What Metric Should We Use For Vehicle Safety?

Okay, specifically when comparing transportation modes. It gets way too complicated when you try to include everything, as the end of my last post shows.

Today, I am specifically inspired by Randal O’Toole’s latest post denigrating light rail transit because it’s unsafe for pedestrians. I don’t take issue with the discussion because I’m a rail fan (truthfully, I’m not, I almost always push bus before rail)., but because of the way Mr. O’Toole presents the safety numbers.

Incidents Per Passenger Mile or Per Passenger Trip?

Oftentimes you will see safety numbers presented 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.

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:

RoadFatalityRatePopulation2012
Source

But that view (even at a state-level) is far too big picture when talking different modes, as Mr. O’Toole is so eager to do.

Ultimately, 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. Mr. O’Toole’s whole post was focusing on non-occupant safety, so let me give a visual example. Let’s say I’m a pedestrian walking along Lake Street in the LynLake area. 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 drove 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 a lot longer, but we’re not talking about accessibility vs mobility right now, we’re talking about safety.

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, some 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. To understand how compact vs distant land uses and their associated safety impacts with people taking longer and longer trips by auto, we should compare incident rates per passenger mile to the rate per passenger trip.

Evaluating those modes’ deadliness relative to how many people they’re moving from A to B (regardless of how far A & B are apart) is the important statistic. Otherwise we could just convince everyone to drive 100 miles for every trip (bump that denominator up!), not lower the pedestrian fatality total, and look like we’re all of a sudden 10x safer than we currently are (2009 average car trip length is 9.7 miles). It’s misleading, almost intentionally in favor of automobiles.

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

Fatality_Rates_Mode_TripvMile

Takeaways:

  • Occupant death rates for transit of all modes is clearly much lower than driving, whether taken as per-mile or per-trip.
  • Drivers of cars 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/jump in front of trains.

A similar trend is observed when comparing injuries by mode:

Injury_Rates_Mode_TripvMile

Cars look roughly the same as every other mode per passenger mile, but are clearly more dangerous than transit. The ratio of occupant:non-occupant injuries for transit is much closer to passenger cars than deaths (meaning, trains and buses kill due to their weight/inertia when a car-ped/bike crash may have meant the person walked away).

Takeaway

Mr. O’Toole advocates for mobility over accessibility, and is willing to let drivers & their vehicle occupants take their lives into their own hands to achieve longer distances traveled. This is 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. Again, it doesn’t matter if my grocery store is 1 or 4 miles away, but only if I die or are injured making that trip.

From a non-occupant standpoint, his argument has merit no matter how you look at it for light rail transit. 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. More grade separation in heavily-trafficked pedestrian areas, better station design, etc are all important in reducing 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

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