Avoiding entrapment and burn-over
By Jim McLennan, Bushfire CRC Safe Decision Making and
Behaviour Project, La Trobe
University
Many wildland firefighter deaths and injuries in Australia and
overseas have resulted from firefighters being surprised and
overtaken by a fast-moving fire— both on foot and in
vehicles, in both grass and forest environments.
If the problem is defined as one of failure to accurately
predict the time taken for a fire to travel from location A to
location B, then there are two aspects of the problem. First, the
fire may have travelled faster than anticipated (underestimation of
its speed). Second, the fire may have been closer than expected and
had less distance to travel (overestimation of distance). And
finally, it may have travelled faster than anticipated AND had less
distance to travel.
Human Visual Perception: Size, Distance, and Movement of
Objects
Unlike bats, whales and dolphins (which use sound waves to
located objects in space) we humans cannot sense directly how far
away an object is, although we mostly behave as if we can directly
perceive distances. In fact, we have to (unconsciously and
automatically) construct judgements of distance by making
inferences that draw upon a combination of visual cues and past
experiences.
In the natural world, humans see by
means of light reflected off objects. This light enters our
eyeball through the pupil at the front of our eye, passes
through a focussing lens, and is projected onto the retina at
the back of the eye. From there, nerve impulses travel to
particular areas of the brain for processing to construct what
becomes our picture of the world in view. Note that while the
world we live in is three-dimensional, what is on our retina
is a two-dimensional image. Our problem area of interest,
speed and distance of a fire, is problematic precisely because
we must use a two-dimensional image to construct a
three-dimensional understanding of what is happening.
When judging size and distance of near objects we can take
advantage of having two eyes, making use of binocular
processes (accommodation, convergence, and retinal disparity).
However, at the distances involved in estimating wildfire speed and
distance, these binocular processes are not relevant. The only
direct information about a fire comes from the image of the fire
and associated objects (vegetation, terrain, structures) on our
retina. We use change in the size of retinal image in combination
with other cues to estimate speed of motion. We use size of retinal
image in combination with other cues to estimate size of an object
and distance from the object. Familiarity with the area and
visibility of features of known size or distance assist speed and
distance estimation greatly. This article assumes such assistance
is not available.
Underestimating How Fast a Fire is
Travelling
There are two issues here. First, estimating the speed of
advance of an approaching fire. Second, estimating the likely speed
of forward spread of a fire, given the wind, fuel, and terrain
slope conditions.
The task of estimating the speed of an approaching fire is
extremely difficult. The reason for this is that we rely on the
increase in size of retinal image to estimate speed of approach.
While this is not inherently difficult with familiar objects that
maintain a constant shape, such as oncoming motor vehicles when we
cross a road, fires change their shape moment by moment and we are
thus unable to make accurate judgements based on retinal image size
change. The implication is that estimates of the speed of an
approaching fire are likely to be very inaccurate.
In endeavouring to estimate the likely speed of forward spread
of a fire given the wind, fuel, and terrain slope conditions, we
face two challenges: (a) making accurate judgements about the
degree of slope, fuel characteristics, and the wind speed and
direction; and (b) making accurate predictions about fire behaviour
based on these judgements.
Concerning (b), we can note the operation of a linear rate
of change bias—humans (apparently) make predictions based on
an assumed linear rate of change, whereas rates of forward spread
of a fire may change exponentially (Note 1) with small changes in
slope, fuel characteristics, and wind speed/direction.
Dr Mary Omodei and her research team have investigated (b) and
found compelling evidence that experienced firefighters are
vulnerable to a linear rate of change bias, and thus likely to
underestimate speed of advance of a wildfire when slope increases.
There does not seem to be evidence available about (a)
—accuracies/inaccuracies in estimating terrain slope, fuel
characteristics, and wind speed. However, anecdotal accounts
suggest that firefighters pay attention to what could be called
local “average” wind strength and may fail to give
sufficient attention to the strength of wind gusts.
Overestimating Distance from a Fire
It is likely that distance estimates
of fires in forest environments may be very inaccurate. While
our primary cue is the size of the image of the object in
question projected onto the retina at the back of our eyeball,
the same sized image can be produced by a small object up
close and by a large object a long way off. In the present
discussion, a nearby fire with small flame heights can produce
the same sized image on the retina as a fire which is further
away but has larger flame heights.
It is thus quite possible for a relatively near fire with
(currently) small flame heights to be misperceived as a fire with
larger flame heights but further away. Given that small changes in
wind, fuel characteristics, and slope may result in dramatic
increases in fire intensity and activity, such an apparently simple
over-estimation of distance from a fire can have serious
consequences.
In their well-known paper The Dead-Man Zone, Cheyne,
Gould and McCaw (2001) wrote that:
We believe there is a common tendency to overestimate
the distance to a fire when observing through the forest. On
Project Vesta fires, it was nearly impossible to see the flames in
a forest containing a 2 metre scrub layer and an intermediate tree
layer when the fires were 100 metres distant, even when there was
intermittent crowning in the overstorey. When flames were clearly
visible, even experienced observers consistently overestimated the
distance between the flames and some reference point such as the
plot boundary, at times by more than 100%.[http://www.csiro.au/files/files.p1ih.pdf]
If their proposal is correct, presumably there are some
processes by which simple inaccuracy in distance estimation is
transformed into systematic overestimation (Note 2). There is no
evidence available that unequivocally identifies such
processes.
There are some effects known from laboratory research into
visual perception that may be relevant, but it is uncertain how
well they apply to the perception of fires in natural
environments.
1. Interposition, Overlay, and Occlusion
Effects: If an object is partly screened from view
(occluded) by a second interposed, or overlaid object, then the
occluded object is judged to be more distant than the occluding
object. Distant objects are more likely to be occluded than near
objects. This suggests that target objects which are occluded by
many other objects are likely to be judged as being further away
than are target objects occluded by few other objects. In forests,
fires are likely to be occluded by a great many objects—trees
and other vegetation. This may to lead to a fire being perceived to
be further away than it is in reality.
2. Context, Foreground, and Background Effects:
Laboratory studies show that size and distance estimates can be
affected greatly by the context in which an object is viewed.
Examples include Ponzo Illusions and Delboeuf Illusions. Context
effects have been proposed to explain the so-called Moon Illusion:
the moon is perceived (by most people) as being larger (by about
three times) when it is near the horizon than when it is high in
the sky. One suggestion is that when viewed over
‘filled’ visual space (near the horizon) the foreground
provides visual cues that the moon is very far away and thus it is
perceived as being larger. In relation to fires, the implication is
that fires viewed over rich visual foregrounds (particularly likely
in forest environments) may be judged as being further away than
they are in reality.
3. Texture Effects: A cue to judging how far
away is an object involves the amount of fine detail (texture)
perceived. Generally, distant objects have little perceived
texture, while near objects are more likely to reveal texture. In
the case of fire, smoke haze is likely to obscure details about the
fire and thus the fire may be judged to be more distant than it is
in reality.
Discussion
The above account of points 1 through 3 comes from the general
field of the psychology and the physiology of visual perception At
this time we have no evidence that they explain why wildfires might
be perceived as being further away than they are in fact.
Factors other than simple visual perception need to be taken
into account. Given that wildland firefighters almost always
operate in crews or teams, social factors are likely to play an
important role in dynamic risk assessments about likely fire danger
(including predictions of both rate of advance of fires, and
distance of fires). Such social factors are likely to include: (a)
the perceived competence of the leader; (b) group cohesion;
and (c) group norms about risk and danger.
There is abundant evidence that as individuals’ mental
workloads increase their attentional resources available for
additional or peripheral tasks, such as monitoring for threat,
decrease. Fire crews in the field are mostly engaged in fire
suppression-related tasks and may concentrate on these to the
exclusion of monitoring the environment for signs of a
deteriorating situation. This suggests the importance of having
personnel on the fireground whose designated task is that of
‘lookout’ to be alert for early warning signs of a
possibly deteriorating safety situation.
Conclusions
1. There is a need for systematic research to investigate and
improve accuracy of judgements of speed and distance of fires in
natural environments.
2. Emphasis should be given to the importance of deploying
designated ‘lookouts’ on the fireground. Current
guidelines for ‘lookouts’ should be reviewed for their
usability. Detecting early warning signs of developing danger
should be emphasised in wildfire safety training.
3. Perhaps there is value in accepting as ‘doctrine’
what I propose as ‘R. H. Day’s Conjecture’:
Any estimate of the speed or distance of a fire is likely to be
inaccurate. Therefore assume that the fire is travelling much
faster than you think it is, and that it is much closer than you
think it is, and act accordingly!
Note 1: “Exponential change” is change which occurs
at an increasing or decreasing rate, rather than at a constant
rate. That is, the change occurs more and more rapidly (or more and
more slowly) over time.
Note 2: Professor Day noted that it may be the case that fire
distance estimates are simply inaccurate. That is, they are as
likely to be underestimates as overestimates, but because the
possible consequences of overestimation of distance from the fire
are so serious, only overestimates are noted.
Acknowledgements
Several people associated with the Bushfire CRC Program D: Safety and
Volunteerism Projects in the School of Psychological Science at
La Trobe University contributed to this paper.
Thanks to Ruth Beatson, Adrian Birch, Peter Hayes, Alina
Holgate, Mary Omodei and Ross Day.
For more information, contact j.mclennan@latrobe.edu.au
(This article first appeared in the Winter 2009 issue of
Fire Australia magazine.)
