20mph Speed Limits by Common Consensus, Not by Physical Infrastructure (or why 20mph by traffic calming is doomed to fail)

The concept of reducing default urban speed limits to 20mph in urban areas, except on selected through-routes, is an idea which has recently become a significant public issue. People are increasingly realising that slower traffic speeds improve the environment and in the event of collisions, significantly reduce harm to unprotected road users. This change in public mood is reflected by an ongoing Holyrood consultation seeking to implement 20mph on the majority of Scotland’s urban roads (1).

Existing surveys and polls suggest that most people are already in favour (2) however, there is a frequently expressed concern that simply lowering the speed limit is not enough. To make people comply, traffic calming infrastructure such as speed bumps and chicanes must also be installed, else most drivers will ignore the slower limit. This sentiment is frequently expressed in on-line comments sections of newspaper articles discussing 20mph.

Whilst traffic calming seems the obvious way to ensure 20mph enforcement, is it in fact desirable or even sensible? No other speed limit is enforced by physical traffic calming so why should 20mph uniquely be enforced by built infrastructure? Let’s examine further.

The Problem of 20mph Enforcement by Built Infrastructure
Even if we were to decide that the best way to make drivers comply with 20mph speed limits is to install speed bumps and other infrastructure, the simple truth is that building traffic calming on all our urban roads would be extremely expensive, costing about £60,000/km(3). That is not to say that we shouldn’t consider traffic calming in areas of specific concern, but universal installation would be impractical. In Glasgow alone there are about 1,400km of residential roads, and if you consider the summed length of all the residential and urban roads in all Scotland’s cities, towns and villages, the financial cost would be huge. Once installed, no-one would like it and we would bump our ways from hump to hump to hump, breaking our suspension.

An additional and often overlooked aspect is that traffic calming infrastructure is not ‘fit and forget’. Once installed it takes a hard pounding and quickly degrades. I live in the Mansewood area of Glasgow where traffic calming was installed in 2014. In the short period since we have already lost 4 of the 55 installed speed cushions due to degradation. Rather than replacement, the council has simply opted for installation of standard road surface.

Speed Compliance Without Traffic Calming
In an ideal world, everyone at all times would obey the posted speed limit. We all know the reality that some drivers speed, though it is also true that the lower the speed limit, the lower the average speed. Real world data shows that if default urban speeds are reduced from 30 to 20mph, without installing traffic calming, then some drivers may speed, but average speeds reduce (4), not perfect but a significant and cost-effective improvement.

Ideally no vehicles should break the speed limits, so how can we improve compliance? The Norway model is worthy of contemplation as an example where there are very high penalties for speed violations (5). I am not saying that we should necessarily have new draconian speeding laws, but if, for example, drivers who speed at 30mph in a 20mph zone are penalised as breaking the speed limit by 50%, rather than by a seemingly innocuous 10mph, then their sin should be treated accordingly; high fines and high levels of points on licence. Consider that 30mph in a 20mph zone is proportionately the same as 105mph on a 70mph road. For speeding in 20mph areas to be taken seriously, we need the Police, Courts and Procurators Fiscal to buy in to the idea of appropriate speeding penalties.

The above notwithstanding, one of the main problems of compliance with existing 20mph limits is that they typically only apply to small geographically isolated areas. Consequently it can be confusing for drivers who are often unaware that the road they are on is 20mph. If a national urban 20mph speed limit were implemented, this confusion would be removed. Additionally, the police will not be able to sit aloof as they sometimes do for local schemes. Assuming that the proposed national 20mph scheme becomes law (1, 2), then the resultant publicity will greatly assist the public to buy into the need for complying with it, and stimulate the Police and Courts to consider appropriate punishments for infringement.

A Common Consensus
In the final analysis we cannot build our way to 20mph speed compliance though infrastructure. It’s simply too expensive to install, let alone the problems of maintenance or its deleterious effects on vehicle suspension. We must create compliance by social consensus; create a common understanding that 20mph is the right maximum speed for urban driving, backed up by suitable penalties for those who infringe. We need residents, drivers, politicians, the press, the Police and the Courts to buy into acceptance of 20mph. Compliance will not happen overnight but I am sure that in 10 years we will all wonder why we thought that 30mph on residential streets was ever a good idea.

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