Does cutting speed limits to 20mph actually work?

20’s plenty. Image: educators.co.uk/Flickr/creative commons.

A new speed limit of 20mph has been proposed for roads in central London. The plans, which would reduce the limit to 20mph within the Congestion Charging Zone, are part of the “Vision Zero” strategy, which aims to “eliminate deaths and serious injuries from London’s transport network by 2041”.

The main reason for reducing traffic speed is to lessen the likelihood of a collision – and to reduce the severity of road traffic casualties. Research indicates that if a pedestrian is struck by a vehicle at 24mph, they have a 10 per cent risk of dying. This goes up to 25 per cent at 32mph, and 50 per cent at 41mph. A reduction in speed of as little as 1mph is associated with a reduction in casualties of up to 6 per cent.

Yet plans for the 20mph limit in London have been controversial. Some question the impact it will have on increased traffic congestion and air pollution. Retailers are concerned it will discourage customers from visiting the city centre due to increased congestion. Others question the need for a 20mph limit in areas where congestion means it is rarely possible to go any faster than that. So what’s the evidence on 20mph limits so far?

Many look to the example of Bristol to support the introduction of 20mph limits. Headline findings have so far suggested that “four lives a year are saved” in the city, which reduced speeds in 2014 and 2015. It has also been estimated that Bristol has saved over £15m per year due to lower casualty rates.

Elsewhere, results from pilot schemes in Edinburgh and Portsmouth indicated an overall reduction in speed of between 0.9mph and 1.9mph on roads where 20mph limits were implemented. In Portsmouth, an average reduction of 6.3mph was seen on roads that were characterised by speeds of over 24mph before the lower limits were introduced. The city also showed a 22 per cent reduction in reported road casualties where 20mph maximums had been introduced.

The pilot research has also indicated that 20mph limits in Bristol and Edinburgh have led to increases in people choosing to walk (up to 10 per cent) or cycle (up to 5 per cent). Other studies have shown increases in perceived road safety, and the pleasantness of residential environments.

But others are more swayed by what has happened in Manchester, where the supposed benefits of 20mph limits have been questioned. There, a drop in the number of accidents in 20mph zones was not as great as it was on some faster roads. In light of these findings, Manchester City Council reviewed their 20mph scheme and withdrew funding.

An overall review of previous research concluded that 20mph schemes can reduce accidents, injuries and traffic volume, and improve perceptions of safety, while being cost effective. However, of the ten studies included in the review, only two focused specifically on speed limits (use of 20mph signage without physical interventions such as speed bumps).

The review also found a lack of evidence assessing the impact of such schemes on addressing social inequalities – road casualties are higher in the most deprived areas.

What is less clear are the potential risks of implementing such schemes, particularly for noise and air pollution. Some argue that lowering traffic speed will lead to increased congestion and consequently increased air pollution. On the other hand, if 20mph speed limits are successful in encouraging more sustainable travel modes then this will result in a reduction in air pollution.

The evidence to date is somewhat limited because of a lack of robust, long-term evaluations. In particular, there has not been enough investigation into their economic impact and cost effectiveness.

Looking at the signs

So what can London learn from the evidence so far? The first lesson is that communication is key – it is vital the public is kept up to date with plans. It is also important to have an evaluation plan which includes interim analyses, so that any changes made are based on solid evidence.


And that evaluation should be a broad one. As well as measuring changes in traffic speed, and the number and severity of collisions, include other measures such as changes in behaviour when it comes to travel mode and liveability.

In our increasingly congested cities, more must be done to move towards sustainable modes of travel – and for society as a whole to become less reliant on cars. However, changing social norms will take time, and in order to be successful, local authorities must ensure they engage with the general public when implementing ambitious transport plans.

London is the latest city in an ever growing list to implement 20mph speed limits. At the “Is 20mph plenty for health?” research team, we would encourage London to learn lessons from the other cities – and to share its knowledge as we continue to build the evidence base for 20mph speed limits.

The Conversation

Ruth Hunter, Lecturer, School of Medicine, Dentistry and Biomedical Sciences, Queen's University Belfast.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

 
 
 
 

What's actually in the UK government’s bailout package for Transport for London?

Wood Green Underground station, north London. Image: Getty.

On 14 May, hours before London’s transport authority ran out of money, the British government agreed to a financial rescue package. Many details of that bailout – its size, the fact it was roughly two-thirds cash and one-third loan, many conditions attached – have been known about for weeks. 

But the information was filtered through spokespeople, because the exact terms of the deal had not been published. This was clearly a source of frustration for London’s mayor Sadiq Khan, who stood to take the political heat for some of the ensuing cuts (to free travel for the old or young, say), but had no way of backing up his contention that the British government made him do it.

That changed Tuesday when Transport for London published this month's board papers, which include a copy of the letter in which transport secretary Grant Shapps sets out the exact terms of the bailout deal. You can read the whole thing here, if you’re so minded, but here are the three big things revealed in the new disclosure.

Firstly, there’s some flexibility in the size of the deal. The bailout was reported to be worth £1.6 billion, significantly less than the £1.9 billion that TfL wanted. In his letter, Shapps spells it out: “To the extent that the actual funding shortfall is greater or lesser than £1.6bn then the amount of Extraordinary Grant and TfL borrowing will increase pro rata, up to a maximum of £1.9bn in aggregate or reduce pro rata accordingly”. 

To put that in English, London’s transport network will not be grinding to a halt because the government didn’t believe TfL about how much money it would need. Up to a point, the money will be available without further negotiations.

The second big takeaway from these board papers is that negotiations will be going on anyway. This bail out is meant to keep TfL rolling until 17 October; but because the agency gets around three-quarters of its revenues from fares, and because the pandemic means fares are likely to be depressed for the foreseeable future, it’s not clear what is meant to happen after that. Social distancing, the board papers note, means that the network will only be able to handle 13 to 20% of normal passenger numbers, even when every service is running.


Shapps’ letter doesn’t answer this question, but it does at least give a sense of when an answer may be forthcoming. It promises “an immediate and broad ranging government-led review of TfL’s future financial position and future financial structure”, which will publish detailed recommendations by the end of August. That will take in fares, operating efficiencies, capital expenditure, “the current fiscal devolution arrangements” – basically, everything. 

The third thing we leaned from that letter is that, to the first approximation, every change to London’s transport policy that is now being rushed through was an explicit condition of this deal. Segregated cycle lanes, pavement extensions and road closures? All in there. So are the suspension of free travel for people under 18, or free peak-hours travel for those over 60. So are increases in the level of the congestion charge.

Many of these changes may be unpopular, but we now know they are not being embraced by London’s mayor entirely on their own merit: They’re being pushed by the Department of Transport as a condition of receiving the bailout. No wonder Khan was miffed that the latter hadn’t been published.

Jonn Elledge was founding editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.