In April 2019 a heavily-used bridge across the River Thames in London was closed indefinitely due to structural problems. Local media were full of alarm, warning about the likely traffic congestion that would result. But, curiously, several months later, the signs are that the opposite may be happening. Pollution levels in key nearby centres have gone down, a strong indication that fewer cars are on the roads. Could this be the latest sign of one of the best kept, and counter intuitive secrets in urban planning, that less road space doesn’t increase congestion but leads to a drop in vehicle numbers? In a world looking to quickly cut carbon emissions it’s an insight that could prove revolutionary.
Imagine if we closed some roads to cars and traffic congestion actually reduced as a result. This sounds counter-intuitive; yet, it is exactly the effect that was revealed by research in the 1990s in a number of cities around the world. This result was described as ‘traffic evaporation’ in the seminal 1998 UK study of 100 locations. The report showed that after a ‘settling in period’, where road capacity was reduced for private cars there was a 25% average overall reduction in traffic. Case-by-case outcomes varied substantially, but in many cases, when you reduce road capacity, existing motor traffic doesn’t just find another route. Some of it ‘disappears’, or ‘evaporates’.
By the 1990s, the city of Copenhagen had already adopted what has since become their long-term strategy of reducing space for cars while developing public transport options. By cutting parking spaces, removing road lanes, and banning certain through traffic, in 1998, 80% of all journeys were made on foot, and 14% by bicycle. Car traffic in the city core was hugely reduced and congestion was no longer a problem. Long-term planning and vision is of course vital, but change can happen quite quickly on the ground. In Kajaani, Finland, the High Street, which was taking 13,000 vehicles per day across the main square, was closed completely. Traffic in the adjacent streets rose, but only from 1,000 to 6,500 vehicles per day. Other surrounding streets saw no change in traffic flows. This means that over half the former traffic in the main square simply ‘evaporated’. Similar patterns emerged in a range of European cities studied, from Nuremberg to Oxford, and from Strasbourg to Wolverhampton.
These ideas have taken some time to percolate through into widespread urban planning – perhaps through a mix of mistaken beliefs about how traffic flows work and the strength of the car lobby – but there is no reason they could not be implemented quickly, given the political will. Even the US, where the car was made king, cities have started to look at how to make better use of space previously designated for cars, such as parking lots. After San Francisco implemented a pilot project with real-time data on parking availability and dynamic pricing for spaces, an evaluation found that the amount of time people spent looking for parking fell by 43%. This was part of San Francisco’s Smart City initiative, which has been looking more widely at how to move around the city in the most efficient way for people and for the environment. As urban designers realise the danger of moving people to the suburbs, hollowing out city centres and creating areas of deprivation, city space is increasingly needed for homes, businesses, learning and leisure facilities. The car can appear to be convenient for individuals, but planners and politicians alike are waking up to the fact that they are an inefficient use of space and resources for society as a whole.
Wider relevance
What is interesting about traffic evaporation is that it is all about the behaviour of people and our inaccurate assumptions about how they behave in any given scenario. There is no constant steady demand that we can point to as ‘traffic” for any site; instead, the flow of vehicles is revealed to be a fluctuating, highly changeable body made up of individuals making complex decisions. When alternative means of travel are available, people therefore shift surprisingly quickly to whichever mode works for them to get round the particular travel problem they face – traffic jams, strikes, road blockages etc. For shorter journeys, walking or cycling is the usual substitute, with buses, trams and trains taking up those travelling the longer distances.
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Context and background
In 1994, the UK Government-commissioned Sactra (the Standing Advisory Committee on Trunk Road Assessment) report provided evidence on the impact of new road building on local traffic levels. The report revealed that when new road capacity is provided, overall traffic levels in the vicinity of the scheme may actually increase. The evidence does not offer a reliable means of predicting the extent of this traffic increase, but case studies in the report suggested that it is typically around 10% in the short term, and 20% in the longer term. This went against popular thinking, which imagined that new roads were simply satisfying a steady demand – the approach of so-called ‘predict and provide’ – not stimulating extra demand. It established once and for all that the ‘induced traffic’ phenomenon – building new road capacity generates more motor vehicle trips – was a reality and should be taken into planning decisions.
However, policymakers tend to favor highly visible physical investments, such as building roads. Sometimes roads are built or upgraded even when demand hardly justifies it. Improving capacity on ‘strategic’ roads like the M25 around London has often just encouraged short car trips within the local area, which in turn leads to the new capacity filling up, and longer distance traffic being delayed again. Congestion continues – with many other, arguably more important, problems (air pollution, noise pollution, severance, road danger) worsened.
Research in the US supports this claim, with findings that the more highway capacity a given metro area had, the more miles its vehicles travelled on them. A 10% increase in capacity, for instance, meant a 10% increase in vehicle miles, on average.
Enabling factors
This kind of transition will demand vision from the planning authorities and a steely reserve in the face of those lobbying for additional road and/or car use. The cities featured in the 1998 report case studies all decided to take action at the municipal level, bringing together planners, developers and transport authorities. This kind of joined-up thinking is the ideal way to progress such a scheme – and perhaps town or city level is the best way to proceed, where decisions can be taken more quickly. The only drawback could be that smaller jurisdictions may not have the resources to argue their case against the powerful car lobby and the public’s lack of awareness of what happens when you build more roads.
An incremental approach gave residents in those cities with advanced thinking time to adapt, and to change from driving and parking their cars to walking, using bicycles and public transport. The sooner we start making these changes, the longer we have to put them into practice in a smooth and equitably way. Some face particular mobility challenges, such as people with disabilities, and provision for cars – whether electric, conventional or self-driving – will continue to be a part of traffic management planning, but finding ways to encourage more use of alternative modes of transport for mass transit – public transport, cycling and walking – must be the goal of any sustainable urban policy. This will need time, planning and investment.
Demand for urban space for living in a more pleasant environment is a major driver of change, and is one that is likely to increase – particularly as our populations become more urban and city homes become smaller and more expensive. Public, outside space is being reclaimed by communities who feel it belongs to them – and not just to the car. It is also being clawed back by city authorities keen to make the most of their land, largely through the policy of reducing parking. The Swiss city of Zurich declared as early as 1996 that there would be no more parking: officials placed a cap on the amount of parking spaces that would exist there, putting in place a trading system by which any developer proposing new parking spaces would be required to remove that many parking spaces from the city’s streets. The result has been that the city’s streets have become more amenable to walking, cycling and public transport. Other cities around the world are thinking similarly; São Paulo got rid of its minimum parking requirements and implemented a maximum that could be built into specific projects. Beijing, Shenzhen and Guangzhou are hoping to emulate San Francisco’s dynamic pricing approach.
A 2011 study by the University of California, estimated there are upwards of 800m parking spaces in the US, covering about 25,000 square miles of land. And a study showed that drivers cruise looking for parking for an average of 3.3 minutes. Based on the number of parking spaces in the area studied, this adds up to 950,000 extra miles travelled over the course of a year, burning 47,000 gallons of gasoline and emitting 730 tons of CO2. In London alone in the United Kingdom parking spaces are estimated to take up nearly 20 square miles of land.
Scope and evidence
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