Research Rest Stop │ Ignoring Induced Demand

Every Wednesday, The Mayor’s Office of Transportation and Utilities highlights some new and interesting research in the fields of transportation, energy, or sustainability.


Traffic congestion is often described as sand flowing through a funnel – each grain of sand is an automobile.  One grain has no difficulty traveling through the funnel, but as the number of grains increases, it builds up in the funnel and each grain of sand must wait before it can flow through.  It is the same with traffic; one car on an empty road has no difficulty traveling through, but when thousands of cars are added to the same road, traffic congestion builds up and each car must wait its turn. 


Traffic Congestion in Philadelphia.
Photo Courtesy of The Temple News

The intuitive solution is to widen the road – an extra lane would allow more cars, thus less traffic.  Unfortunately, this will not solve the problem. Widening the road or increasing capacity, causes a phenomenon known as Induced Demand.  In short, Induced Demand means that because a newly widened road has more space, it becomes more attractive to other drivers and before long the new road is filled to capacity and there is even greater traffic congestion. 

Although Induced Demand is not a new idea, two recent studies have shown that many cities do not account for this phenomenon when planning new routes.  As a result, expensive road expansion projects overstate potential time-saving benefits and do not solve the problem they initially address.

What can we do in Philadelphia to help reduce traffic congestion, especially in traffic-prone areas like Center City?  Instead of demanding more space for cars, use public transportation, bike, or walk to your destination and step away from traffic congestion altogether. 


Want to know more? Read the full reports:

Victoria Transport Policy Institute Report

European Journal of Transport and Infrastructure Research Report


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