Why the “induced demand” argument against roads is no good.

I read a lot of forums and articles about transport policy where most of the people agree with each other. The splintering of discussion into groups that agree is a common and well-understood phenomenon that is amplified on the internet.

Listen up, Melbourne

As well as getting excited by optimal stop spacing, one thing people in these circles agree on is that roads are bad. The most common argument made against roads goes like this:

“Essentially, if you widen roads to reduce congestion, people who were avoiding the road because of congestion will find it more convenient and take more trips, thus increasing traffic again.

So what do you have then? A big expensive project to eliminate traffic, and more traffic.” Streetsblog

“New roads will create new drivers, resulting in the intensity of traffic staying the same.

Mann explains how this counterintuitive reality can possibly be true: “As it turns out, we humans love moving around. And if you expand people’s ability to travel, they will do it more, living farther away from where they work and therefore being forced to drive into town. Making driving easier also means that people take more trips in the car than they otherwise would.” Planetizen

“Research indicates that generated traffic often fills a significant portion of capacity added to congested urban road. Generated traffic has three implications for transport planning. First, it reduces the congestion reduction benefits of road capacity expansion. Second, it increases many external costs. Third, it provides relatively small user benefits because it consists of vehicle travel that consumers are most willing to forego when their costs increase.” Victoria Transport Policy Institute

While I support many of the ideas this argument is rolled out to support, I find the argument itself utterly unconvincing. If you want to argue against investing in roads, you’ll need a better argument. Here’s why.

1. Public Transport will behave exactly the same.

If you widen a road, it will encourage more people to drive on it, bringing congestion to an equilibrium level* on that road, and spilling congestion into other parts of the network.

Likewise, if you increase the frequency with which a public transport service runs, it will attract more people to ride on it, bringing congestion to an equilibrium level and spilling congestion into other parts of the network.

This means that induced demand operates as an argument not to invest in any popular/crowded transport where crowding levels may be deterring travel. I’m not sure that’s what proponents of the induced demand argument intend.

*The equilibrium level is not necessarily the same amount of congestion as before, it’s the level where congestion deters travel.


2. Induced demand is good.

Induced demand means previously people had latent demand for travel, but they were unable to satisfy it. Now they are able to access jobs, get to shops that sell items that match their needs better, visit friends and family more. Any argument about transport investment that objects to people doing more travel should be treated with suspicion, if not contempt.


3. Focusing on congestion is the wrong way to look at transport policy.

If your approach to solving a city’s transport problem is to seek out choke points and jams and try to untangle them, you may end up fighting unwinnable battles against geometric problems. You will become frustrated, growing ever more sure that transport is a zero sum game with no easy answers.

But the answer is not necessarily eliminating every queue – there are planners who believe congestion is perfectly acceptable, a sign of popularity and even of success. (1, 2, 3,)

Forget congestion. If you make your yardstick access, you focus on what people want: What places can I get to?

A rational approach to transport planning in a city would be to measure access: From each address, a sum of all the jobs, services and other addresses that can be accessed, averaged across a time period encompassing peak and off-peak.

That number would be aggregated across the entire city to create a total score. Then, you would provide incentives for bureaucrats to improve that number. These bureaucrats must have more than just transport levers at their disposal.

They should also be able to make decisions on zoning and land-use, and road pricing. Improving access can as easily mean moving jobs to people as people to jobs. If a particular investment is expected to have spill over effects that worsen transport times in a far-flung part of the network, the access measure should pick this up and permit that effect to be compared to its positive local effect.

Build a new school, access improves. Add traffic light priority for buses, access improves for some at the expense of others. Price a road, access improves for high-value trips at the expense of lower value trips. Add a new lane on a freeway, access improves locally and probably diminishes elsewhere.

Focusing on access allows trade-offs and comparisons to be made. But often, improved access will involve turning latent demand for travel into real trips. We shouldn’t object too loudly to that.

The point of a city is that people want access to other people, and services. Let them at them.

More on this topic:

Trevors in Traffic: a PR strategy for congestion charging.

Selling the street: A land use hypothetical…

New competition report tries to go hard on road pricing, but is naive

Could plans London didn’t use be put to work in Melbourne?

This tweet came up in my feed a couple of days ago, with the accompanying pictures.

bridges london ramps londonThe man who tweeted them, Brent Toderian, is a big name in urbanism, based in Vancouver. Of course he is horrified by the way roads and cars dominate the skylines.

But I saw something in these pictures that redeemed them. I think they are possibly quite smart.

The use of apartment buildings as props for very high freeways was the main aspect that caught my eye. It would not be feasible to drop a road on top of existing buildings. But it may work where you take an existing road, build apartment buildings in its space, and then put the road on top of them.

In talking about this plan, I’m especially thinking of Hoddle Street, which is Melbourne’s Achille’s heel. A north-south road near the city that is choked with traffic almost all the time, it has been the subject of countless studies into how to improve it, with little permanent improvement..


What might work is an idea from left-field, like that illustrated by Sir Charles above.

Here are the upsides I see.

1. Urban infill.

If you take Hoddle Street and fill it with high rises that support a freeway, you get a quick bump in inner city density. Furthermore, the land belongs to the government, so they ought to be able to actually make profit on the property development.

2. Better street level traffic.

The street level near Hoddle Street is a horror story. It’s very unpleasant to walk on because of the traffic – noise, fumes, and on the occasional place where they can build up a bit of speed, the fear you’re going to be killed. The houses along Hoddle Street look cheap and poorly maintained – there is obviously a price discount for being on the road. There is very little retail and not a single cafe or shop with outdoor seating on its whole length.

If the roadway was way up in the air, (and a couple of the many many traffic lanes were retained for a more normal scale street), the street level might be a bit more comfortable for actual use. Cyclists could use Hoddle Street again. Shops might open up.

3. Better amenity because the elevation is so high.

The complaint about elevated roads is the same as the complaint about freeway overpasses. They leave dark empty spaces that are a blight on the urban environment. But if you put the road high enough up in the air, the sense of being closed in will disappear. Even better, if the supports for the freeway are not just concrete pillars but actual apartment buildings, the passive surveillance of these areas would be much better.

I would imagine that top-floor apartments, right under the roadway, would be less attractive as they would get less sun and more rumbles from above. But if they had balconies on the sides the road does not extend, there is no reason they could not be full of light.

The shadow from the entire structure will be vast over a wider area than a smaller freeway, but by having it so high, there will be no single space that is permanently shadowed and unattractive for licit activity.

The obvious downside is the ramping. The illustration shows a six storey building, which would be about 20 metres tall. To descend 20 vertical metres a car needs 120 metres of ramp,( the preferred gradient for public car park ramps is 1:6.)

That would probably have to circle around the building to descend, as per the pictures.. And I’m not convinced the value of the apartments encircled by those exits would be so high. Still, not every building would be an exit, and some exits could ramp away from the buildings.

A more complete set of things being proposed by Sir Charles Bressey 80 years ago can be found here. Nb. I do not agree with turning Trafalgar Square into a multi-storey parking garage. 

China Series Part 4: City-shaping

This is Part 4 in this week’s China Series. You can see the previous parts here: Part 1 Part 2 Part 3 

China has learned many valuable lessons from its growth. Among the biggest: you can’t just respond to demand for a certain kind of transport.

beijign traffic sunset
Too much of a good thing

Beijing built a series of ring roads between the 1980s and today. There are six in the city.

Loads of new tarmac coincided with a boom in wealth. That meant an explosion in car ownership and traffic that got out of control.

Screen Shot 2014-01-23 at 11.48.34 am

China has tried to respond with rules to limit traffic and car ownership, such as quotas. But they have not always worked.

killer smog
Killer smog, 2013

beijing subway interior

But that does not mean Beijing has given up. When I was in Beijing in 2003, there were just three subway lines. Now there are a dozen.

New stations are popping up everywhere like a game of whack-a-mole. On our holiday in 2013, we picked up a subway map (actually it said subwang) at our accommodation, and it was already out of date.

beijing subwang

This is something Australia could learn from. When you build a road to solve a traffic jam, that road will likely last until the collapse of the civilisation it supports.

Sydney’s George St is now over 200 years old. There are just a handful of examples of freeway removal worldwide. A road lasts longer than a building, longer than the technology that uses it, longer by far than the average road engineer.

What you are doing –  in the long run – is not “solving a traffic jam” but shaping your city.

People like to talk about induced traffic from new roads – “if you build it they will use it.” I don’t doubt this is partly true, but I think the long-run effect of a new road is far greater than whether or not you get your traffic jam back within 18 months.

This is why I am so excited about the prospects of improvements to rail networks. They can also last a very long time, and have long-run positive effects, not least of which is discouraging the building of more roads.

train stations beijingBut while Beijing’s improvements are underway (see right), Melbourne’s are just on paper.

The city-shaping effects of an efficient metro system in Melbourne would be huge. But a great deal of political change will have to happen for it to get built.