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Congestion and its solutions
Russell Degnan

Yesterday saw another two articles added to the clamor for a solution to West Gate Bridge congestion problem by first, noting that average travel times have almost doubled, and second, putting the blame on the number of new houses in the south-west region. I'd like to begin however, by discussing what congestion actually is, because how you think of it, affects what you think we can do about it.

The standard definition of congestion is when "the volume of traffic on a roadway is high enough to become detrimental to its performance". This implies that the congestion problem occurs when there is insufficient road-space to allow traffic to flow freely. Ergo: build more roads.

Looked at another way though, congestion at a point in the road occurs when the out-flow of traffic is less that the current in-flow of traffic. This is useful because it not only tells you why congestion occurs, but also why the speed drops so substantially. To explain further, under normal conditions the out-flow of traffic will equal the in-flow:

No problem. However, when the out-flow becomes less there is an issue. This can happen for any number of reasons: more traffic, an accident at that point of the road, a persistent bottle-neck (such as the West Gate Bridge) or a lane merge, or because of a subtle slowing of the traffic (people slowing down to merge with an on-ramp, or gawking at something on the side of the road).

Regardless, what occurs is a queue, cars at the front still leave at the standard flow-rate, cars arriving later must wait, reducing traffic speed at that point. This is why there is a substantial drop, instead of merely a minor slow-down. Cars are waiting for others to clear that point.

The road will remain congested until the in-flow is once again less than the out-flow. It will then begin clearing from the front, but takes time for the traffic to regain speed. Thus, even a short and small initial slowdown in traffic flow can cause a queue and substantial congestion.


What matters therefore is not the volume of traffic on the road, but the relative flows on either side of a congested point. This makes a solution much more complex. Consider for the West Gate Bridge:

1. Increase road space by building a tunnel or bridge

At the point of congestion the traffic flows equalize again, and the time becomes a constant (the distance travelled / the speed of movement). However, by doing this the in-flow to the roads after that point have increased substantially. And in this case, those points are the city itself. All this achieves is a movement of the queuing point to nearer the city centre.

2. Decrease the speed or the traffic leading to the bridge

People do seriously propose this, so it is worth considering; often by placing traffic lights on on-ramps. This time, the in-flow of traffic has been reduced instead of increasing the out-flow. The bridge is no longer congested; instead, the roads leading to the freeway form queues. Theoretically better? Certainly cheaper, but you can do the maths yourself.

3. Do nothing

But... it's congested. Correct, but roads -- transport generally -- is affected by the 'equalisaition' principle. We choose to travel by a method and to a destination based on the time, the cost, the convenience and the comfort (generally in that order). Oddly enough, for most people, the time spent commuting tends to be quite constant -- around 30min. This is why a large increase caused by congestion causes such angst; people have decided to buy houses based on their ability to get to work in 30min. When that jumps out to an hour parliamentary seats change hands.

However, as Hayek would gladly tell you, the high or low price of a product sends a signal that you should (or can) use less or more of that product. Changes in the amount of time spent travelling send the same signal: a longer period says to find either another destination or another method of travel; a shorter period says that this is a good way of travelling and you should do more of it (the Induced Demand Effect). Over the long-term -- in which we are all dead -- roads will find a natural level of congestion, and land-use and travel patterns will reflect the time people are willing to spend travelling to places. As argued here and elsewhere, suburban sprawl is generally the result of faster travel over greater distances.

Therefore, any changes to the speed travelled eventually find the equilibrium of what people will pay. Roads (and railways, and for that matter, cycling paths) will naturally congest unless you have so many of them that you couldn't conceivably fill them.

4. Do something

Government policy is not tuned to doing nothing however, and so the one final solution is to try and tempt people on one of the other travel factors. Public transport advocates love talking about convenience and comfort but as Banister says in the book Transport Planning:

"No matter how attractive public transport is, no matter how close facilities are located to the home, no matter how expensive petrol is, people will still use their cars."

London's congestion charge however, has comprehensively proved one thing: cost considerations will kick in for some people -- and by some, we mean enough to reduce the in-flow of traffic to a congested road or area -- if you make the charge high enough, and if the money is used to fund an alternative that is vaguely competitive with the car time-wise.


With the possible exception of Singapore, who've been quietly applying these principles since 1975, congestion charging has been fairly ad-hoc and problem specific. Large headlines screaming about road chaos and expensive but ultimately futile public/private works projects need to be reassessed. The principles of economics will always apply even if the relevant factor is normally time, not cost. It would be nice if transport planners took even the slightest bit of notice before creating said chaos.

Planning 4th October, 2005 15:04:35   [#] 

Comments

Jeffrey Smart will be pleased
If we build more roads (eg. overpasses and raised ring roads) he will have more to paint!
BridgeGirl  5th October, 2005 09:23:30  


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