Traffic Evaporation: What Really Happens When Road Space is Reallocated from Cars? - The City Fix


Road development throughout the 20th century was based primarily on the premise that more infrastructure eases traffic. But evidence shows that road building, instead of reducing congestion, actually increases traffic. When travel time by car is reduced and convenience increased, coupled with the appeal of the private vehicle as a continued indicator of wealth and standing, people are inclined to make more car trips. A recent working paper by researchers from the University of Barcelona, using data from 545 European cities from 1985-2005, confirms that capacity expansion efforts over two decades led to more vehicle traffic, not less, and congestion was not relieved.


A reverse effect to traffic generation is the phenomenon of “traffic evaporation”: traffic that disappears when road space is reallocated from private vehicles to more sustainable modes of transport like walking, cycling and public transportation. While traffic evaporation has been well-documented for more than 20 years, most decision- and opinion-makers are still under the impression that reducing car lanes will make traffic worse.


In 2001, researchers Cairns, Atkins and Goodwin published a paper in Municipal Engineer reviewing 70 road space reallocation cases, including testimonials from 200 traffic engineers and planners in multiple countries. The researchers concluded that predictions of unbearable traffic as a result of reallocating space away from private vehicles were, in most cases, alarmist. People adjusted their behavior in ways that traffic models did not accurately predict. When lanes were reassigned from car traffic to higher-capacity modes – sidewalks, bike lanes and bus or rail lanes – traffic issues were less severe than expected, and traffic volumes were significantly reduced.


Naturally, there were strong variations on this effect depending on the local context, background conditions and how the road space reallocation projects were planned and implemented, but the general results were more positive than negative. There was not a traffic apocalypse. Traffic was reduced not only for the roads where lanes were reassigned, but on nearby streets too in most cases. Out of 63 areas, 51 saw traffic reductions ranging from 147% to 0.1%, with an average decrease of 22%. Only 12 cases saw traffic increases ranging from 0.4% to 25%.


Why did this happen? According to the authors of the study, the projects in several cases included new traffic management plans, such as coordination of traffic signals, to make traffic more efficient. But in many cases, individual drivers changed not only their routes but their departure times, “flattening” out peaks in travel times. Other drivers changed their destinations (e.g., shopping in a different location) or consolidated their trips (a concept known as “trip chaining”), shared their vehicles with others or teleworked more often. Several months after the interventions, more people started moving to and working in areas with greater access to modes of transport other than cars, and even developers changed their plans. Road space reallocation seemed to ignite behavior changes and break habits of private car usage that may not have been broken otherwise.


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