When Marion Terrill, of the Grattan Institute, set out to find out how much commuting times had worsened in Sydney and Melbourne, she discovered something you’ll find very hard to believe. But it would come as no surprise to transport economists around the world.

Everyone is sure traffic congestion has got much worse in recent years. This is only to be expected since Sydney’s population grew at the annual rate of 1.9 per cent, and Melbourne’s rate grew even faster, at 2.3 per cent, between the censuses of 2011 and 2016.

Illustration: Glen Le Lievre

Both cities have grown much faster than the Australian population overall. People are crowding into our big cities, much to the disapproval of many people already living there.

Why are they piling into already-crowded cities? For reasons economic geographers call “economies of agglomeration”. One way for countries to get richer is for their businesses to pursue economies of scale; another way is for businesses and their workers to pursue the gains from agglomeration – a fancy word for piling things together.

There are three kinds of agglomeration economies. They come from matching (in a big city, people are more likely to find a job, while businesses are more likely to find the particular workers they need; there can be greater specialisation), sharing (less idle capacity in, say, car parks, or waiting around for customers), and learning (more workers for you to see and imitate; knowledge and know-how shared face-to-face).

Sharing, matching and learning can occur in two ways. When a lot of firms in the same industry gather in the same city, or just because a lot of people and firms are located together, making the city large enough to justify, for instance, heart and lung transplant centres.

Of course, along with the great benefits of crowding together go the costs of crowding together - such as feeling terribly crowded.

There are more people per square kilometre living in the centres of our big cities than there were five years ago. Sydney’s population density has increased by 23 per cent – and Melbourne’s by a mere 46 per cent.

And surely more crowding means more traffic congestion. But this is where Terrill and the co-author of her report, Hugh Batrouney, found their first strange fact. Between the last three censuses, from 2006 to 2016, there’s been virtually no change in the distance between where people live and where they work, measured as the crow flies.

Next surprise came from the HILDA survey – household income and labour dynamics in Australia – which, among other things, asks people how long they spend commuting.

Although the traffic really is heavier, making trips less pleasant, this doesn’t prolong the time of the trip as much as we think it has.

In the four surveys between 2004 and 2016, for both Sydney and Melbourne there was no change in the fact that a quarter of workers had one-way commutes lasting no longer than 15 minutes. One half of workers had commutes no longer than 30 minutes.

When you take it up to the experience of three-quarters of workers, there was some increase over the years in Sydney, but only a small increase in Melbourne. Other figures, from Transport for Victoria, tell a similar story.

So, we all think the increasing traffic volume is leading to greater delay and, hence, longer commute times, but the best available actual measures of commute times say they’re little changed.

Find that hard to believe? Well, as I say, few urban economists would. Why not? Because it fits well with what they call “Marchetti’s constant”. Marchetti was an Italian physicist credited with discovering the empirical truth that the average time spent by a person on commuting is about an hour a day – 30 minutes each way.

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The amazing truth of this “constant” has been shown by many studies of many cities around the world.

And it fits with another empirical regularity known as the “Lewis-Mogridge position”, formulated by those gents in 1990: “traffic expands to meet the available road space”.

The government notices that traffic is particularly congested on a certain road, so it builds a big new expressway. When it opens, the time taken to get from A to B falls dramatically. But when people realise this, more of them stop travelling to work by public transport and start going by car.

So many people do this that the speed gain disappears within months, even weeks. The time taken to get from A to B goes back to about what it was before the expressway was built.

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The only change is that a higher proportion of workers are able to go by car. The traffic jam is often just shifted to another place on the road network.

Getting back to road congestion in Sydney and Melbourne, how can the gap between what we think has happened and what actually happened be explained?

One possible part of the explanation is that although the traffic really is heavier, making trips less pleasant, this doesn’t prolong the time of the trip as much as we think it has.

But the main explanation – both in Oz and in other countries – is that commuters adapt to the greater congestion.

They take evasive action by moving to a job that’s closer to home, or moving to a home that’s closer to the job. Or they stop going by car and start using public transport.

One thing that really has changed with our bigger cities is more crowded trains and buses.

It’s as though each of us has our own internal, unconscious regulator that draws the line at 30 minutes and, when that limit is exceeded, prompts us to take steps to get travel times back down to where they should be.

Terrill and Batrouney are clear on this: in neither city was enough new infrastructure built between 2011 and 2016 to explain why the huge population growth didn’t lengthen commute times.

The government didn’t fix it, you and I did. Which says we ought to be wary of thinking the obvious – and only - solution to greater crowding is great spending on transport infrastructure.

Ross Gittins is the Herald’s economics editor