Crowd Science

Module 7, lesson 3 · 50 minutes

Capacity: the Green Guide method

How a sports ground’s safe capacity is calculated: four separate capacities, two factors that discount for condition and management, and the rule that the lowest number wins.

You’ll be able to

  • compute holding, entry, exit and emergency evacuation capacities and take the minimum;
  • apply the P and S factors correctly;
  • explain where each number in the method comes from, and how far to trust it.

Introduction

A terrace has 1,000 m² of standing space. The Green Guide’s maximum standing density is 4.7 people per square metre. So the terrace holds 4,700 people. That answer is wrong twice over.

It is wrong first because 4.7 p/m² is the maximum for a terrace in perfect condition, perfectly managed, and few are. It is wrong again because a terrace is only as big as its exits: however many people it can hold, it can only admit as many as it can empty in time. The Green Guide’s method makes both corrections, and its central rule is one sentence long: the capacity is the lowest of four numbers.

Four capacities, one minimum

The Green Guide computes four capacities for each part of a ground and takes the lowest as the final capacity:

  • Holding capacity is how many people a viewing area can safely accommodate, from its usable area or seats, discounted for its condition and management.
  • Entry capacity: how many can be admitted through the turnstiles in a given time.
  • Exit capacity is how many people can leave through a section’s exit routes within the egress time.
  • Emergency evacuation capacity: how many can be evacuated to a place of safety within the emergency evacuation time.

final capacity = min(entry, holding, exit, emergency evacuation)

The minimum rule is the bottleneck principle of module 1 applied to a whole ground. A chain of capacities passes only what its weakest link passes.

The terrace used in this lesson’s worked example. The exit and turnstile figures follow the SGSA’s published worked example; the area and factors are chosen to match this lesson’s quiz. Schematic, not to scale.

Holding capacity and the P and S factors

The starting point for standing areas is a maximum appropriate density of 47 people per 10 m², which is 4.7 p/m². That figure is then reduced by two factors, each scored from 0 to 1:

  • The P factor scores the physical condition of a viewing area, and the S factor the quality of its safety management, each from 0 to 1; the Green Guide applies the lower of the two. P covers the physical condition of the terrace or seats.
  • The S factor covers the safety management of the area.

The rule that trips people up: the lower of P and S is applied; they are never multiplied. A well-built terrace badly managed is limited by its management, and a well-managed terrace in poor repair by its condition.

D = min(P, S) × 47 per 10 m²; standing = (A / 10) × D; seated = seats × min(P, S)

A
usable standing area, m²
D
appropriate density, people per 10 m²
P, S
physical and safety-management factors, 0–1

Worked example. The terrace has A = 1,000 m², P = 0.9, S = 0.8. The lower factor is 0.8, so D = 0.8 × 47 = 37.6 per 10 m², and the holding capacity is 1,000 ÷ 10 × 37.6 = 3,760. Had the factors been wrongly multiplied, 0.9 × 0.8 = 0.72 would have given 3,384, an under-estimate that would look cautious but is simply not the method.

Entry, exit and emergency capacities

Entry. The SGSA’s worked example rates a turnstile at 660 people per hour. Six turnstiles admit 3,960 per hour, so filling the 3,760-person terrace would take about 57 minutes. Entry capacity is about whether people can get in safely before the event starts, without a crowd building up outside; this lesson’s calculator counts one hour of turnstile throughput and also shows the time to fill.

Exit. As in module 6, exit capacity is width × time × rate, with rates of passage of 82 p/m/min on level routes and 66 on stepped ones, and a normal maximum egress time of 8 minutes. The terrace’s four stairways of 1.5 m give (4 × 1.5) × 8 × 66 = 3,168. A section with 12 m of level exits would give 12 × 8 × 82 = 7,872.

Emergency evacuation. The same arithmetic with the emergency evacuation time, which the Guide sets between 2.5 and 8 minutes, and at no more than 2.5 minutes where the fire risk is high. Routes onto a pitch or other emergency-only routes can be added. If the terrace were judged high fire risk, its stairways would evacuate only (4 × 1.5) × 2.5 × 66 = 990 people in time.

Final capacity. min(3,760 holding, 3,960 entry, 3,168 exit, 3,168 emergency at 8 minutes) = 3,168. The exits bind. The terrace could hold six hundred more people, but it could not empty them in time.

  1. Check the defaults against the worked example: holding 3,760, exit 3,168, final 3,168.
  2. Raise S from 0.8 to 1.0. What happens to the holding capacity, and to the final capacity?
  3. Read the line under the chart. Add the level exit width it suggests (try one level route of that width) and watch which capacity binds next.
  4. Set the emergency evacuation time to 2.5 minutes, as for a high fire risk. Which capacity binds now, and by how much?
  5. Switch to “12 m of level exits” and confirm 7,872. Then tick “alcohol served” to see a council’s reduced rates at work.

Raising S from 0.8 to 1.0 changes nothing when the exits bind. Improving the stewarding of a terrace whose stairways are too narrow raises a number that was never the limit. Find the binding capacity first, then spend money on that.

The method and rates are from the Green Guide 5th edition and the SGSA’s 6th-edition worked example. Entry is counted here over one hour at the worked example’s 660 per turnstile per hour. The “alcohol served” rates of 72 and 56 p/m/min are one council’s interpretation of Purple Guide practice, not the Green Guide.

A seated stand has 8,000 usable seats, P = 1.0 and S = 0.85, and exits giving 14 m of stepped width with an 8-minute egress time. What is its capacity, and which constraint binds?

Holding = 8,000 × 0.85 = 6,800. Exit = 14 × 8 × 66 = 7,392. The holding capacity binds at 6,800 (assuming entry and emergency capacities are higher). Here better management, raising S, would raise the capacity, up to the exit limit of 7,392.

Where the numbers come from

Every number in the method has a history, and a professional should know it.

  • 4.7 p/m² is the Green Guide’s maximum for standing viewing areas (5th edition §2.9), and the 6th-edition worked example uses “4.7 spectators per square metre”. It is a standing, watching density, not a moving one: module 1’s flow lesson and module 2’s thresholds lesson explain why that distinction matters. Still adds a caution: the Guide “fails to highlight that the demographics (individual sizes) need to be factored into the density analysis”.
  • 82 and 66 p/m/min date from the 5th edition (2008), which followed BS EN 13200-1. The 4th edition (1997) used 109 and 73. In Still’s words, the 109 figure “was adjusted DOWN to 82 people per metre per minute to bring the Green Guide – 5th Edition – in line with the rest of the world”.
  • 8 minutes is the normal maximum egress time, which the 6th edition calls the “Zone 2 travel time”; the emergency evacuation time is 2.5 to 8 minutes.
  • 660 per turnstile per hour is the figure in the SGSA’s worked example.

Outside sports grounds, practice differs. One council’s published reading of the Purple Guide plans standing audiences at 2 p/m², uses 82 and 66 p/m/min (72 and 56 where alcohol is served), and sets exit times of 10, 8 and 6 minutes for low, medium and high risk. The same 1,000 m² that the Green Guide could credit with 4,700 people would plan for 2,000.

Still’s rule from the last lesson applies here: “If a system fails a first pass analysis (sometimes called a rough cut capacity analysis) then it should be rejected as being ‘not fit for purpose’.” The Green Guide calculation is a fast, transparent first pass. If an area fails it, no amount of later detail should rescue it.

A promoter wants to use a 6,000 m² showground for a standing concert and quotes the Green Guide: “4.7 per square metre gives us 28,200.” What three objections would you raise?

(1) 4.7 p/m² is a sports-ground maximum before the P and S factors; the lower of the two must be applied. (2) The capacity is the minimum of four numbers, and exits, entry and emergency evacuation must all be calculated; one of them will probably bind lower. (3) For an event outside a sports ground, councils typically plan at 2 p/m² (12,000 here), and Still notes that 4.7 does not account for the size of the people in the crowd.
  • “Capacity = area × 4.7.” That is the starting density before P and S, and only one of four capacities.
  • “P and S are multiplied.” The lower one is applied; they are never multiplied.
  • “Improving management always raises capacity.” Only if holding capacity is the binding constraint.
  • “Stairs run at 73 p/m/min.” That is the 4th edition. Since 2008, 66.