Vision Zero calls for 30km/h limits for a solid reason: physics.

Let’s talk speed limits

Earlier this year there was some noise made in city council about reducing residential speed limits to as low as 30km/h. In general I would say that the idea was met with scepticism from councillors and the public alike. Since Councillor Carra brought the issue up again on Twitter while at RailVolution, I was inspired tackle the discussion that I had avoided back in April.

The main driver for discussing this change is pedestrian safety. For that reason, the discussion that follows is based on the premise that we should protect the most vulnerable (and important) modes of transportation but understand that there is a risk to travelling and moving around a city that will never be zero. Our goal for this post is to see if reducing residential speed limits from 50km/h to 30km/h will significantly lower the amount of pedestrians seriously injured or killed by vehicles.

.@wklumpen I’d suggest it’s internal streets: anything that doesn’t have painted lines. Higher order streets need diff engineering solutions — Gian-Carlo Carra (@gccarra) October 11, 2016

Would this proposal really make a difference? Are there things we can do in addition that would make it even better?

I’m going to argue that it certainly would make a difference, though to truly capture the spirit of what we’re trying to achieve we are going to need to do more than just change the default speed limit.

And no, it doesn’t include more enforcement.

The Theory

The standard case for lowering the speed limit to 30km/h has a relatively simple physics explanation. You can read a detailed description of the theory on Raise the Hammer, but the basic principle is this: The more kinetic energy a vehicle has when it collides with a pedestrian, the more likely the pedestrian is to be killed or seriously injured. From physics we know that kinetic energy grows by the square of the speed of the object, meaning that if you double an object’s speed, you quadruple its kinetic energy, quadrupling the chance of death in a collision.

On top of that, a slower moving vehicle is able to stop faster (more physics), and travel less distance before the driver is able to react (again, physics), further reducing the chance of an accident. When these two factors combine, it leads to some drastic results, detailed in the aforementioned Herald article:

The chance of severe injury is 50 per cent and the chance of death is 21 per cent when a pedestrian is struck by a vehicle going 50 km/h. But the chances of severe injury and death go down to 30 per cent and 12 per cent respectively, when struck at 40 km/h, and dip even more significantly, to 18 per cent and six per cent respectively, when struck at 30 km/h.

So why stop at 30km/h? Why not reduce to 20? It turns out that 30km/h is about the point where we hit “diminishing returns”, as evidenced by this super awesome graph from the World Resources Institute

A car that is moving slower is less likely to hit a pedestrian. A car that is moving slower when it hits a pedestrian is less likely to kill or seriously injure it.

The Practice

To sum up:

Notice that we have not yet concluded that simply lowering residential speed limits from 50km/h to 30km/h will achieve the goal of drastically lowering pedestrian fatalities. What remains are two questions that are significantly more difficult to answer:

Does lowering a speed limit actually cause people to slow down? How many of these collisions would actually occur on streets where this policy would be in effect?

To answer the first question, many people postulate that enforcement is the only way to get people to obey these limits, and that it won’t help. Of course, there will be a fraction of drivers that will not obey the speed limit (I think ultimately what slows people down are streets that are designed to be slower and safer. Calgary suffers from over-engineering, and so to be truly effective a speed limit reduction would have to be accompanied by a design change. People already drive slower in areas where streets are narrow and parked cars abound (the Beltline, for example), and faster in neighbourhoods with extremely wide streets with gradual curves and very few intersections. In order to be truly effective, the speed limit reduction must come with a re-design of what a residential street really is.

The second question is much more city specific. If the majority of pedestrian collisions are happening in places where speed limits would not be changed (major arterial roads, perhaps), then the policy might not be as effective as intended.

So let’s take a look at 10 years of pedestrian collision data in Calgary, mapped by intersection. The darker the red, the more severe/frequent the accidents are:

First off: the data has a few flaws, some intersections near malls and other areas are over-counted (notice the dot right in the middle of Nose Hill, for example). So instead of looking at individual intersections, let’s take a page from New York City and look at corridors, since a speed limit reduction would in theory be effective along longer stretches of road where cruising speeds are high.

Immediately, I notice a few “hot areas”: McLeod Trail South, Centre St. North, and 17 Avenue South pretty much directly across the city. All of these roads are relatively major, and certainly not residential. The policy we’re looking at would not improve these areas, which account for a pretty big chunk of the problem.

Downtown is also a hotbed of dark red. I would stipulate that this is due to a few factors: the high amount of pedestrian and vehicle traffic in general, one way streets, and the over-engineering of major core roads such as 4th, 5th, 6th, 9th, 11th, and 12th Avenues. If I had to pick one area to pilot this policy, it would be in the city core. One way streets are already falling out of popularity, and during peak congestion periods traffic doesn’t move much faster than 30km/h anyway so an argument for “traffic chaos” is pretty much out the window.

Lastly, let’s have a look at what I like to call the “curvy” neighbourhoods, or neighbourhoods that have very similar street names. In every quadrant of the city we can see a collection of collisions in residential areas, but the NE quadrant stands out to me as a little more dense than the rest. This may be due to there being more pedestrians in general in the northeast.

Conclusions

We spent a fair bit of time looking at different aspects of a speed limit reduction. For me, the argument for reducing traffic speed in cities is strong, and reducing speed limits is obviously a practical first step in accomplishing this. With that said, I would suggest considering any policy change that leads to speed limit reductions with the following ideas:

Reducing speed limits alone will slow down a certain fraction of drivers. Engineering and re-designing roads to match the desired speed limit will slow down nearly everyone. All residential road design and refurbishment from this point on should be wary of over-engineering.

Reducing speed limits in residential areas will protect a significant amount of pedestrians, especially children. To truly be effective, though, any Vision Zero policy that calls for speed reduction should include a targeted analysis of the most dangerous corridors. Speed reductions along these corridors are likely the most effective of many potential solutions.

Speed reductions along these corridors are likely the most effective of many potential solutions. The Downtown, Beltline, Victoria Park, and East Village core area are prime candidates for a targeted pilot project.