Building your way out of Congestion & why it doesn’t always work

For decades engineers and city planners have known about a phenomenon known as Induced demand that relates to increased road traffic when more infrastructure is put into place.

Based on the simple economic principle of supply and demand the idea is that when you increase the availability to users, the demand will increase to fill that gap. What this usually means is that when you build a new highway you don’t actually reduce congestion you actually increase it because more people will opt to drive their cars on the new road because of the added convenience.

Shortening travel times by building new highways can encourage people to live at greater distances than they would have normally considered for a daily commute, this then creates a situation where they are then dependant on their car for travel and increase the average Km’s driven per car.

So knowing this how can city planners use it to their advantage? well by increasing the supply and convenience of public transport you can increase it’s usage.  The inverse is also true in that closing roads can actually reduce demand. A New York State Department of Transportation study showed that 53% of the traffic which had used the West Side Highway was not displaced, but simply vanished after part of the highway was torn down.

Induced Demand in Australia

Indeed this study looking at Sydney’s M4 motorway found an increase in traffic after the opening of the Highway even after accounting for other factors that may have increased the traffic flow.

While this paper cannot conclude conclusively that the M4 Motorway
had negative economic effects for the people of Western Sydney, it does raise the
possibility that it didn’t generate any significant or positive benefits either.

Induced Demand is a known factor when building new motorways and yet it is still something that appears to to be lacking in the design phase of modern roads in Australia

For major projects, it is recommended that transport modelling and economic appraisal
account for additional trips and redistribution of trips that result from the project.2 These
induced trips can both positively and negatively impact the overall benefits of road projects.
Negative impacts could occur where induced trips lead to road recongestion. This has been
shown to be material in demand modelling undertaken for a major scheme in Victoria. The
proponent intends to refresh its analysis allowing for induced and redistributed traffic in an
update to the business case. Excerpt from the 2014-2015 Assessment Brief of Westconnex

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