What are we talking about when we talk about severance?

Post by Dr Sadie Boniface

On our travels to potential case study sites we have seen the different ways in which busy roads have the potential to sever communities and neighbourhoods.

We have talked a lot amongst ourselves about which types of barriers cause severance (roads, railways, rivers/bodies of water), and also the characteristics of infrastructure that are relevant for severance (see Jenny’s previous blog post). While we discussed at length what these barriers might look like, I’ve been thinking that we haven’t talked about what this might mean for the resultant severance.

This is important for Street Mobility because it will affect the kinds of questions we ask participants when we are trying to measure severance. Participants might not describe their community as being ‘cut off’ as a result of infrastructure, but it might be ‘split in two’ by a busy road . The effects of severance will also not be felt in the same way if there is a different community on the other side of the barrier.

I can think of three kinds of severance that we might see in our case studies.

The first of these is that we might see a community bisected by infrastructure. An existing community could be split by the widening of a road, a new road building built, or other transport changes resulting in more and/or faster traffic. Residents might avoid crossing the busy road, meaning that they do not walk around their neighbourhood as much or see as many people in their community. This could be made worse if most of the shops or facilities are concentrated on the other side of the road. Travelling by bus could also be a problem, as the bus stop is likely to be the opposite side of the road for either the journey there or back.

How a community could be bisected by infrastructure
How a community could be bisected by infrastructure

The second type of severance we may come across is a barrier bordering two distinct communities on either side, inhibiting interaction between people from the two communities. As in the previous example, residents might not be willing to cross the road to access goods, services and people. The barrier could actually emphasise differences between the communities, meaning people are even less inclined to cross the busy road. This also describes what might happen in the first example long-term.

How severance may define the boundaries of two adjacent communities
How severance may define the boundaries of two adjacent communities

The third and final type of severance is a community that is isolated by a combination of infrastructure barriers. A less extreme example would be to imagine a community that can only be accessed by a single route or entrance. The barriers could be different types of infrastructure (as in the picture below), or all the same. In this example, the community itself is not split or divided, it is separated from the wider area, which may hinder access to facilities such as schools, hospitals, or places of work.

How severance might lead to a community becoming isolated
How severance might lead to a community becoming isolated

While I think the first example is unequivocal, there are definitely good arguments for not calling the other two community severance. We’d like to know your views – which of these would you call community severance, and why?


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.

What kind of severance to measure?

Post by Dr Jennifer Mindell

One of our first tricky decisions is what level of severance to study in this first project.  Should we study the obvious, major severance that can be seen easily where a railway line or a busy A road or motorway cuts through an urban or suburban area? This may be so bad that people don’t even try to get across; are resigned to the noise and the dirt from the road; and live their lives on ‘their’ side of the obstruction. It affects their quality of life and maybe their mental wellbeing and even their physical and mental health, but could anything realistically be done to change it?  Are options, such as diversions for pedestrians, underpasses, or bridges no better than not crossing the roads?

Should we look at the more subtle effects on residential streets (what Appleyard calls ‘neighbourhood streets’), where people might expect to live in relative peace, with easy access to their neighbours across the road as well as next door, and where the streets used to be used as social spaces until rat runs and general increases in traffic levels discouraged them?  Appleyard and Lintell also found that people on heavily trafficked streets reporting their ‘home territory’ as only their own home, or even just the rear part of their home.  In lightly trafficked streets, residents often described their whole streets as their ‘home territory’.

There are effects of community severance for people of all ages.  For example, playing and traveling independently outside improves children’s physical and psychological development.  However, because we cannot do everything at once, we are focussing on people aged 50 and above for this first study.  Once we have chosen our case study areas, we will be asking groups of local residents what they think the important issues are, to ensure we develop a tool that measures what they think matters.

What is community severance?

Post by Dr Jennifer Mindell

Community severance happens when a railway line or major road splits a community, so people cannot get to the goods, services, or people they want to.  It can also happen where the speed and/or volume of traffic stop people being able to cross the road easily.  This is probably more common. It can make it difficult to get to workplaces or to schools and colleges. It reduces the number of people that local residents meet in their everyday lives.

These social networks, of friends and family but also of acquaintances and neighbours, are important for health.  Appleyard and Lintell in San Francisco in the 1970s showed that the number of friends and acquaintances local residents had was lower the higher the amount of traffic on their street.1  This is explained very well in this video:

More recently, Joshua Hart showed the same in Bristol.

People with fewer social contacts are more likely to have poor health, to be admitted to hospital, and to die younger. Obviously, people who are ill may be less able to go out and therefore see other people.  But this effect on health is found even if you allow for age (because older people are at higher risk of poor health and also may go out less) and consider only people who are healthy at the start of the study.  In fact, the effect on health is of a similar size to the effect of stopping smoking, which is one of the most effective things to improve health.

In our Street Mobility project, we will be developing tools to measure community severance.  Until we can measure it, it is very difficult to say how widespread a problem it is; what makes it better or worse; what the effects of a local road ‘improvement’ will be; and how community severance affects health.2


   1.   Appleyard D, Lintell M. The environmental quality of city streets: The residents’ viewpoint. American Institute of Planners Journal 1972; 38: 84-101.

   2.   Mindell JS, Karlsen S. A review of the evidence on community severance and its impacts on health. J Urban Health 2012; 89: 232-46.

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