Whether or not it’s possible to “build your way out of congestion” is a fundamental question that transport planners all around the world grapple with. On the one hand, we seem to have been building a lot of “transport stuff” over the past few hundred years yet our cities seem as congested as ever; while on the other hand it seems quite crazy to suggest that building transport projects is pointless as no matter what you build, you’ll never actually be able to make a difference to congestion. Perhaps a useful term to consider is that we have to “run fast to stand still”, and that transport infrastructure needs to be constructed to ensure that, at the very least, congestion doesn’t get worse. But that’s not how it’s sold to us. The transport projects that a lot of money is currently being sunk into are sold to the public for their ability to reduce congestion, and if the politician is being particularly optimistic, to eliminate it altogether.

For an example, let’s have a look at some of yesterday’s announcement about additional spending on road infrastructure in and around Christchurch:

Less congestion, shorter travelling times and improved road safety are three of the major benefits Canterbury motorists can expect from the biggest road construction programme in the region’s history…

…NZTA Board member Garry Moore said the construction programme was designed to improve and future-proof access to the Christchurch International Airport and Lyttelton, Port of Christchurch.

“Our goal is to make life easier for Cantabrians by delivering reliable, safe and efficient road access to the airport and the port for the people and businesses of the region. The future economic growth and prosperity of Canterbury, depends on people and goods reaching these destinations efficiently, safely and on time.”

Mr Moore said the construction programme would deliver a wide range of benefits including:
• More efficient movement of increasing freight volumes through the region
• Improved access to Christchurch’s CBD, airport, Lyttelton Port and other facilities such as hospitals
Reduced congestion leading to more reliable journey times
• Improved road safety, with lane-separated state highways, and traffic diverted from local roads
• Substantial investment in the local construction industry flowing into the regional economy.

Now to be honest I don’t know much at all about these motorway projects in Christchurch, and they very much might be essential – I simply don’t know the details enough to make a judgement in that respect. However, what is clear is that we’re being sold some big promises here for the $600 million or so that will be spent on these roading projects over the next decade. Are they likely to be realistic promises? Can roading projects really actually reduce congestion and improve travel times in the longer term?

I suppose at first glance it seems logical that building more roads, or widening existing ones, would reduce congestion. The more or wider roads you have, the greater capacity the transport system has and therefore the greater number of cars that can travel through it without the system becoming ‘overloaded’ – ie. congested. I suspect that many people think of a transport system being similar to a stormwater system, in that there is a certain amount of cars (water) that needs to be provided with space to get from A to B, and if there’s not enough capacity for them to get there – then you’ll have congestion, which is bad.

Googling the term “build your way out of congestion” shows that there are a reasonable number of groups that vehemently believe that you can achieve this task (although they generally appear to be anti-transit pro-sprawl groups). NZTA and the current government certainly seem to believe that building your way out of congestion is possible – as shown in the press release above (as well as just about every other press release about motorways we’ve seen in the last year or so). But it is true? How does the argument that each project is going to “make things better” stack up against the reality that congestion seems to keep getting worse, or at best, simply stays the same?

The big “elephant in the room” that seems to be ignored here is the concept of “induced travel demand“. I have talked about induced demand before, but basically it can be summed up as “if you build it, they will come”. Because new and wider roads offer a ‘superior product’ compared to what was there before, they attract more people to use them at the times people find most convenient. So therefore all that increased capacity can end up disappearing pretty quickly as people change routes, make their trips at peak times instead of off-peak, switch from public transport to driving or just make a trip that they wouldn’t have bothered with before.

Todd Litman, of the Victorian Transport Policy Institute, has put together an excellent paper on induced demand and its planning implications that explains the phenomenon in excellent detail. Here’s the abstract from his paper on the topic: The introduction contains a bit more information on what induced/generated traffic is, and the economic theory that sits behind the concept: In a bit more “user friendly” language, here are a few examples of transport decisions that relate to induced/generated demand: Interestingly, research has shown that over many many decades average commute times have apparently stayed the same, at around 75 minutes per day in the USA, even as transport technologies have rapidly improved. Of course people have been able to travel much further in that alloted time, but there certainly doesn’t seem to be any reduction in the length of commutes. However, that statistic is useful in attacking the use of ‘time-savings benefits’ in the cost-benefit analysis of projects rather than arguing that you can’t build you way out of congestion. If one was able to travel 125km within the 75 minute commuter allotment, traffic would be travelling pretty quickly and in all likeliness there wouldn’t be significant congestion.

But what this natural equilibrium of both commuting time and level of congestion means is that it’s damn hard to actually reduce congestion through road-building, because when you make an improvement to a road by widening it, the ‘latent demand’ that wanted to use that road but was put off doing so by congestion, is set free – and much (if not all) of that additional demand is eaten up very quickly indeed. This is well illustrated in the diagram below: Bringing this back to Auckland, we can see on a few of our motorway routes that capacity has been reached. Taking the Harbour Bridge and the SH16 causeway as two examples (because both are involved in potentially hugely expensive project to increase their capacity, we can see that over the period of 2004-2008 the number of vehicles crossing the Harbour Bridge reduced from 161,900 to 153,324 per day, while the number of vehicles going along the SH16 causeway between Waterview and Patiki/Rosebank Road went from 90,815 to 87,262. Of course 2008 was a rather strange year for traffic volumes, as petrol prices of more than $2 a litre really did end up forcing people off the road. But even if we look at the more “normal” 2007 figures, there really was little or any growth in traffic numbers between 2004 and 2007 on these two routes. Which to me suggests that they are probably considered to be approaching ‘capacity’. Of course it would be really interesting to find out the peak hour flows and how they’ve changed over the years, but unfortunately NZTA does not publish that data.

The point of mentioning the Harbour Bridge and the SH16 Causeway is that if/when we do add additional capacity – in the form of widening SH16 and building another crossing of the harbour (in whatever shape or form it turns out to be) then it seems inevitable that we will “let go” a lot of this latent demand, a lot more people will end up driving at peak times, and before long the roads will probably be just as congested as they are now.

Getting back to Littman’s article, aside from the ‘broken promises’ of motorway projects that were supposed to reduce congestion, but turn out to do nothing of the sort, there are quite a number of “costs” associated with induced traffic, that often are ignored by the current cost-benefit analysis system: There is a lot of other good stuff in the paper, and I could probably quote the whole thing quite happily, so I do suggest having a good read of it. However, I shall jump to the conclusion – as it provides some excellent overall points: So can you “build your way out of congestion”? Thanks to induced demand, I would think the answer is probably no, unless you built a truly unbelievable amount of roading so that all the latent demand in the whole city could be accommodated, even at peak hour. And to be honest, that seems like a pretty tremendous waste of resources. While congestion is annoying, it’s also sending us a message that perhaps we shouldn’t be driving along this road at this time of the day in this vehicle.

Interestingly, induced demand also applies to public transport (a better service will encourage more users). However, unlike the situation with private vehicles, induced demand is actually generally a good thing for public transport as it means higher patronage, lower costs-per-user and lower subsidies being required. Another fundamental advantage I suppose.

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  1. good day,
    Traffic roblems in large cities can only be solved by one method; private car reduction. I do not believe in road pricing /economic toll barrier. I believe in PRIORITY of buses and service vehicles during the rush hours, this is TrafficLogistics, I have developed a “concept” Trafikklogistikk, see http://trafikklogistikk.com, please contact me on knut@trafikklogistikk.com if you would like to hear more or the best if you want to develop this further together with me.

    Knut Bøe, Norway

  2. Hmmm… interesting. Yes I agree that giving priority to certain vehicles makes a lot of sense. Transit lanes on the southern motorway, which turn into truck lanes off-peak seems like a good way to improve the efficiency of that route.

  3. In terms of the CSM in Chch, it will probably take (a lot of) congestion off the current truck routes such as Main South–CBD via Blenheim Road, however, Blenheim Rd is a straight line into the city, and the motorway is a long curve away from that straight line.

    Secondly, it avoids all the current industrial areas at a fair distance, as the satellite photo on Chch Transport shows. The final section across from Curletts road is the current Southern Motorway.

    It also avoids all the residential areas, and again takes such a curvy route it’d be hard to see it become more preferred above the current straight-line routes in to the city, but most residents work in the areas nearby, which the motorway goes around. Obviously as well it’s no use to the buses.

    It only appears to be an improvement only for trucks going to Lyttleton port or central Chch, and coming from outside the city from the south, and wanting to avoid the industrial areas in the west – noone else will use it.

  4. Speaking of anti-transit pro-sprawl people, saw Wendell Cox was getting airtime on Stuff today blaming the RMA for high house prices.

  5. Yeah I saw there was another demographia report out or whatever its called. Of course high house prices are nothing to do with NZ’s obsession with stand-alone housing. Its a human right to be able to do laps around your house.

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