Low traffic neighbourhoods are a type of cul-de-sac

Except without the biggest downsides of cul-de-sacs

1. Cul-de-sacs are streets with only one entrance to cars. They are popular with homebuyers and residents, because unless you live there or are visiting, there is no point driving in. Ths means they are quiet, the air is cleaner, and they are safe. This, in turn, means neighbours know one another better, and there is generally a stronger sense of community. Cul-de-sacs are popular with residents.

2. However, cul-de-sacs also add ‘circuity’ to a system. If they lack cut-through alleyways, then they make walking journeys extremely long (see diagram below), encouraging extra driving. By definition, cul-de-sacs generate roadspace that doesn’t accommodate traffic in the network. Cul-de-sacs, done badly, can generate traffic.

3. If a cul-de-sac does have alleyways, then it’s like a grid for pedestrians, but a cul-de-sac for drivers (would you prefer the 7 min drive below or the 3 min walk). This is the principle behind ‘Low Traffic Neighbourhoods’ in the UK, and ‘superblocks’ in cities like Barcelona. By closing streets to certain traffic, they make many people’s streets into what are effectively cul-de-sacs, with less pollution, more safety (e.g. for children to play in the street), and less noise. By adding circuity for cars but not other transport modes, these types of cul-de-sacs may not encourage driving. Why drive five minutes when you can walk it in three? Cul-de-sacs, done well, need not generate traffic.

4. Each individual LTN or superblock, when it’s introduced, closes various roads. Since people still want to get around, one intuitive prediction is that this just means more traffic on the remaining open streets. Evidence had appeared to reject this, appearing to show not just driving redistributed around the neighbourhood from the closed interior streets to the open boundary streets, but also reduced driving overall. However, close inspection of the evidence suggests this probably isn’t true. Because an individual LTN only affects a small fraction of most drivers’ journeys, it doesn’t affect their decision to drive much, and people tend to just shift their routes to the now-more-congested boundary streets. Each individual LTN largely shifts driving around.

4. However, each LTN introduced likely reduces the total amount of driving in the city a small amount. When roads do not charge users for driving, the main regulation on how much people drive is the speed they expect to go at – this has tended to keep London road speeds around 8-9 mph for about 100 years. If LTNs reduce driving speeds on arterial boundary streets, LTNs would be expected to lead to reduced driving overall, even if each individual one barely affects local driving. This is the flipside of the standard result that in high income countries with no road pricing and moderate or high population density, more roads don’t reduce experienced traffic or increase road speeds, but they do allow more people to get through the equally congested roads (so-called ‘induced demand’). Thus, although each LTN worsens traffic on the boundary roads nearby, if every residential neighbourhood was an LTN, traffic on all boundary roads would be no worse than beforehand. A city with more LTNs is one with less driving.

5. The costs of traffic are likely nonlinear. Going up from a cul-de-sac to a through road appears to have larger effects on house prices and community than going from a congested main road to an especially congested main road. Plus, more people in total live on interior streets than on main streets. Finally, those who chose to live on a boundary street generally already opted for higher traffic, suggesting they are more tolerant to it. This suggests that just because traffic is shifted to other streets where people live, it could still be a net improvement. Traffic shifting is not necessarily bad, and can be good.

6. In theory, we could solve the problem of excessive traffic on boundary streets with pricing systems (in fact – this could improve traffic to be much better than before LTNs). This would be much, much easier (and cheaper) than charging road pricing on every road in the country, plus more defensible politically. Indeed, if all residential neighbourhoods had the option of being LTNs, then achieving fast flowing roads across the country with tolls would be much more straightforward. Rural roads could be exempted; leaving only motorways, ring roads, and arterials, like the boundary streets relevant to LTNs. In practice, it’s reasonable for those on boundary streets to complain that this may not happen. LTNs make road pricing easier. Road pricing could solve boundary road congestion.

7. Another way we could compensate those on boundary roads for the short term cost to them is a policy like halving their council tax. Since those on interior streets are enjoying gigantic benefits, this should be considered fair.  And it should make LTNs more resilient and lasting where they do indeed have net benefits. Since very few people live on boundary streets, this would be cheap. Residents of boundary streets of LTNs should be given discounts on their council tax.

   • As an example, Ashburnham Triangle in Greenwich, London is a historic low traffic neighbourhood, with through access blocked for decades. It includes at least 150 terraced or semi-detached houses, each worth over £1m, plus about 50 social and private rented flats worth about £300,000. Each has perhaps a fifth of its value tied up in it being low traffic – in total over £30m of proprty value (or £750,000 of rental value each year). Far fewer people live on the much more commercial boundary streets (though one of them is Liz Truss) – completely exempting the perhaps 100 properties from about £1,500 council tax per year would come out at far lower than the uplift on all the beneficiary properties. Of course, actually capturing this uplift to use it would be difficult with today’s institutions.