Every weekend we dig into the archives. This post by Peter was published in October 2014.

Wired magazine recently published a good, succinct explanation of induced traffic. It’s worth reading in full as it hits upon an incredibly important, often overlooked fact: it’s not possible to eliminate congestion by building more roads. Here are a few of the more interesting excerpts:

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 1960s, 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. These findings imply that the ways we traditionally go about trying to mitigate jams are essentially fruitless, and that we’d all be spending a lot less time in traffic if we could just be a little more rational.

But before we get to the solutions, we have to take a closer look at the problem. In 2009, 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.S. cities between 1980 and 2000, and the total number of miles driven in those cities over the same period.

“We found that there’s this perfect one-to-one relationship,” said Turner.

If a city had increased its road capacity by 10 percent between 1980 and 1990, 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 1990 and 2000, the total number of miles driven also went up by 11 percent. It’s like the two figures were moving in perfect lockstep, changing at the same exact rate.

Los Angeles: Sitting in traffic after ignoring supply and demand for over 50 years.

In their excellent paper on the topic, Duranton and Turner describe this as “the fundamental law of road congestion: New roads will create new drivers, resulting in the intensity of traffic staying the same.” Their research also digs into a couple of other related and equally interesting phenomena:

  • Better public transport provision doesn’t actually reduce road congestion – but it does enable more people to move without being affected by congestion
  • Reducing road capacity has no measurable impact on congestion – if less road space is available, people take public transport or active modes instead, or avoid making low-value trips.

Urbanist.co also has some further discussion of Duranton and Turner’s work. The economists go on to suggest economists’ favourite answer to congestion: road pricing. (If you’re interested in reading more about that topic, Stu Donovan and I have written several posts about the economics of road pricing.)

So what can be done about all this? How could we actually reduce traffic congestion? Turner explained that the way we use roads right now is a bit like the Soviet Union’s method of distributing bread. Under the communist government, goods were given equally to all, with a central authority setting the price for each commodity. Because that price was often far less than what people were willing to pay for that good, comrades would rush to purchase it, forming lines around the block.

The U.S. government is also in the business of providing people with a good they really want: roads. And just like the old Soviets, Uncle Sam is giving this commodity away for next to nothing. Is the solution then to privatize all roads? Not unless you’re living in some libertarian fantasyland. What Turner and Duranton (and many others who’d like to see more rational transportation policy) actually advocate is known as congestion pricing.

Incidentally, I like Turner’s “Soviet Union” metaphor a lot – I’ve said on occasion that we’re running our transport system like a Polish shipyard.

Lastly, it’s incredibly important to consider induced traffic when making policy recommendations. As I wrote in my review of Alain Bertaud’s talks in Auckland, keeping commute times down is an important part of maintaining an efficient urban labour market. Some people seem to have taken Bertaud’s recommendation that policymakers focus on keeping average car commutes under 30 minutes (and PT commutes under 45 minutes) as a call for more roads. This is a superficially appealing but deeply wrongheaded idea.

Induced traffic means that building roads to keep commute times down will not work. And it will be expensive. While there is often a good case for specific road improvements to remove key bottlenecks or improve safety – the Victoria Park Tunnel comes to mind – Duranton and Turner’s work shows that a strategy of building lots of roads will not succeed in minimising commute times. An alternative approach is needed.

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15 comments

  1. That was a good, succinct post. The billions we’re spending on adding road capacity to Auckland (SH ‘improvements’, etc) and what the developers are building too (which must make up a fair chunk of the 40 km of new roads added each year) are all inducing more traffic.

    AT, when designing your cycleways, I hope that you will first look at how to reduce the traffic volumes and speeds, because otherwise the money we’re spending on cycleways is pushing against the money we’re spending on roads. To build a safe cycle network you need to first reduce the traffic volume and speed.

  2. One piece of roading infrastructure which is in the planning that I feel needs another look (again) with this post in mind is the Reeves Road Flyover. The quoted main reason for it is to take vehicles off Pakuranga Rd and avoid putting them through the Ti Rakau Dr intersection to allow the efficient movement of buses on the new busway. But surely, part of the idea of putting the busway in is that people might decide to take the bus instead of drive and therefore cars would be removed from Pakuranga Rd anyway?

    1. Yes. In the same vein, the LR along the NW needs to be on reallocated traffic lanes not on a new strip of land beside the motorway. Putting in new PT infrastructure and not reallocating road space to it is committing people to high levels of traffic flow on our local roads. We need to take the opportunity to reduce that traffic while increasing accessibility, if for no other reason than it is the most cost-effective way to increase safety for other road users.

      1. Heidi, a great post. Yes there is another way and as you suggest that could be building PT infrastructure on existing roads; and if the reduced road capacity becomes too congested free it up using congestion charges; and apply the congestion charges to subsidising the PT option to further encourage use of it.

      2. Agree with both the posts from nzroo and Heidi. The renders I’ve seen for the Reeves Road flyover show it to be the ugliest structure you could possibly imagine. I recall that the flyover was going to be put off until the indeterminate future in favour of the earlier extension of the busway along Ti Rakau Drive – until that is the local (commuting) community raised such a noise that AT buckled at the knees and put the flyover back into an earlier stage of the development. As I see it, this was purely because of noise, not because it was actually needed earlier than the busway. I still live in hope that it might get put back on the back burner and then quietly forgotten.

        As for the NW motorway – I absolutely agree that having already provided a whole bunch more lanes it should be a simple matter just to appropriate two for LR. What it takes is courage – and a strong argument which shows that by doing so you are providing the equivalent of x more vehicle lanes at very little cost.

        When will AT and NZTA grow the balls to face down the motoring lobby and make decisions that allow the greater throughput of people using the existing constructed resources? They’re prepared to do it along Dominion Road, so why not the NW motorway?

  3. I would like to see a proper analysis of the orgy of motorway widening in Auckland over the last decade. It seems to me that this increased capacity on the motorways, including massive off ramps leading to the arterial network has just shifted congestion from the motorways (which are generally reasonably free running) to the rest of the road network which gets horribly clogged. This is where the induced extra traffic fetches up – on ordinary roads which have not had their capacity increased to handle the extra cars. Even a few extra cars per cycle can soon gridlock an intersection. So arguably the billions spent in the name of congestion busting by “improving” the motorway network have actually made the situation worse. This is just my impression – would be good to see some numbers.

    1. Yes, completely agree. I guess the figures from AT for congestion partially shows what you’re talking about: “Around 24 per cent of the arterial road network is now congested at peak times compared with 18 per cent two years ago… 11 per cent of the arterial road network is now congested in the interpeak, up from 8.8 per cent this time last year” (From AT’s SOI).

      The extra traffic counts the AELB organised after the WC will help. Thanks.

  4. Graeme, I completely disagree because it is so intuitive that it doesn’t need figures to prove it. If more cars are on our motorways they have come from somewhere. If fuel usage is growing in Auckland every year, and it is, then it is most likely it is because there is more vehicle kilometres travelled each year. No, now is the time to do something about it. AT have those fuel use figures, which are about 19% over their target. They know what they have done has not worked and yet they have been part of a RLTP that delivers more of the same.

    1. If you have a look at the 2012 – 2041 Integrated Transport Plan, you can see that they were projecting increased levels of congestion : htps://i.imgur.com/tf1ypGo.png Of course, this will be based on modelling that doesn’t include the trips generated by the increases in road capacity, so the congestion levels could be a lot worse.

      committed funding investment = spending a fortune on widening roads
      fully funded investment = spending an even larger, outrageous fortune on widening roads
      Where was the projection for the plan that involved reducing vkt, reducing carbon emissions, and investing INSTEAD in public and active transport modes?

  5. What would happen if the parking in the CBD and other congested areas were to be either removed or if they are not contributing to congestion have charges imposed on them that would leave 55% vacant at any time?
    Would that reduce demand for road space?
    Would that enable all carriageways to be used full width for moving buses bikes pedestrians and goods service vehicles?
    Would that encourage business to provide off street loading areas to service their buildings?

  6. In reply to Take-ite: you clearly do not understand the principle of induced traffic which is that when the capacity of a road or intersection is increased then it will provide an incentive to travel more on the route(s) it forms part of. This is less about redistributing traffic from one part of the network to another (assuming a zero-sum game) but more about inducing an increase in travel across the entire network. Over the last decade the number of cars on Auckland’s roads has grown even faster than our brisk population growth and my contention is that the substantially increased capacity of the motorway network is a contributing factor.
    Intuition will not cut it in persuading the powers that be – we need conclusive data.

    1. Graeme, let me spell it out for you – data of itself does nothing. My son can see that he is poor because he has only $200 on his bank statement. Does that stop him spending – no.

      Everyone understands induced demand – look at the NZTA website re the additional harbour crossing – they get it – 55% induced demand.

      Forget consultant’s reports – fix the problem!

  7. Nice post Peter, again 🙂 – it’s one of our go-to’s for explaining induced demand.

    It’s a crucial point about modal shift only happening if we constrain or otherwise slightly worsen the driving experience *as well as* improving the alternatives. For cycling, the quintessential example is Stevenage in England. A super writeup here https://www.theguardian.com/cities/2017/sep/19/britains-1960s-cycling-revolution-flopped-stevenage

    (poor network designer Eric Claxton’s frustration and dismay is palpable!)

    1. That’s an excellent article, thanks. I’d put Albany in Stevenage’s typology: while progressive locals might think “it’s ok, we just need some better cycleways” such additional infrastructure won’t fix its built-in car-centric design, and over time, the few cyclists persevering will just battle against increasing traffic endangering them wherever cyclists and traffic cross or mingle.

      For the isthmus, reclaiming the limited road corridor space we have back from car dominance to providing for people will improve the alternatives while naturally slightly worsening the driving experience. Except there’s one thing I don’t get:

      How do the Netherlands and Denmark manage to simultaneously provide a positive driving experience? They reformed from car-bias to people-focused design, just as we want to do. And they ended up with good design for all modes. How was this achieved? Is it actually only a few years of drivers feeling like they’re losing amenity before everyone sees the benefits?

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