I have argued that Australia will see high population growth in the coming decades and that we must prepare our cities for that change now.

I made the point that decentralisation and densification must be priorities. I also kept banging on about the need for infrastructure upgrades. We can be pretty darn certain that your city, your local government area and your suburb will look different in a decade than it does now – remember that in forecasting we will spend most time on how to adapt to the most likely scenarios.

For this week’s column, we are looking at a cool Italian physicist to learn about the future of traffic and congestion in our Australian cities.

There is no specific population size that dictates when a city relying on a single central business district (CBD) runs into problems. Smaller cities get away with a single CBD and a few industrial sites. Traffic flows reasonably well. Obviously, the geographical layout, the availability of different modes of transit, levels of density, and economic specialisations of cities impact how long the single CBD model works for a city.

As a city grows, as more jobs get squeezed into the one and only CBD, as the existing traffic network reaches its limits, as house prices near the city centre skyrocket and force people to the urban fringe, the population becomes dissatisfied. Life was better in the past! Traffic is a nightmare! We must stop population growth!

Hail Cesare!

This tipping point for the transport network was probably reached in Melbourne and Sydney at somewhere around four million residents – let’s not argue about the exact point in time when traffic started to suck.

In the opening paragraphs, I promised you a cool Italian physicist. His name is Cesare Marchetti. He died in April this year at age 95, but his Marchetti’s Law (or Marchetti’s Constant) will live after him.

Here’s Marchetti’s insight: Throughout history and across the world, the average human was willing to travel for about an hour per day (regardless of the mode of transportation). We choose our place of residence with the goal to maintain a reasonable commute time of right around an hour.

Global time use data shows that the magical one hour mark is surprisingly universal. It’s hilarious, how even people living two hours from a CBD try to adhere to the one hour per day travel time by only commuting to the office once or twice per week (resulting in an average commuting time of an hour per day). The amount of time we are willing to commute is about an hour as long as we have any choice in the matter.

The paradoxical element of Marchetti’s Law is that it is near impossible to beat the issue of traffic congestion.

Let’s say your commute takes 30 minutes each way. Assume your city adds sparkly new roads, fresh rails, widens to eliminate traffic bottlenecks, even introduces jetpacks and flying cars. Turns out you would still spend an hour in traffic each day.

Hobble the sprawl?

As transportation technology improves and travel speeds increase, people are willing to move further away from the job centre to maintain the one-hour commute while being able to afford better homes. Intuitively we measure our commute in minutes rather than kilometres.

Rather than reducing commuting time, we collectively choose to live at greater distances from our workplaces when faster modes of transportation become available, and our cities keep sprawling. This puts more people on the roads and into the trains that carry them to the CBD. All of a sudden we are back to longer commuting times…

One solution would be to stop a city from growing, or at least to slow growth to a trickle, and then invest in traffic infrastructure. However, a stagnant city quite quickly turns into an aging city. This brings a lot of challenges with it, especially if the older population wants to enjoy decent access to medical services, carers, hospitality, and entertainment services.

The negative effects of an aging population are numerous, and traffic would be the least of this city’s problems.

Such a city would see decreased economic productivity, increased demand for healthcare, without economic growth pensions would be stagnant at best, and the whole social security system would need to shrink. Public finances would be strained, leading to higher taxes, or reduced public spending on other essential areas like education or infrastructure.

This wouldn’t be attractive for the much-needed younger workers and they would look for more attractive locations. The resulting general skills shortage would lead to a massive shortage in the aged care sector. Plenty of problems that would be much bigger and more urgent than fixing a bit of traffic.

Australia, in my humble opinion, is almost certainly going to continue countering the negative effects of our aging population by continuing to take in a large number of migrants. This means we must rethink which parts of our national city network should grow faster than others.

We learned that human-centric urban development allows for all workers traveling to the CBD to do so within 30 minutes. That’s partially where the whole talk about 30-minute-cities comes from. If too many people fall out of this 30-minute commuting zone they become unhappy.

A big city cannot keep growing while relying on a single CBD without making life less desirable for its population. Retrofitting a single-CBD city into one with multiple functional CBDs is an expensive and long process – just ask Melbourne and Sydney.

Go bush, young families

It would therefore be preferable to channel population growth into secondary cities (Adelaide and Perth) that can take more population for now without massive needs for infrastructure investments. Smaller satellite cities will need to grow at much higher rates than they currently are.

This means councils need to do their part in terms of rezoning to get comfortable with medium-density housing, they need to make more land available while protecting valuable and limited farmland, and they need to not be frightened by local NIMBYism. The states need to increase their investments in regional infrastructure.

In the big cities struggling with traffic at the moment (Melbourne and Sydney), we need to limit the number of jobs located in the primary CBD while increasing housing stock in the primary CBD. This would guarantee job growth in the secondary CBDs and minimise the number of people commuting to the primary CBD. Local residents who walk to work are a dream for urban planners.

Traffic between the secondary CBDs and the primary CBD must flow for the system to work (there is still regular collaboration happening between job clusters) – we are talking about outrageously expensive infrastructure here. Melbourne for example needs to build a Suburban Rail Loop into an existing city.

This is not just expensive but hard to pull off politically because tens of thousands of residents will be opposed to having a railway plonked into their backyard – which leads to endless delays in providing this much-needed piece of infrastructure. Best to add a rail loop before it impacts too many people.

The opportunity to get traffic infrastructure right from the get-go is higher when we are deciding to build out secondary cities now.

Whatever we do though, don’t expect any of the massive traffic investments to shorten your personal commuting time, Marchetti’s Law takes care of that.

Demographer Simon Kuestenmacher is a co-founder of The Demographics Group. His columns, media commentary and public speaking focus on current socio-demographic trends and how these impact Australia. Follow Simon on Twitter, Facebook, LinkedIn for daily data insights in short format.

This story first appeared in our sister publication The New Daily.