Sunday, 13 December 2020

Why Is Twenty Plenty?

The widespread adoption of 20mph (30kph) across many parts of Europe is not a coincidence or a fad, it is based on science and is a key part of delivering sustainable safety or vision zero.

Back in 2013, I wrote about the "how" of 20mph limits and in this post, I want to have a little look at the why. Before I go on, it is worth reiterating a point I am often making and that is speed limits are a product of mass motoring. Where motor traffic isn't allowed, then speed limits are a meaningless concept. It's also worth mentioning that 20 is a round number and in many cases, a lower limit would have even more positive outcomes. The 30kph limit elsewhere is of course lower at 18.6mph and even that slight difference is a little safer.

Under the sustainable safety approach, modes with significantly different speeds will be separated. It's taken as read that we wouldn't expect people walking to share the road with people driving at 30mph or more and so it's no coincidence that the Dutch designers will extend this to separating people cycling to. Sustainable safety will also keep people walking and cycling separate as well, unless there is a good reason for integration such as a public square at a destination point where people cycling will be dropping their pace.

A Dutch street with a 30mph speed limit. Cycling
is in protected space. The side street is 20mph and
cycling is integrated once more.

20mph doesn't necessarily mean that a road or street will be "safe" because with significant volumes and high levels of HGV traffic there is still a high level of risk exposure to those outside of the motor vehicles. However, 20mph is all about risk reduction and it comes from the intersection of physics and human biology - more specifically the ability of the body to withstand an impact (and I don't apologise for the blunt language).

Perhaps one the best articles I have read comes from "Unsafe at Many Speeds" by ProPublica. It's based on a US report from 2011 which uses US crash data from 1994-1998, but the laws of physics and human biology don't worry about that and there are some stark findings. The immediate thing we see is that the relationship between the risk of death and the impact speed is non-linear;

I've reproduced the graphics from the article above. This is risk of death vs impact speed from a car or a light truck. The top graph is the average risk for all ages within a 95% confidence range and the bottom graph has the risk for people aged 70 (dotted line) and people aged 30 (solid line).

What this all shows is that once we get over about 15mph, the risk multiplies for every 1 mph of speed increase and the effect is more profound on older people who's bodies (in general) cannot sustain impacts as well as younger people, even at low speeds. The data doesn't deal with impacts from heavy good vehicles because people are simply going to be crushed by them, even at low speeds. This is why people should be separated where there are high percentages of HGVs even if overall traffic flow is lower.

It's worth reading the article and looking at the interactive graph which helps you understand this non-linear relationship. Once we get to high speed crashes, then few people stand a chance of survival. We often hear from pro-speed advocates that pushing low speeds is flawed because of the relatively low numbers of crashes where a driver hits a pedestrian. This is about debating absolute numbers rather than talking about risk and it doesn't take into account situations where people are kept off the streets from a fear of being hit because of speeding drivers.

Sustainable safety also helps those inside motor vehicles in two ways. For drivers, the act of moving more slowly allows them to processes the cognitive load coming at them from an urban environment - reading signs, people crossing in front of them, people pulling out of side roads etc. In other words, there is more time to take in and react to what is going on around them, even if it is taken evasive action from someone else disobeying the rules - 20mph is a speed which forgives mistakes. The second way is that the closing speed between vehicles is reduced in a way which helps protect the occupants of the vehicle from death and injury.

The other useful benefit of 20mph in urban areas is that it allows designers to be more radical and space-efficient with their designs in terms of narrower lane widths, tighter turns, reduced forward visibility and so on. In designing in this way, we can reinforce lower speeds rather than enforcement them because the environment guides behaviour rather than just a sign at the side of the road. That doesn't mean we have change things at the same time as bringing in a 20mph limit, just that it deals with the chicken and egg by making a decision to start change.

Despite the positive outcomes of having this as a standard speed limit, there is still substantial push back from citizens and politicians which seems to hinge on a couple of misnomers. First, they will say that going to 20mph will make journeys longer in time, usually giving some local example. This only really works when we talk about free-flow roads with grade-separated junctions - places where we would never suggest 20mph. 

In urban places we are more interested in looking at journey time reliability (the same journey being predictable in time) then speed because of the complexity of how the networks operate. Unless on an urban trunk road with grade separation, you're going to meet with "friction" from side streets, traffic signals and all of the things which impact journey times. Even if you raised the urban speed limit to 50mph everywhere, you're not going to get away from the friction and the risk of collisions significantly increase.

Of course, arguments about having higher speed limits at night are often advanced on the basis that there are fewer people around, but this would still increase the risk exposure of those who are actually around (regardless of mode) and potentially an increase in noise (especially tyres). It also ignores the fact that urban places aren't generally designed for higher speeds and those driving will not be competent to handle the cognitive loading from moving faster (and in the dark) compared with say a motorway situation - many people overestimate their driving skills.

Then we have the whole argument around people who claim the gearing on their cars make it "hard" to drive at 20mph and in fact having to drop a gear (maybe) means they are more polluting. There's lots of debate around this (maybe slightly partisan in some cases), but it is pretty much impossible to separate out of the variables an the elephant in the room is missed that motor traffic is polluting (the debate isn't on a level playing field). 

If 20mph is being used at the same time as reducing motor traffic then it's not even a point of discussion and the ability to drive within any speed limit and the prevailing conditions is about roadcraft not about the mechanics of vehicles per se. If one is travelling more slowly, then there will be less gear changing and braking requires less dumping of energy (and therefore brake pollutants). Smooth driving is the important point, not the speed limit.

The discussion needs to be turned on its head. We shouldn't be having to justify 20mph in urban areas (including villages). It should be the natural choice with the idea of having a higher limit being the point which needs to be defended by those advocating for it.

Now you can disagree with me here and you can have your own opinions. However physics and human biology don't care, so maybe you need to think a little more about why you're happy with higher limits and whether or not your apparent convenience should be placed more highly than the safety of other people.


  1. You mention the cognitive overload as speeds increase and there is a known tipping point for the brain to close down the focus on from peripheral vision to a more directional and narrower angle

    As speed increase the brain needs the assistance of clear road markings, and ultimately the risk management of a guided system, with higher levels of regulation (signalling, exclusive use of the moving space) - a railway - as speeds increase further

    There have been trials where road markings have been removed and without indicators of priority, and positioning drivers speeds drop, to around 18mph perhaps the evolved top speed for unassisted human movement - as the maximum running speeds for hom sap peak at around 20mph (a 4 minute mile = 15mph)

    With our ancestors running around at up to 20mph, little 'accidents' like running into trees, or falling over are the equivalent of crashes we might have in the modern world, with a body that has adapted to survive banging into things at .. up to 20mph. Those who were badly damaged in such crashes didn't breed, and so the body features we have today are of a robust package which can take the knocks and react to things happening as long as the speeds are under 20mph - some parts are even sacrificial 'fuses' that break or bruise rather then have damage to more critical parts - notably here the collar bone. It can also inform protective action in a crash - a foetal 'tuck' with the head against the chest, legs protecting one side, spine & rib cage the other, with arms crossed over the nape of the neck protects the brain with the rest of the body, or the feet hitting first, and the legs bending from straight to squat provides a natural shock absorber

    There might be some intersting analysis from minor cycle crashes to see whether this who have done judo, or a contact sport where people collide or fall over (or both) suffer fewer broken bones or other injuries, and from this

  2. Seems like a no brainer tbh, would be good to see some data from the Scottish Boarders on changes in speed and behavior following the blanket roll out of 20mph limits. Imo, 20mph limits in urban areas are a lot like parking restrictions, they can be painted and signed as such, but are they observed if not enforced or made more difficult through the introduction of infrastructure? (speed bumps, bollards etc.) 20mph speed limits are a police enforcement issue, which leads to a difficult decision for society, do we divert already stretched resources to enforce this rule, is there a bureaucratic knock on effect for police officers? Does a cost benefit analysis of this approach show it is efficient? Yes you cannot put a price on additional safety benefits, but will these benefits actually be realised in real world conditions? Are traffic calming measures required in order to promote this change, which I think is the case presently when introducing 20mph zones?

    Despite these issues, the general public can actually help with this problem, for instance on a one lane road all it takes is 1 driver to reduce their speed and all those behind are forced to follow by default. This is the case despite any formal introduction/change to existing speed limits. Painting 20 roundels on the ground and installing signage is not a magic wand that will save more lives, massive behavior change is required and anyone that drives in urban areas can help to contribute to this change right now!

  3. As I understand it, from government stats, 85% of drivers, if allowed, will drive faster than the 20mph limit. This goes up to 90% at the weekend...

  4. Currently living in Munich where streets that are largely for housing and small shops are 30 kph. Even in the out of the ring development the roads are narrow. There is room for two way traffic but parking is allowed, in many cases on either side, but not directly across from each other. You can't expect to do 30kph because you have to negotiate with oncoming drivers based on who is first and whose side the cars are parked. Between that and intersection right of way laws you rarely get speeding, or even sustained 30 speeds. Plus there doesn't seem to have a need for actual traffic enforcement.