A widely accepted definition of community severance is how the process by which the way a city is shaped leads to reduced access to goods, services and people. These obstacles are typically transportation infrastructure — notoriously highways, but also rail lines and even parking lots.

Transportation infrastructure is supposed to make us all more mobile, but it is becoming clear that not everyone benefits from the dividends of that mobility. Also known as the "barrier effect," the idea of community severance is gaining currency as our cities speed up, but our community bonds dissolve.

Physical barriers

Once, making my way on foot across the Georgian capital of Tbilisi, I arrived at a riverfront road with a rush of cars flying by. After five full minutes of waiting for a gap to be able to cross and continue my journey, I gave up the fight and hailed a passing taxi.

We have all encountered barriers traveling through the cities we live in, particularly when on foot or by bicycle. Have you ever tried to walk the circuitous route to cross an industrial estate? Or tried to bike to an out-of-town mall? You are likely to get the distinct impression that this environment was not designed for your two feet or two wheels.

Severance can particularly affect the elderly and children. Older urban residents often don't make the necessary speed of 4 feet per second required to complete a pedestrian crossing designed for the more mobile people in our cities. Meanwhile, children now have more restrictions than ever on their local mobility and capacity for play on the streets outside their home, leading to serious public health concerns.

Social barriers

We are used to hearing statistics on the links between traffic and road accidents. But social rather than physical harm first came to the attention of urbanists in the 1970s when Donald Appleyard published his book "Livable Streets."

Appleyard compared three streets in San Francisco one with heavy traffic flows, one moderate and one light. Using a set of fascinating graphics documenting social ties, he showed that the heavier traffic on roads outside, the fewer friends and social contacts residents had.

Thus, the concept of community severance was born. As the video below shows, for the first time there was a coherent argument demonstrating the "invisible harm" done by traffic that disrupts communities rather than knits them together.

Barriers in the mind

Most interesting perhaps is that the "barrier effect" speaks not only of physical but psychological barriers. Psychological barriers can take the form of perceived hazards for children, or the availability of walkable green streets.

One of Appleyard's other findings was that the presence of heavy transportation infrastructure outside your door can even influence how you perceive and describe your "home territory," which shrinks when severed by busy roads.

How to combat severance

So what to do in the face of this growing evidence?

One of the favored solutions in the cities of the modernist 1960s was to simply provide passage across multilane streets by building (costly) underpasses and footbridges. But more recently that approach has come under criticism.

The Street Mobility Project at British university UCL has done a lot to explore the impacts of severance. One of their findings is that most people prefer to walk further to a crossing with pedestrian lights rather than use a subway or footbridge.

Instead, new road design standards are being used to help reduce the social impact of transport infrastructure. Movements like Complete Streets and Green Streets are designed not only to accommodate multiple modes (cars, bicycles, pedestrians) but also to restore the social function of a street, so persuasively described by Jane Jacobs in the 1960s.

Redesigining streets in this way can build metaphorical "bridges" among communities, rather than physical ones.

The challenge ahead

Cities today are vying to become more walkable, pedestrian friendly, more "green." But it is not only for environmental reasons that mobility obstacles can impact our quality of life.

Under the pressure of urbanization, our roads may be widening, but it appears our social horizons are narrowing. Perhaps this is part and parcel of the modern condition. Or perhaps it is something that those in charge of our cities can do more to understand and address.