. That doesn’t really come through here, it’s about the mode doing the killing, not the mode being killed.

    • Alex Cecchini
      Alex Cecchini May 21, 2015 at 10:57 am #

      Well, yes. If we want to say that driving a car is dangerous for pedestrians relative to moving the same number of people in a train or bus, we need to say that for X number of people taking a trip in a car, Y number of non-occupants will die.

      Yes, a huge number of pedestrians are killed each year by cars. No argument. But there’s a HUGE number of passenger trips taken by car. There have been, what, 2? pedestrian fatalities due to collisions with the Green Line (thinking the one a few weeks ago and the one back in September). Many people ride the Green Line (35-38k a day). But how many car-on-pedestrian deaths have there been in the last year in Minneapolis and St Paul? There were 35 across the entire state in 2013.

      So yes, today, when you walk out the door you’re more likely to be killed by a car. But if you got 100% of those people out of cars and into 20 theoretical light rail lines to handle huge shift in mode change, we would expect an increase in pedestrian fatalities. Once you’re in the LRT, you’re way safer per trip you take than in a car. And certainly, retrofitting LRT station/platform areas is way cheaper than the entire road system we’ve built.

      • Jeff May 21, 2015 at 11:59 am #

        Apart from the suicide variable, I wonder, also, whether the fact that LRT is an emerging mode in the USA (or at least my sense is that it is) plays a factor in the higher non-occupant mortality rate. I wonder how the domestic rates to rates in countries with mature LRT/train infrastructure. Seems to me that if you could control for those two variables, LRT might look safer for non-occupants than cars.

  2. Alex Cecchini
    Alex Cecchini May 21, 2015 at 10:59 am #

    I should also note that Bill wrote a piece I missed (until yesterday) on this topic a few years back. Go read it, he’s better at writing than me 🙂 http://streets.mn/2012/11/20/saying-goodbye-to-the-passenger-mile/

  3. Monte Castleman
    Monte Castleman May 21, 2015 at 11:05 am #

    Interesting how there’s been complaints that there’s been “too much” traffic control for pedestrians on the University of Minnesota transit mall. Yet traffic control other places hasn’t stopped people from walking in front of trains. Maybe since some people think pedestrians are never at fault for anything we should slow trains down to 20 mph, or however slow it takes to be able to stop if one wanders onto the tracks. After all it’s all the engineer’s fault for making trains dangerous to pedestrians.

    • Alex Cecchini
      Alex Cecchini May 21, 2015 at 11:51 am #

      I tried to make this a pretty pragmatic take on safety, so I hope comments don’t get too testy. But there are obviously some things that maybe didn’t come out clear enough/didn’t make the cut:

      1) “2/3 of heavy rail and 1/3 of LRT non-occupant deaths are suicides.” For LRT, it’s actually 40% of non-occupant deaths are suicides. While there are *some* pedestrian suicides on roadways, the ratio isn’t approaching anywhere near this, so we can effectively take that rate down a peg.

      2) I didn’t include “light trucks” in the “passenger car” calculation. Light trucks include SUVs, light-duty pickup trucks, etc. Annual VMT for light trucks were 77% of the miles by passenger cars in 2011, so a sizable chunk of total VMT, and are more dangerous. Including them in the “cars” numbers brings up the risk to non-occupants by 16% (though still less than bus/LRT).

      3) LRT is pretty new in this country. People aren’t accustomed to it, engineers maybe are not as good at designing places (and stations) that make co-mingling of pedestrians and trains as safe as other parts of the world.

      4) That said, LRT is also the unfortunate compromise thanks to high US transit construction costs. I’d say most people would love grade-separate tracks for trains the same way we did with freeways through urban/dense parts of our cities 60 years ago.

      5) This risk rate ignores vehicle pollutant deaths, which would certainly put cars well above electrified rail.

      6) Cars are still extremely dangerous to people inside them. Taken as per passenger-trip, you’re 13x more likely to die than being in a bus, 12x vs heavy rail, and 3.5x vs LRT.

      7) I also didn’t include large, freight trucks, which have non-occupant fatality rates higher than bus or heavy rail and close to pre-suicide-removal LRT rates. A hidden consequence of big, wide streets for us to turn into Taco Bell drive-thrus is that they’re also possible for 40′ semi haulers to drive down (often delivering goods to those drive-thru chains). The small restaurant down the street from me gets goods delivered by a small 2-axle delivery truck.

    • Wayne May 22, 2015 at 7:58 am #

      Trains don’t often run red lights or make turns without checking for pedestrians.