New study finds that road closures can alleviate congestion in dense urban areas

By Chet Edelman

Historically, transportation policy addressing vehicle congestion has entailed increasing road capacity—whether it be through widening existing roads or building new roads altogether. However, research consistently reveals that these policies have the opposite effect. Rather than alleviate congestion, adding road capacity only creates additional demand, leading to almost no improvements in traffic flow. Even if the total number of cars on the road were to be held constant, the addition of a new road in some instances may actually lead to worsening traffic. In fact, a new study reveals that cities may be able to improve vehicle travel times by closing certain road segments completely. Using the theoretical framework of the Braess Paradox, the study’s researchers model how blocking off selective streets in downtown Winnipeg can reduce overall vehicle travel times, a change which in turn enables new car-free spaces to be reclaimed as parks or pedestrian plazas.

Braess’ Paradox, a theory first hypothesized by German mathematician Dietrich Braess in 1968, states that traffic flow in some instances can actually be worsened when a new road is built. The basis behind the paradox is that while a new road may initially provide drivers with a shortcut to their destination, eventually more drivers will act in their own self-interest, leading to a traffic flow equilibrium in which travel times are actually longer than before. The solution, in this case, is to close off the road and restore previous conditions.

Across the world there have been several examples of this phenomenon in action. In New York City, for example, the closure of Broadway through Times Square not only led to overall reductions in localized vehicle congestion, it allowed the street to be reclaimed as a popular pedestrian-only plaza.

While the Braess Paradox is proven to occur in real-life, identifying where exactly it may be applicable is challenging. Shutting off a single street creates ripple effects that can influence traffic circulation and demand across an entire road network. To account for these secondary and tertiary effects, the study’s researchers developed an algorithm that simulates the resulting traffic flow and demand from different combinations of street closures. Using the downtown Winnipeg street network as a real dataset, the researchers found that by selectively blocking certain streets in the heart of the city’s central business district, overall vehicle travel time in the area actually decreased.

The benefit of such road closures extends beyond just improved traffic flow. As is the case with Broadway in New York City, car-free roads have the potential to be redesigned as green spaces, parks, or vibrant walkable areas. Considering how many urban areas lack adequate access to parks and public spaces, reclaiming selective roads helps to chip away at this underlying issue. By identifying roads to which the Braess Paradox is applicable, policymakers can get the best of both worlds with improved vehicle circulation and additional public spaces—an outcome that would be unlikely through conventional road widening.

Chet Edelman is a Project Assistant at SSTI.