A Guide to Social Distancing for Essential Businesses
With COVID-19 social distancing measures in effect, food stores and other venues that must remain open are limiting the number of people who can enter inside. In people flow, the number of people supported by an area at one time is known as carrying capacity. Currently, WHO recommends a distance of 6 feet between individuals in order to minimize human-to-human infection virus spread. Hence, at the first glance, one could easily calculate the maximum number of people allowed inside a venue by dividing the venue’s total area by 36 sqft (as this is the “safety box” that must surround each person to meet WHO’s recommendations). However, this calculation uses flawed logic for the following simple reason: Uniform distribution of people is not possible without enforcing where individuals must stand at every moment. Some areas will always be more crowded than others due to demand. For example, taking into consideration a grocery store, the cashier line and areas around high demand items will be more crowded than other places.
Essentially, the venue’s total carrying capacity should be determined by taking into account peoples’ movements between these areas. To do this, the overall capacity of the store must be determined by the area that has the minimum capacity, i.e. “bottleneck area,” a well-known concept in people-flow modeling. In considering these movements to be dynamic rather than static, the total carrying capacity of the space is a time-dependent concept: so, the amount of time people spend in areas will help determine the areas’ carrying capacity.
To sum up, there are two important factors to take into account when determining carrying capacity and bottleneck areas:
1) What percentage of the people visit an area
2) How long people stay in each area
Using the following simplified example of a grocery store, we can illustrate the factors’ effect on carrying capacity.
Example: A grocery store (total area = 5,760 sqft)
The figure below depicts a sample store with four sections (A, B, C and D). The percentages represent the amount of people who visit the section out of the total amount of people who enter the store.
Figure 1A: Visual Representation of the Grocery Store Layout
Cashier Area: 720 sqft, 100% visiting here, with each visitor spending 3 minutes on average
High Demand Area A: 1,440 sqft, 40% visiting here, with each visitor spending 9 minutes on average
High Demand Area B: 1,440 sqft, 70% visiting here, spending 8 minutes on average
Low Demand Area C: 720 sqft, 40% visiting here, spending 5 minutes on average
Low Demand Area D: 1,440sqft, 20% visiting here spending 4 minutes on average
Using 36 sqft per person requirements discussed above, one might assume that the total number of people that could be in this store at the same time would be 160 (dividing its total area by 36 sqft). This figure, however, does not consider the fact that not all customers visit all areas in the store, nor does it account for the duration they spent in the store.
However, if each person takes 3 minutes in the cashier area, and the area can hold at most 20 people at a time (i.e. 720 sqft divided by 36 sqft), 400 people can be accommodated by the cashier area in in one hour (i.e. 20 persons every 3 minutes). Table 1 below shows similar calculations for each grocery store area in the simplified example.
In the table,
· Each area’s capacity is equal to the area’s sqft divided by 36 (space required per person)
· Normalized Avg. Dwell Time is equal to Avg Dwell Time multiplied by the % Visited
· Hourly Output is equal to the Area Capacity multiplied by 60 (minutes), divided by Normalized Avg Dwell Time
So, under the current assumptions, when no panic buying, the bottleneck area is the cashier area, which means it determines the store’s overall carrying capacity. In this case, the store should allow for 400 persons to enter the store every hour. However, health authorities have also advised to minimize the number of people waiting outside. For this reason and taking into account our 400 people/hour cashier line, it can be recommended that that 20 persons be allowed to enter the store every 3 minutes.
However, this number can change, based on the areas of the store that may receive an influx of customers due to changing customer behavior (i.e. stock buying). For example, let’s say that Area C above is where toilet paper is kept. So, 90% of all customers who enter the store start using this area. In this case, the bottleneck that will determine the store’s carrying capacity becomes Area C, as opposed to the cashier area. The overall carrying capacity of the store then drops down to 267 customers per hour, as can be seen in the calculations shown in Table 2 below:
In order to counteract this shrinking of capacity, the store can increase Area C’s capacity to increase the overall capacity of the store. However, if capacity in that area cannot be increased, the store should limit the customer arrivals at 267 customers/hour (or allow in about 26-27 customers in every 6 minutes) to ensure proper social distancing inside the store.
The above calculations are a good approximation for calculating the approximate carrying capacity without using sophisticated models. They can be used with confidence by lay persons with ease. However, a more accurate calculation by considering detailed operational parameters such as the number of cashiers, the layout of each area, and variability of service times between customers is only possible by using a simulation model.
While no one knows how long such social distancing measures will need to be kept in place, this model provides an easy way to calculate the safe amount of customers to be allowed stores. Obviously, these recommendations can be used for any sort of enclosed space, and is not limited to grocery stores. By properly calculating carrying capacities, businesses can help to limit the spread of the virus by preventing over-crowding in small spaces and continue to serve their communities safely.