Proximate and distal causality

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Chapter 3 - Causality (extract)

“Shallow men believe in luck or in circumstance. Strong men believe in cause and effect.” (Ralph Waldo Emerson, Essayist, 1803 - 1882)

"We learn from history that we do not learn from history." (Georg Wilhelm Friedrich Hegel, Philosopher, 1770–1831)

Introduction

In this chapter, we examine causality of crowd related incidents, with specific focus on the reasonably foreseeable crowd risks for planned events. One of the first questions we are asked, during expert witness cases, is “was the incident reasonably foreseeable?” and “at what point in time could the incident have been avoided/averted”.

As we defined in the opening chapters of this book, the event process can be split into three distinct phases, planning, approval and operations. To answer the questions above we need to review the concept of “reasonably foreseeable” in the context of a layperson, an adequately trained crowd manager, a licensing officer/inspector and an expert. 

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Crowd forces

"Crowd forces can reach levels that almost impossible to resist or control. Virtually all crowd deaths are due to compressive asphyxia and not the "trampling" reported by the news media. Evidence of bent steel railings after several fatal crowd incidents show that forces of more than 4500N (1,000lbs) occurred. Forces are due to pushing, and the domino effect of people leaning against each other.

"Compressive asphyxia has occurred from people being stacked up vertically, one on top of the other, or horizontal pushing and leaning forces. In the Ibrox Park soccer stadium incident, police reported that the pile of bodies was 3m (10ft) high. At this height, people on the bottom would experience chest pressures of 3600-4000N (800-900lbs), assuming half the weight of those above was concentrated in the upper body area.

"Horizontal forces sufficient to cause compressive asphyxia would be more dynamic as people push off against each other to obtain breathing space. In the Cincinnati rock concert incident, a line of bodies was found approximately 9m (30ft) from a wall near the entrance. This indicates that crowd pressures probably came from both directions as rear ranks pressed forward and front ranks pushed off the wall.

"Experiments to determine concentrated forces on guardrails due to leaning and pushing have shown that force of 30% to 75% of participant weight can occur. In a US National Bureau of Standards study of guardrails, three persons exerted a leaning force of 792N (178lbs) and 609N (137lbs) pushing. In a similar Australian Building Technology Centre study, three persons in a combined leaning an pushing posture developed a force of 1370N (306 lbs). This study showed that under a simulated "panic", 5 persons were capable of developing a force of 3430N (766lbs)."  From Fruin Causes and Prevention of Crowd Disasters

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During investigation for a number of expert witness cases I've work on there is typically a need to differentiate between the Proximate (immediate) and the Distal (underlying) causes of the incident. 

Causality is a complex legal issues and there needs to be a clearer definition of the crowd dynamics, in the context of crowd related accidents and incidents. For example the definitions of "reasonably foreseeable" and "competent person" performing the risk assessment, approving the site, signing off on the safety aspects of a major event. As there are no standards for training, how should we define a “competent person”?

For the purposes of further discussion here are some definition I've used to differentiate types of causality and competence.

A proximate cause may be defined as an event which is closest to, or immediately responsible for causing some observed result. This exists in contrast to a higher-level ultimate cause or distal cause, that may be thought of as the fundamental reason something occurred. From various references including the Stanford Encyclopaedia on causation in law: 

Proximate cause: The most direct, effective or substantial cause of an accident or incident; relevant where the negligence of more than one person contributed.

Distal causeThe larger context in which individuals carry out their actions. For example instances in which an agency substantially increases the probability of harm by conduct that falls below the standards of behaviour established by law for the protection of others against unreasonable risk of harm. A person has acted negligently if he or she has departed from the conduct expected of a reasonably prudent person acting under similar circumstances.

From wikipedia.

Causation in English law concerns the legal tests of remoteness, causation and foreseeability in the tort of negligence. It is also relevant for English criminal law and English contract law.

In the English law of negligence, causation proves a direct link between the defendant’s negligence and the claimant’s loss and damage. For these purposes, liability in negligence is established when there is a breach of the duty of care owed by the defendant to the claimant that causes loss and damage, and it is reasonable that the defendant should compensate the claimant for that loss and damage.

Reasonably foreseeable

If we look to the proximate and distal causation in major incidents (mass fatalities) there are three stages to consider; the planning stage, the approval stage and the operations stage. One could argue a case as "reasonably foreseeable" for many major crowd related incidents if we can test the design using standard flow/capacity calculations. We call this a first pass approximation, or rough cut capacity analysis. 

For example, Fruin (1993) outlined all the basic principles for understanding risks to crowds. From Fruin’s “Causes and Preventions of Disasters” published in “Engineering for Crowd Safety” 1993:

“Crowds occur frequently, usually without serious problems. Occasionally venue inadequacies and deficient crowd management result in injuries and fatalities.”

In the same paper Fruin describes “The view from the crowd”:

“(...) When crowd density equals the plan area of the human body, individual control is lost, as one becomes an involuntary part of the mass. At occupancies of about 7 persons per square meter the crowd becomes almost a fluid mass. Shock waves can be propagated through the mass sufficient to lift people off of their feet and propel them distances of 3m (10 ft) or more.” 

Later, in the same paper, Fruin describes the forces involved in crowd related disasters.

“Crowd forces can reach levels that (are) almost impossible to resist or control. Virtually all crowd deaths are due to compressive asphyxia and not the "trampling" reported by the news media. (...) Forces are due to pushing, and the domino effect of people leaning against each other (...) Horizontal forces sufficient to cause compressive asphyxia would be more dynamic as people push off against each other to obtain breathing space.”

Again from the same paper under the title “Prevention of crowd disasters by crowd management” Fruin states:

“Most major crowd disasters can be prevented by simple crowd management strategies. The primary crowd management objectives are the avoidance of critical crowd densities.”

Under the heading “Movement Pathways” Fruin states that:

“Arrangements that result in unbalanced use of egress or ingress routes, dead ends, or similar confusing and irregular pathway choices, are not acceptable. (...) Dispersed and equally balanced ingress and egress points are preferred over a single centralized location. The influence of external facilities on the volume and direction of movement must be considered.” 

A balanced ingress (or egress) system would be a system in which the entry and exit points are distributed around a site and not focused on one shared system. 

One can argue that the concepts, described above, are sufficient to define crowd safety and crowd risk analysis in the complex and built environment. Therefore, a reasonably competent person, with little more than the basic principles and applications outlined above, should be able to conduct a crowd risk analysis and define crowd safety for a major event (and/or places of public assembly).

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Some citations on crowd forces

1. Tolerable force against 100m flat bar 623N (Men) up to 800N when they could push back

This is extracted from : "Crowd Pressure Monitoring" by IHG Hopkins, SJ Poutney, P Hayes and MA Sheppard published in "Engineering for Crowd Safety" RA Smith and JF Dickie (Editors) in 1993 by Elsevier.

2. Dangerous Crowd Pressure

“When pushed against a 100mm wide flat bar, the tolerable force was typically 140lbf (623N) for men. When allowed to push back against the bar, to reduce loading on the rib cage, this force increased to typically 180lbf (800N). The tolerable force or women was reported to be significantly less. ....

.... the method was essentially subjective in that it established the discomfort or anxiety threshold of the volunteer.”

2. Crush Asphyxia 6227N for 15 seconds

3. Crush Asphyxia 1112N - if sustained for 4-6 minutes

Also from 

"Crowd Pressure Monitoring" by IHG Hopkins, SJ Poutney, P Hayes and MA Sheppard published in "Engineering for Crowd Safety" RA Smith and JF Dickie (Editors) in 1993 by Elsevier.

4. Dangerous Crowd Pressure

“Firstly, death was estimated to have occurred 15 seconds after a load of 1440 lbf (6227N) was applied. In the second case it was estimated that death occurred after 4 - 6 minutes of applying a load of 250lbf (1112N).

5. 3 people pushing can exert a force of 1370N

6. 5 people pushing can exert a 3430N force

are from “The Causes and Prevention of Crowd Disasters” by John J. Fruin, Ph.D., P.E. United States of America. (Originally presented at the First International Conference on Engineering for Crowd Safety, London, England, March 1993. Elsevier Science Publishers B.B. © 1993)

“Force - crowd forces can reach levels that almost impossible to resist or control. Virtually all crowd deaths are due to compressive asphyxia and not the "trampling" reported by the news media. Evidence of bent steel railings after several fatal crowd incidents show that forces of more than 4500 N (1,000 lbs.) occurred. Forces are due to pushing, and the domino effect of people leaning against each other.

“Compressive asphyxia has occurred from people being stacked up vertically, one on top of the other, or horizontal pushing and leaning forces. In the Ibrox Park soccer stadium incident, police reported that the pile of bodies was 3 m (10 feet) high. At this height, people on the bottom would experience chest pressures of 3600-4000 N (800-900 lbs.), assuming half the weight of those above was concentrated in the upper body area.

“Horizontal forces sufficient to cause compressive asphyxia would be more dynamic as people push off against each other to obtain breathing space. In the Cincinnati rock concert incident, a line of bodies was found approximately 9 m (30 ft) from a wall near the entrance. This indicates that crowd pressures probably came from both directions as rear ranks pressed forward and front ranks pushed off the wall.

“Experiments to determine concentrated forces on guardrails due to leaning and pushing have shown that force of 30% to 75% of participant weight can occur. In a US National Bureau of Standards study of guardrails, three persons exerted a leaning force of 792 N (178 lbs.) and 609 N (137 lbs.) pushing. [9] In a similar Australian Building Technology Centre study, three persons in a combined leaning an pushing posture developed a force of 1370 N (306 lbs.). [10] This study showed that under a simulated "panic", 5 persons were capable of developing a force of 3430 N (766 lbs.).

[9] Fattal, S.G., Cattaneo, L.E. Investigation of Guardrails for the Protection of Employees From Occupational Hazards. Nat. Bur. Stds. NBSIR 76-1139, July 1976, 114 pp.

[10] Horizontal Loading on Handrails. NBTC Tech. Rec 514, Nat.Tech. Centre, New South Wales.

7. 20% compression of the chest cavity can break ribs - this is around 3000N (from medical bone fracture analysis) so we can see that there is a potential for a 5 person deep push to break ribs. 

is from “Biomechanics of Chest and Abdomen Impact” (David C. Viano, Albert I King) 2000 CRC Press LLC (which I’ve attached)

8. Gross Bone Fractures (Pathological and physical parameters) are around 6920N - which is above the concert and football barrier tolerances (5400N)

is from - Patellofemoral Joint Fracture Load Prediction using Physical and Pathalogical Parametres” Proceedings of the 1998 SAE International Congress and Exposition. SAE 980358 by PJ Atkinson, CM Mackenzie and RC Haunt.

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Prediction of Human Crowd Pressures (Ris S.C. Lee, Roger Hughes) Accident Analysis and Prevention 2006 Elsevier. Is a collection of the above citations, on one paper.

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