Canberra’s future: medium density

What are the effects and impacts of our increasing population on traffic congestion in our beautiful city? In Canberra, over 80% of the working population drive to work. That percentage is higher than in any other city in Australia! Assuming this bad habit remains, our continued car culture will heavily impact on already heavy rush hour traffic. A circular city modelled gives us some insight into Canberra’s future.


Congestion within this article is related to the population increasing, and has the unit people per hour per km.

  • Congestion growth is linear with the population growth – it’s a proportional relationship.
  • Congestion is inversely proportional to the distance from the centre of the city. Move from 1 km to 2 km from the city centre and the congestion halves, from 2 to 4 km it halves, again, and so forth. Every time we double the distance we have driven from the city centre, the congestion halves.
  • Finally, congestion is inversely proportional to the duration of the peak period. Double the length of the peak period and the congestion halves.

Congestion rises rapidly as the road’s carrying capacity is approached. Should we all leave for work at the same time, we will overload our roads. In principle, cooperative behaviour reduces congestion. However, our commute times are not coordinated between commuters and peak periods are determined by core working times and personal circumstances. Fear of congestion makes us change our behaviour reactively. As a society, we generally lack the willingness to proactively cooperate to reduce congestion (Tragedy of the Commons).

In ACT Transport traffic modelling, the congestion curve for modelling is shown below. The left axis (vertical) shows the “proportion of free flow speed”. The horizontal axis is the amount of traffic (volume) the road carries in relation to it designed capacity (volume/capacity). When the amount of traffic on the road has reached its designed capacity, the volume/capacity ratio has the value 1 (representing 100% load). So a value of 0.8 means that the traffic volume is 80% the designed capacity.

The “proportion of free flow speed” is very high where the road is carrying less than 80% of its designed capacity. That is what our traffic planners desire. Above 80% the congestion worsen noticeably. At the designed capacity, this curved predicts that 50% of the drivers will experience congestion – either slowing or stopping. With further small increases – a few more hurrying to work – the congestion rapidly increases and the “free flow” rapidly decreases to 10%. Such sudden changes are not uncommon in the real world and known as tipping points (Tipping Point by Malcolm Gladwell).

Source: a recent report of AECOM for TCCS, ACT Transport Corridors Phase One: Regional Social, Economic and Transportation Analysis, ACT Transport Corridors.

The model

This model predicts that with a population of 400,000 people, 1 km from the city centre, 5 arterial roads approaching the city, and a 2 hour peak period, we will see traffic densities on these arterial roads of 5,000 people/hour. If 1 person sits in 1 car, as is currently the case, this is 5,000 cars per hour on each of 5 arterial roads radiating from Civic (a simplification).

What happens when the population of Canberra almost doubles in the next 30 years.  If we wish to keep traffic congestion at the same level, the peak period will have to double. The city in this model has the same footprint (surface area) as today.

When we say we wish to double the population and keep the same urban footprint, we are talking about infill and urban renewal. In our model, infill accounts for 100% of the new developments and, importantly, NO greenfield developments. The ACT Greens suggest this at the last election, however, the current Parliamentary Agreement states targets 70% infill and 30% greenfield developments.

The average commute in Canberra is already approaching an hour (50 minutes). If Canberra’s peak period is currently 2 hours, we have to double that to 4 hours by 2050 to achieve similar congestions levels. Even then, we are assuming that people will spread their commutes evenly over that period AM and PM peak periods – say from 6:00 am to 10:00 am, and 3:00 pm to 7:00 pm. Clearly, this is dependent on Canberra’s employers changing their HR policies! We can expect that more people choose to drive in the middle of AM and PM peak periods and therefore congestion is greater at these times.

The best case scenario for twice the population


  • double the population
  • no new greenfield estates
  • same city (surface) area
  • same road area
  • peak periods twice as long

The situation

This leaves only infill and urban renewal. Cars are kept underground, as parking space is a constant. A square metre space is worth twice what it used to be. Land prices should double. Parking prices should double.


  • population will double in the ACT in the next 30 years
  • double prices of land will make apartments more attractive
  • double prices of parking will make public transport more attractive
  • double duration of peak hour will make active travel more attractive


  • high density and shorter distances make cars less useful
  • bicycles more attractive
  • walking more attractive
  • living close to work more attractive
  • congestion remains about the same
  • commuting done by those who don’t have a choice (real or perceived)

The doubling of the population will see the single family home on the decrease. Canberra in the future will be a medium density city of 3 or 4 storey buildings, with taller ones in town centres and along light rail corridors. This design is reflected in city states.

Paris has changed in recent years and an example of modern city develop – including a growing cycling culture.

Paris. Photo by Shvets Anna on

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