Why do we need more motorways

Maybe transform them all into double-decker highways with cars zooming on the upper and lower levels. The concept is called induced demand, which is economist-speak for when increasing the supply of something like roads makes people want that thing even more.

Though some traffic engineers made note of this phenomenon at least as early as the s, it is only in recent years that social scientists have collected enough data to show how this happens pretty much every time we build new roads. But before we get to the solutions, we have to take a closer look at the problem. In , two economists— Matthew Turner of the University of Toronto and Gilles Duranton of the University of Pennsylvania—decided to compare the amount of new roads and highways built in different U.

If a city had increased its road capacity by 10 percent between and , then the amount of driving in that city went up by 10 percent. If the amount of roads in the same city then went up by 11 percent between and , the total number of miles driven also went up by 11 percent.

Maybe traffic engineers in U. But Turner and Duranton think that's unlikely. The modern interstate network mostly follows the plan originally conceived by the federal government in , and it seems incredibly coincidental that road engineers at the time could have successfully predicted driving demand more than half a century in the future. A more likely explanation, Turner and Duranton argue, is what they call the fundamental law of road congestion : New roads will create new drivers, resulting in the intensity of traffic staying the same.

Intuitively, I would expect the opposite: that expanding a road network works like replacing a small pipe with a bigger one, allowing the water or cars to flow better. Instead, it's like the larger pipe is drawing more water into itself. The first thing you wonder here is where all these extra drivers are coming from.

I mean, are they just popping out of the asphalt as engineers lay down new roads? The answer has to do with what roads allow people to do: move around.

As it turns out, we humans love moving around. Making driving easier also means that people take more trips in the car than they otherwise would. Finally, businesses that rely on roads will swoop into cities with many of them, bringing trucking and shipments.

As long as driving on the roads remains easy and cheap, people have an almost unlimited desire to use them. You might think that increasing investment in public transit could ease this mess. More road space just increases the distances people travel. Data from the National Travel Survey Typically, within five years after a road is upgraded, the traffic volume increases to a level where congestion is similar to before the upgrade.

In the case of piecemeal road upgrades, congestion is more often displaced than temporarily reduced. This is why widening roads does not reduce congestion: it just increases the distances people travel. Is more travel good for the economy? In most respects, no: someone who commutes 50 miles to work is no more economically active than someone who commutes 5 miles.

However, there is one respect in which travelling further has an economic benefit: it gives employers a larger catchment pool to recruit from, increasing productivity and facilitating business expansion. This is the main economic justification for investment in roads and rail.

Politicians hear their constituents complaining about congestion; they hear businesses complaining about recruitment. They put two and two together and conclude the answer is to increase road capacity. Economists dutifully provide a cost-benefit analysis to support their decision.

So, widening roads does not reduce congestion in the long run. Any reduction is temporary, negated by increased congestion elsewhere, or both. There is an economic benefit of allowing some people to travel further to work, but that benefit is unjustifiably small. Most carbon emissions derive from converting fossil fuels to energy in vehicle engines, boilers, power stations, blast furnaces and so on.

Replacing these with carbon-free alternatives will take decades. A considerable body of evidence is now available to confirm this. But, despite this indisputable fact, many road-improvement decisions continue to be based on the assumption that extra space will not generate new traffic. A significant change occurred in when a report by the UK Advisory Committee on Trunk Road Appraisal confirmed road building actually generates more traffic. Some congestion discourages people from driving suppresses latent demand , but with no congestion traffic will fill road space over time, particularly in or near urban areas.

Interestingly, the opposite can also work. Where road space is removed, demand can be suppressed and traffic reduces without other neighbouring roads becoming overly congested. Read more: Could electric car batteries feed power back into the grid? One of the best examples of this is the closure of the Cheonggyecheon Freeway in the middle of Seoul, South Korea. This suppression of latent demand works best when good alternative ways of travel are available, including high-quality public transport or separated cycle lanes.

The short answer to the question about road building and expansion is that new roads do little to reduce congestion, and they will usually result in increased emissions. Portsmouth Climate Festival — Portsmouth, Portsmouth. Edition: Available editions United Kingdom.