Lowering Speed Limits Speeds Up Journeys

I have found myself, far too often, arguing with people about the speed limit in the UK. My position--the correct one--is that the speed limit should be halved, from 30 miles-per-hour (mph) to 15mph. My assertion is that this will speed up journeys. The consensus--incorrectly--is that the opposite would result. While I generally loath the Freakonomics/behavioural economics 'one weird trick'-ism that characterised the field around 2010, in this instance, lowering the speed limit is actually one weird trick to speed up journeys, while also--I suspect--reducing road casualties.

My argument has three pillars. But before getting to them, let us address what I will call the 'intuitive' case; the case that prompts so many who hear my conjunction to laugh in my face. Imagine driving down a straight, 1-mile road. At 30mph, this journey will take around 2 minutes. If we halve the speed limit, we intuitively double the time. Ergo, I am an idiot. At least, this is the most common confusion when this matter is raised in the pub.

Yet, this 1-mile stretch of 30mph road is an idealised case, and does not represent typical--by which I mean average--driving conditions (throughout this post I will make various assertions, and most often will be asserting things on average, though might not always specify as such). For instance, in a 1-mile stretch of road, there may be around four sets of traffic lights. It seems quite reasonable to me that a typical driver must stop at, say, two of these four. And, for the stake of convenient maths, let us say a stop at a traffic light lasts around 30 seconds. Now, in our idealised example, we are still driving for 2 minutes (at 30mph), but we are stopped for two lots of 30 seconds, or 1 minute. Our 1-mile journey now takes 3 minutes, resulting in an average speed of only 20mph. From just two sets of traffic lights! Of course, driving speed is not a step-function, but a continuous one. We must speed up and slow down for traffic lights. As such, while we may reach a maximum speed of 30mph, we do not constantly achieve this maximum. Often, we will be driving less than 30mph as we accelerate from, and decelerate towards, our two sets of traffic lights. Again, for mathematical ease, let us assume that this pattern of speeding up and slowing down adds another minute to our 1-mile journey. Well, just by accounting for traffic lights, the average speed has been reduced to around 15mph. How quaint, is it not, that those who vocally oppose 15mph quite often seem to be stuck at this speed, anyway?

My contention is that the average speed may often be lower, still. The intuitive case only accounts for traffic lights. It does not account for closed lanes, unexpected impediments, or even turning corners. These are all things that are much harder to model, in terms of impact on average speed, but that nevertheless can be expected to reduce the average speed. If you do not believe me, time your next car journey. Then, look up the distance travelled. Simple division will give you your average speed. Assuming your journey did not involve long stretches of motorway (which I will return to, later), I am quite confident your average speed will be much closer to 15mph, or even 10mph, not 30mph.

How, then, might halving the speed limit reduce journey times? The first pillar is the economist's classic argument about incentives and preferences, made most famously by Anthony Downs in Stuck in Traffic. People drive because they are incentivised to. Part of this incentive package is the convenience of driving--it drops you off straight at your door. Part of it is the availability of alternatives--some people do not have public transport options available to them. Part of it is intrinsic--people enjoy driving. But a major part of the incentive package is efficiency--driving is often faster than alternatives, or is at least perceived to be. For instance, on a good day, it takes me two hours to get to Leeds from Manchester via train, as measured from the time I leave the house. That trip (again, on a good day) can take only 40 minutes in the car.

Downs' argument, which most economists should agree with, is that lowering the speed limit makes driving a worse option compared to existing alternatives. Well, strictly speaking Downs prioritises a Pigouvian tax on driving, but the behavioural effect is likely to be similar. For Downs, such an intervention is essential to reduce congestion. But logically, from a speed perspective, the reduction in traffic arising from fewer drivers should also allow those who continue to drive to achieve the maximum speed limit a much higher percentage of the time. Of course, if the new maximum speed is now longer than the previous maximum, it does not immediately follow that journeys will be faster. Reducing the number of cars on the road must have other compensatory effects, which I will return to these in due course.

The second pillar is the question of traffic lights. Traffic lights destroyed the average speed in the intuitive case. They are a major choke-point for achieving higher average speeds. Fortunately, by halving the speed limit, a great campaign to eliminate traffic lights can be undertaken. At lower speeds, pedestrians become less reliant on systems to stop traffic. Similarly, drivers become more capable of negotiating junctions and other intersections on their own. Traffic lights are an automated traffic management system. But people can self-organise traffic and achieve better outcomes. However, this can only be done at low speeds. High speeds reduce reaction times, placing too much strain on self-organisation. If we remove traffic lights in our intuitive example by capping the speed limit at 15mph, drivers will spend much more of the time travelling at maximum speed, rather than spending around a third of the total travel time stationary. The disincentive to drive also contributes to this self-organisation approach. Fewer cars mean the risks of self-organisation at junctions decrease, while gaps in traffic become more frequent, enabling pedestrians to act without stopping the free flow of traffic. Unexpected hazards can also be dealt with better at lower speeds, making roads safer while avoiding incidents that create traffic jams. We sacrifice self-organisation for higher speed; yet, we should not forget the tremendous advantages that come from self-organisation.

The third pillar relates to disincentives and self-organisation. While many people assume traffic jams are caused by collisions on the road, or obstacles that constrain available lanes, most traffic jams are caused by nothing at all. Well, this is not entirely correct. Most traffic jams are caused by the clustering effect. Imagine several cars are driving in a circle, and each is told to maintain the same speed, 30mph. Quite naturally, some drivers will drive slightly faster than this. Some will drive slightly slower. And, quite quickly, the cars will begin to cluster, with faster drivers catching up to slower drivers, before having to compensate and slow down, causing those behind them to catch up, then slow down, etc. The natural variance in human driving creates traffic jams out of nothing, and this is pretty much unavoidable. What can we deduce from this variability? Anecdotally, when people speed in a 30-zone, they may drive around 33mph or 34mph. But when they speed on the motorway (UK speed limit 70mph), they will typically drive at 80mph. What we observe is that people do not deviate from the speed limit by an absolute mph-number, but by a percentage. For simplicity, let us say 10%. However, it is the deviation in terms of mph that accounts for the severity of a traffic jam: larger mph deviations will cause clusters to form faster, resulting in more traffic jams.

Attentive readers may already see where I am going with this. At 30mph, some drivers will drive as slow at 27mph and some as high as 33mph (again, this is natural variation, and so anti-speeding campaigns may have limited impact on this behaviour), a difference of 6mph. But at 15mph, we would anticipate slow drivers achieving maybe 14 or 13mph, and speedy drivers hitting maybe 16 or 17mph, a difference of around 3 mph. This means that traffic jams will form much less frequently at 15mph than a 30mph. This is only considering the cluster effect. Fewer cars should also reduce traffic jam frequency, as should self-organisation, which should avoid sudden accidents that create non-cluster effect jams.

The point is that these three pillars show how lowering the speed limit actually help eliminate the common causes of slow speeds: other drivers (disincentives), traffic lights (self-organisation), and traffic jams (the cluster effect). By halving the speed limit, and eliminating these obstacles, drivers will spend more of their time achieving the maximum speed limit which--at 15mph--may actually be higher than the average achieved at 30mph. Reducing the speed limit will speed up journey times.

Let me finish on a couple of tangential points. Firstly, motorways. The trouble with the 30mph speed limit is that it is too fast for roads that must accomodate both cars and pedestrians, resulting in management systems (e.g., traffic lights) that reduce the average speed. But motorways are not be a place for pedestrians. Motorways can suffer other obstacles, namely traffic arising from too many cars. But, for the most part, motorways function as a place where higher speeds should result in faster journeys. So, I have no problem raising the speed limit to, say, 100mph on motorways. Others can probably offer a more informed opinion on this matter. But broadly, faster motorways coupled with slower roads could reduce journey times (of course, faster motorways may encourage more people to drive, resulting in deleterious effects. If this post has made any point at all, it is that seemingly linear relationships often have much more dynamic effects). Collisions at such high speeds may be more significant, resulting in worse crashes (and thus worse traffic jams) and, more importantly, more casualties. But--and I must reiterate how everything here is on average--the reduced casualty rate from lower speed limits on roads is likely to compensate for higher motorway casualty rates. Secondly, demand for journeys. Many conversations I have about transport policy focus on supplying journeys, but if one wants to speed up journeys, or simply reduce traffic, it is essential to reduce the demand for journeys, too. This can be achieved by putting more stuff within walking distance (why people hate 15-minute cities, I have no idea). In a world where driving is obviously about as fast as cycling (again, most people do not think in terms of their average speed, but instead, the speed limit), and not that much faster than walking, the incentives for how we arrange our cities will change. A self-organising traffic system re-emphasises how the city is a place for people, and that roads do not belong exclusively to cars. Such measures might knock us out of our car-centric mentality, and in the long-run, result in more easily navigable spaces, regardless of one's means of transportation.