If you build it, they will come: bigger roads mean more traffic

It’s a good time to remind ourselves of a universal law of urban physics: if you build it, they will come. It applies for all kinds of travel, so it’s bad news for those of us who believe “widen the road, remove the bottleneck, that’ll fix the traffic”.

This is an excerpt from a post originally published on Greater Auckland, reproduced with thanks.

One of the big debates in transport circles is the concept of ‘induced demand’ or ‘induced traffic’. As its simplest, induced demand means that when you make an improvement to a transport corridor, that improvement will encourage more people to use that transport corridor – or more simply: “if you build it they will come”. Induced demand isn’t necessarily a good thing or a bad thing, although its treatment can have a big effect on the justification for transport projects.

A common critique made of roading project economic analyses is that they ignore, or under-estimate, the effect of induced traffic on the viability of that project. Most roading projects are primarily justified on the basis that they will reduce travel times, thereby giving people more time to do other stuff, and also enabling freight to be shifted around a city or country quicker than before. I’m sceptical of ‘time savings benefits’ (explained in World Bank report here) for a number of reasons, of which induced demand is certainly a big factor. The World Bank also talks about how induced traffic should be dealt with when analysing transport projects – with a critical conclusion being that induced traffic can actually halve time savings benefits. I wonder if NZTA takes that into account?

The idea of induced traffic is a bit counter-intuitive.

Traffic on Taranaki St by Automotive News

At first thought it appears obvious that widening a road would make the traffic flow better – after all more lanes means more capacity which means more vehicles are able to pass through it in a given space of time. Hence there really should be lower congestion, right? Well the problem with that approach is that it ignores the longer term effects of induced demand. The wider road certainly does initially lead to less congestion, but that makes it a more attractive route for someone to travel along – particular at the times when the road used to be most congested. This leads to more people using the road, and rapidly erodes the time saving and congestion relieving benefits that the road widening, or road construction, was supposed to bring.

There are a number of different ways in which induced demand occurs, which when combined certainly make it appear very very difficult to ever solve congestion by expanding road capacity:

  1. Time-shift induced demand: This is when people may have previously taken their trip during an ‘off-peak’ or ‘shoulder-peak’ time, but shift to driving during peak hour because there is more capacity available and (at least initially) congestion during peak times has been reduced.
  2. Route-shift induced demand: This is when people may have previously taken a variety of different routes to get to their destination, but then shift to using the one that was previously congested. I do note that this type of induced demand is likely to benefit the roads that were previously being used, but it still contributes to more potentially unexpected traffic using the route in question.
  3. Mode-shift induced demand: This is when people who may have previously used public transport, or walked, or cycled the route switch to using their cars, because the reduced congestion has made that a more viable option.
  4. Changed destination induced demand: This is a longer term effect, where people will potentially alter where they live or where they work to take advantage of the improvements to the corridor. They may not have previously used this road, but its initial benefits will attract people to locate their homes or jobs somewhere near that road.
  5. Change of trip frequency induced demand: This is when people might make trips along a certain corridor more often because of the improvements to that corridor. For example, someone may not worry about undertaking their necessary tasks in separate trips along an improved corridor, whereas previously they might have bundled them together into one trip.

Obviously in some circumstances induced demand can increase the benefits of a project.

An example of that is Auckland’s CBD Rail Tunnel Project, where I think there will be a significant reliance on induced demand to provide the project with its benefits. These benefits will come in the form of mode-shifting people away from private vehicle use (so therefore reducing congestion on the roads), encouraging redevelopment and investment in the CBD, improving peak-hour capacity of the transport network to the CBD and so forth.

However, in other circumstances – most obviously in situations where roading projects are being analysed – induced demand will actually lead to a reduction in the benefits of that project.


[Read the full post on Greater Auckland. Editor’s note: from here it’s Talk Wellington.]


Other NZ cities and towns have been learning the hard way about “if you build it, they will come” – that fundamental law of urban physics.

A classic example: Tauranga.

For the last few decades, lots of public money has gone into building bigger roads in Tauranga to ease its growing congestion.

Lots of money.

And very little has gone into any other way to get around (this Regional Land Transport Plan is typical).

Their public transport is still pretty feeble.
Cycling only happens in special little off-road oases (to which locals invariably drive, bikes on the car, cos it’s not safe to get there by… actually cycling).
A TW staffer’s friend Rosie who walks to work (it’s a pleasant, flat, 20 min walk) regularly gets people pulling over asking if she’s OK: “Are you sure love? You’re just… walking along!”.

It’s a real choice desert up there when it comes to transport, and they’ve got more and more big, 70- and 80-km/hr motorway-grade roads.

And yet, heartbreakingly, induced demand and car-centric suburban growth keep wiping out the benefits of the expensive expanded roads, in just a few short years. They’ve got more road space, but it’s just filled up again with people driving in cars.
Bugger.

Now, finally, the penny’s dropped in Tauranga.

You can’t, in fact, build your way out of congestion.
More road space will not make driving better; in fact it’ll do nothing to help while ruining the quality of life in your town.


So good on Tauranga for stopping denying urban physics. But now they have to ‘fess up to the public that “we’ll make it better by building expensive roads, folks!” was a crock all along. Awkward…

They’ve started the process with these nifty videos. Good luck to ’em!
We like this one…

Auckland, too, has for decades been principally using “build more road space” to deal with traffic. Now they’re grappling with being Off-Ramp City.

Given all of this, we’re flummoxed why Let’s Get Wellington Moving was – when last seen – still talking about widening roads in our inner city to ease congestion. Yes, really.

And yet despite us having overwhelmingly funded private car driving versus all the other ways to get around, people in Wellington are raring to drive less. Lower-than-predicted growth in traffic (fewer people benefiting from a faster driving trip) was a big reason the Inner City Bypass has failed to be worth it, on its own terms.

So we’ve got a lot going for us. Let’s hope we in the Wellington region can learn from other cities’ mistakes.

One comment on “If you build it, they will come: bigger roads mean more traffic”

  • luke says:

    Wellingtonians seem entranced with the 1960s motorway building suburban sprawl model. Any proposal to look at alternatives is met with anger and outrage. Are our change resistant dinosaurs particuarly vocal compared to more progressive cities?

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