Urban Traffic Congestion: unavoidable but self-limiting

My new book published on 1 September is one in a series of short books on policy and economics topics described as ‘essays on big ideas by leading writers’. My contribution is a critique of the inconsistencies of transport policy in recent decades, which I attribute to the shortcomings of conventional transport economic appraisal in identifying the benefits that arise from investment.  This book, Travel Fast or Smart?, is available both in print and as an ebook from Amazon Books

The House of Commons Transport Committee is carrying out an inquiry into urban traffic congestion. I submitted evidence and appeared at an oral evidence session along with other experts.

Traffic congestion occurs in densely populated areas where car ownership is high. There is too little road space to meet all potential demand for car travel while maintaining reasonable flow. Accordingly, many possible car trips do not take place. People are deterred by the prospect of uncertain delays in congested traffic, so they change the timing of their trip, or the destination (alternative shops, for example), or the mode of travel (rail often faster than a car on congested roads), or decide not to travel at all (by working at home, for instance).

The implication of such suppressed demand for car travel is that measures to lessen congestion may have little impact. A congestion charge, as in Central London, would deter some drivers, but they would be replaced by others able to bear the cost who previously would have been dissuaded by the congestion delay. Increasing road capacity, while generally not feasible in urban areas, is seen by many as a solution beyond. However, this permits more and/or longer car trips, the extra traffic thereby restoring congestion to what it had been – hence the maxim that you can’t build your way out of congestion.

Congestion is self-regulating in the long run. As traffic builds up, speed drops and journey times increase. Some drivers opt for alternatives of the kind mentioned above, and congestion stabilises. Urban Traffic Management systems, which operate through adjusting the timing of traffic signals, help to prevent gridlock. However, congestion can be exacerbated in the short run by temporary blockages such as street works or major construction, and also while adaptation takes place to new permanent structures such as cycle lanes.


There is little that can usefully be done to mitigate urban traffic congestion. On the other hand, the growth of urban car traffic can be contained by not increasing road capacity for cars, and indeed can be reversed by subtracting from such capacity. This is the experience of London Congestion Charging, which returned congestion to past levels and yet traffic volumes are lower than they were – because the Mayors took the opportunity offered by the Congestion Charge to take road capacity away from conventional traffic in favour of pedestrians, cyclists and other uses.