Three elements to identify community severance

Post by Dr Paulo Rui Anciaes
An example of a dual carriageway as a barrier.
An example of a dual carriageway as a barrier.

The previous post talked about strategies for identifying suitable cases for studying the problem of community severance.

There is no consensus among practitioners and researchers about suitable indicators to assess the effects of transport on inter-community mobility. This is because the concept of community severance is hard to define. This differs from the case of other local impacts of transport, such as air pollution and noise, which can be assessed with objective measures.

This post suggests three elements for a tentative definition of community severance. As our project develops, other elements will be introduced, in order to arrive at a formal definition of the problem, which can be used as a foundation for future work to assess severance impacts and to test interventions.

These elements are:

1. Existence of barriers to the mobility of pedestrians

2. Mobility needs of the population living nearby

3. Characteristics of the population affected

The concept of community severance usually refers to the barrier effect of linear transport infrastructure such as roads and railways on the mobility of pedestrians. Bicycle or bus traffic may also be affected by this infrastructure and can be included in an extended definition of severance. The concept can also be used to describe the effects of other transport and non-transport infrastructure, such as airports, ports and industrial areas.

Motorways and railways are an absolute barrier to the movement of people, as they limit the number of points where pedestrians can cross.  However, busy roads can also be a barrier, although less severe. In this case, pedestrians can cross in a relatively large number of locations, but the characteristics of the road may have a negative impact on pedestrian safety or be perceived as intimidating or unpleasant.

The magnitude of this barrier effect can be assessed by traffic data, such as total number of vehicles, percentage of heavy goods vehicles (HGV) and average speeds. It can also be assessed by the characteristics of the road (such as number of lanes and lane width) and by the number and type of pedestrian crossings. The change in the infrastructure and in traffic levels over time is also important.

To assess the barrier effect on pedestrian flows, we also need to consider the mobility needs of the population affected, that is, people living on both sides of the road. People working living in the affected area, but working or studying there may also be considered. The unfulfilled mobility needs due to the presence of barriers are the potential trips to certain destinations, such as workplaces, shops, other amenities, stations and bus stops. An assessment of the severance effect must then consider the location of these places in relation to residential area. Additional information, such as school catchment areas, may also be required.

The characteristics of the population living (or working) on both sides of the road or railway are also relevant. This is because some groups are especially vulnerable to losses in local mobility. This is the case of the elderly, children, and car-less households. The analysis of the demographic and socio-economic characteristics of the population living (or working) in a neighbourhood is therefore an important step to identify cases of community severance.

These three elements provide only an initial, partial, view of the severance problem. They were selected because they can be easily measured and mapped, using readily available data, from traffic counts, observation of aerial images or “street view” websites, and demographic and land use maps.