Thread by @bswud

Although induced demand is not a helpful concept, and roads are useful way of generating agglomeration benefits, roads (and private cars) have two big costs.
One is the externalities they cause. Another is the land they use. These have important implications.
As promised, a🧵

First my thread and post explaining how roads satisfy a latent demand for journeys, make housing cheaper by opening up much more land for productive development, support economic agglomeration, and helped start the Industrial Revolution.

The lesson we should take from this is *not*, however, that roads are always the best transport mode, that historical schemes to bulldoze our cities for them were good ideas, and that we should push against every scheme for pedestrianisation and reducing traffic in city centres.

The first issue is that cars take up a lot of space relative to the person they are transporting. Online urbanists have pointed this out so many times that you might think it's a canard, but it is important. Including stopping distances, cars take up the space of 20 people.

This means that if lots of people want to do exactly the same trip, then despite the fact cars have engines, they're often slower at moving lots of people than just walking, as you have to squeeze so much extra material through the available space. fastcompany.com/3063344/these-…

Cars have also got much bigger over time as we have got richer (on the demand side) and more efficient/productive at making them (on the supply side). This makes for a much nicer drive, but it worsens the space costs of driving.

For roads like motorways, which are mostly in the countryside, this doesn't really matter. Road links between cities are very important, and the alternative use of the land is usually just farming, worth about £10,000 per acre (£25,000 per hectare).

Land in cities, by contrast, is very scarce, especially in the centre. Land in Westminster, London's most expensive borough, averages £93 million per hectare, and that's with today's planning constraints. london.gov.uk/sites/default/…

A plot of land in Westminster without any planning constraints would go for quite a lot more. 49 Park Lane went for £3,300 per square foot. At 55 storeys (like the Hilton down the road), 1/2 hectare would have about 1 million square feet of usable space – £3.3 billion worth.

So land in the most prime areas can be over a million times more valuable than land outside cities. This is of course, why we build upwards when land is scarce, and we build at low density in low value areas, since we have ample cheap land. cdn.auckland.ac.nz/assets/busines…

This is also part of the reason why urban railways have often been shifted underground. (The other part of the reason is the second half of the thread: the noise and pollution externalities railways have, especially steam railways.)

Just look how many more people could get down a Manhattan street in the early 1910s compared to today

All of NYC's bridges carried more people daily 100 years ago, before the lanes were given over to cars. The Brooklyn Bridge had 426,000 crossings then vs 178,000 now

London has 2m railway and 5m tube journeys each day – on the 1% of its land that is railways and stations. There are about 6m trips (incl bother passenger and driver) on the 12% of its land that is road.


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The more busy roads you have in a city the more pedestrians get slowed down by constant road crossing. My experience is that walking in NYC is tortuous compared to London as you have to stop at every block. worksinprogress.news/p/notes-on-pro…

This suggests we want to direct driving around the centre of cities with bypasses and ring roads. Roads in the centre should mostly be for access. Where possible, arterials should go underground as they approach the centre, just like metros and cables.

Take Rennes (France) and Bristol (UK), which have core cities and metropolitan areas of roughly the same population. Rennes is neatly ringed, Bristol has major roads driving right through the centre. My story implies the Rennes approach is superior.

So the first problem with roads and private car travel is that they are extremely space inefficient, meaning that they are inappropriate in the centre of cities.

The second problem is that they have large spillover costs, especially noise and air pollution.

Notice anything about this fine-grained map of Nitrogen Dioxide pollution? Things get worse as you get to the centre, but above all, pollution is driven by roads. The same is true for PM2.5s and other air pollutants.

I've written before about how bad air pollution likely is, even though we have almost conquered the absolute worst pollutant, lead. worksinprogress.co/issue/scientif…

Road use is the biggest cause of air pollution in the UK. Everyone who lives near a big road can smell and taste it. uk-air.defra.gov.uk/assets/documen…

Switching to electric vehicles is going to help (probably halving the problem), but they will not completely eliminate local air pollution from cars.

What's more electric vehicles make very little difference to noise pollution above 20mph (though below 20mph they emit only that glorious warm drone sound).

People *hate* noise pollution, so much so that Studies have been seeing its effect on house prices since the 1970s – one 1976 paper found that prices went steadily up for the first 1,150 feet as the house moved away from the I-495 in North Springfield, Virginia.

A more recent Swedish study found that every decibel of extra road noise cut 0.6% from a property's value. Being in the loudest places cut 30% off the property's price. citeseerx.ist.psu.edu/document?repid…

Studies around the world have found similar results (I will share more in a post later). And the same thing is found for aeroplane noise. Each decibel above 45 (roughly the sound of light rain) wiped €1,900 of the value of Dutch properties in 2009. sciencedirect.com/science/articl…

People just really hate noise, and aeroplanes and cars are the two biggest sources. Each decibel is generally worth 0.5% to 3% of home value.

This is why people like cul-de-sacs. They don't want loud, polluting, dangerous traffic going past their front door. They want their kids to be able to play in the street. They want to have tea with their neighbours.

This is why people pay more for homes on cul-de-sacs. 29% more.

This is why developers will build all new neighbourhoods as cul-de-sacs (or something close) if they're allowed to: because buyers prefer them.

Similarly, the Great Estates that once owned swathes of London always blocked through traffic if they could – though their biggest concerns (like driving cattle to market) are not quite the same as we have today.

All this harm from noise and pollution (and danger) means that we should try, where possible, to keep traffic away from residential streets. This means bypasses and ring roads.

When you have a credible bypass, then even without significant driving restrictions in the centre, you will see most of the traffic just go the quickest, most convenient route on the infrastructure meant for it. This is absolutely clear to me from driving around Spain.

This was the idea behind the Buchanan Report – Traffic in Towns: build good dedicated road infrastructure to drag traffic away from city centres and residential streets. It was also the idea behind London's failed Ringways project.

Right now there is a huge controversy over Low Traffic Neighbourhoods in the UK, which take existing residential streets in cities, and turn them into cul-de-sacs for cars (but retain access for quiet, non-polluting modes of transport).

I think that the LTN controversy could be made to go away if it was paired with improving arterial infrastructure at the same time so as well as being a win for locals, it was a win for those who were used to driving through and saw that taken from them without a say.

If we could build tunnels much more cheaply – say, as cheaply as Norway do – then we could make neighbourhoods quieter and cleaner without hurting those who depend on car transport.

As well as trying to shift traffic off minor residential streets, and out of city centres, what else can we do to reduce the harm of roads (which we need as vital infrastructure). One thing is just waiting. Electric cars get better every year. Hopefully they will take over soon.

But this will take a while, and my lungs are suffering now. This is the idea behind the ULEZ, which I think was initially a good idea – even though I had to change my car to be compliant. I'm not so sure about the recent expansion to low density outer suburbs.

There's more. I recently learned that other countries put noise barriers on their roads, especially on viaducts. Here are some pictures from Warsaw and Katowice. They even make roads look nicer if you grow plants on them.

Many sound barriers claim reductions of 15db or more, which is approximately the difference between nearby heavy traffic and the sound of rain.

These two facts: that roads are essential infrastructure, but also cause harm to those nearby, suggests there is space for a win-win political compromise. More roads in the right places, fewer cars in the wrong places. More agglomeration, less noise where it matters.

Roads are really great actually. But although we definitely want more of them, we should try and keep traffic as far away from residential streets and the places we live, laugh, and love as possible open.substack.com/pub/bensouthwo…