Social Distancing Part 1: Space vs Distance
Social Distancing followed 3 key pillars in the government advice. Paraphrased, these are:
stay away from people all together;
do not gather in groups; and
if you do have to be with people, stay at least 2m from them.
Now that lockdown measures are starting to be relaxed in England amongst other countries, the emphasis starts to fall on the last point of this advice rather than the first.
Crowd Dynamics are studying how to analyse social distance and what guidance could be provided to try and facilitate a socially distanced society as we transition away from lockdown. On the face of it, this seems easy to achieve. However, there are some fundamental misunderstandings of the space around people that would easily create situations where social distancing is broken.
Some weeks back (when we were now allowed to sunbathe), the media reported on Brockwell park closing. It was because 3,000 people were in the park and were sunbathing. People pointed out that because Brockwell Park is 50.8 hectares, 3000 people would have 169 square metres per person. Obviously, this is a simplified approach, but it serves as a good example of why calculating the space per person (or small group) is misleading:
169sqm per person would seem like a lot of space to anyone when we are supposed to stay 2m apart. However, some simple analysis shows why this is misleading. This is what it looks like:
169sqm per person spread evenly across a space
So in our example, there is 13m between any 2 people. The 169sqm is already shown to be misleading: it is distance that is important, not area or space per person. We should compare the distance between people, not the space they have.
But 13m is still a lot bigger than 2m right? If you were to allow people to be exactly 2m apart from one another, as soon as anyone moves, social distancing is broken. So clearly you need more distance between people to allow for movement.
If we consider the time it would take someone to break the social distance, we see that it would take 8 seconds walking before social distance is broken. If both people move, it is half this time - only 4 seconds:
Time to break social distance when 13m apart if one person moves
Time to break social distance when 13m apart if two people move
So whether we mention space per person, distance between people or time skews how we view the chance that social distancing may be broken.
Buildings, shops and public spaces are coping with the social distancing by limiting the number of people entering and it is now normal to have ground markings from the front door back as to where a queue should form. It is easy to keep to >2m apart in a single line queue. This is because there is only one distance to think about: between you and the person in front. As soon as these people enter the space, they are put into a space where they must now judge distance in lots of directions, quickly reducing the guarantee of maintain social distance.
Lowering the number of people inside a space reduces the probability that social distancing may be broken. But with current standards of measurement, it is nearly impossible to judge how many people should be in the space and what the probability of breaking social distancing measures actually is.
We need a better measure of social distancing, which is being developed by Crowd Dynamics. Watch this space…