1. iVT International June 2009
DON’THAVE
NIGHTMARES
STEVEN CASEY
iVT International June 200948 49
ACCIDENT PREVENTION
they say it can cause bad dreams, but conversely, a couple of
slices of cheese also hold the key to designing – and using
– lift-trucks for accident-free operation
What in the world, you might
ask, does Swiss cheese have to
do with techniques and
technologies for mitigating error in
materials handling? Granted, iVT is
known for the odd piece of lateral
thinking – but Swiss cheese? The
variety with numerous holes?
Yes! The ‘Swiss cheese model’ of
accident causation was named and
popularised by world leader in error
and accidents, Professor James
Reason of the UK’s Manchester
University. The model has been used
successfully to reduce accidents and
system failures in settings as diverse
as medicine and aviation.
So why not also in materials
handling and its many subsystems
consisting of operators, vehicles,
operating environment and
information technology?
Perhaps while making a
sandwich, Professor Reason reasoned
that slices of Swiss cheese
represented the defences (the slices)
and the weaknesses (the holes in the
cheese slices) within human/
machine systems that lead to
unwanted outcomes and system
failure. The individual slices of Swiss
cheese are the barriers or defences to
help prevent accidents and failure
within systems and organisations.
Defences might come in the form
of the skill and quality of the
human operator, the robustness and
intuitiveness of the operator
interface, the physical environment
in which machines are operated, the
maintenance of equipment, or the
overall nature or management of the
organisation. Properly designed and
maintained, defences lower the
likelihood of error and failure.
But then Professor Reason saw
more. The holes in the cheese –
known as ‘eyes’ by aficionados –
represent individual weaknesses that
make a system error or a failure
more likely to occur, and might exist
in the operator interface, the skills
of the operator, organisational
policy, training, information
availability, maintenance, the design
of a warehouse, or in any other
important facet of the enterprise. If,
by chance, or for whatever reason,
the weaknesses – the individual
holes in the several slices of Swiss
cheese – align, there is what Reason
calls a ‘trajectory of accident
opportunity’. The hazard is able to
pass through the holes in all the
slices (the defences) and the result is
a failure or an accident.
An open and unguarded loading
dock, limited illumination and an
unskilled operator are weaknesses
that, when combined, might lead to
a serious failure. Eliminating
individual holes – or latent
conditions as they are sometimes
called – blocks this trajectory of
accident opportunity.
So what are some of the ways in
which vehicle manufacturers,
systems providers, warehouse
managers and even governments
have reduced the likelihood of
failure or accidents by addressing
individual weaknesses in subsystems
and improving the strength of
barriers that help avert failure?
The ‘Swiss cheese
model’ of accident
causation was named
and popularised by
world leader in error
and accidents,
Professor James
Reason

2. Physical environment
A well-designed physical
environment in which lift-trucks
and other materials handling
vehicles operate can serve as a
barrier to unintended outcomes but,
if poorly designed, can contribute to
failures and accidents. Among the
parameters that can contribute to
performance and safety are aisle
width and obstructions, overhead
obstructions, floor condition,
routeing of trucks and pedestrians,
lines of sight, lighting, noise, rack
design, and the handling of drive-off
hazards such as docks. The
proverbial ‘accident waiting to
happen’ can also be seen as one of
Reason’s ‘accident opportunities’ in
his Swiss cheese model.
The classes of errors possible in a
materials handling environment
include a vehicle colliding with a
load, a tipover, a dropped load or
falling objects, colliding with a fixed
object such as a rack, colliding with
another vehicle, driving off a dock,
colliding with a pedestrian, and
improper use of a truck. One of the
more interesting barriers to many of
these are new ‘smart’ warning
systems that provide operators and/
or pedestrians with necessary
information that would otherwise
not be available.
Delaware-based Sky-Trax has
developed a family of products
based on truck position-sensing
technology, vehicle tracking and
warnings. The American company’s
Fork Alert, for example, is one of
the world’s leading pedestrian early-
warning system for workplaces
where forklift operations could
present a considerable danger to
pedestrians. The system is claimed
to be particularly effective around
blind corners, shared doorways, and
other dangerous areas within
warehouse facilities. Individual
vehicles – be they operator-guided or
auto-guided – are equipped with
roof-mounted beacons that emit a
coded infrared beam.
Receivers mounted in strategic
positions through the facility detect
the invisible, coded infrared beams
of moving and even stationary
vehicles, giving pedestrians and
drivers alike at least four seconds of
advance notice of an approaching
forklift. Warnings can be in the form
of audible alerts, flashing lights,
locking gates, or other safety
warnings that might be used in a
warehouse or manufacturing facility.
Sky-Trax also offers a somewhat
simpler but nevertheless effective
technology. Door-Man is a localised
system designed to ‘address safety
concerns that arise when forklifts
and pedestrians manoeuvre within
the same space’. The Door-Man
intersection caution system is a
fixed system operating on standard
electric power that senses vehicles
up to 80ft away and protects from
one to four aisles, illuminating
strobe-lighted signs and sounding an
audible alarm when other vehicles
are approaching.
A reflective 26in mirrored
overhead dome is also part of the
system, providing important visual
reference and feedback for observers.
A more comprehensive approach
is provided by the Sky-Trax forklift
truck safety system, part of the total
Sky-Trax system. Components
include invisible infrared beacons
and a wireless link on every vehicle,
fixed optical position markers
mounted at strategic locations
throughout the facility (scanned by
the optical position sensors on each
vehicle), a central controller that
collects the position data from all
vehicles, fixed information displays
or displays mounted within vehicles,
and auditory- and visual-warning
devices. The system tracks the
location of every vehicle, displays
this information in various
locations, warns pedestrians of
nearby vehicles and alerts drivers
about other vehicles in the vicinity.
The technology can also be used
to improve overall system
performance by identifying the
vehicles that are the closest to
pickup points and convey pickup
instructions to their drivers. Forklift
activity, efficiency and number of
loads can also be monitored.
In contrast to the optically based
Sky-Trax systems, RFID technology
can be used to develop similar
capabilities as long as radio
interference is not an issue. Optical
systems such as Sky-Trax have also
been combined with RFID pallet and
iVT International June 2009 51
ACCIDENT PREVENTION
left: Using position-
sensing technology, vehicle
tracking and warning
systems, Sky-Trax helps
prevent accidents to
pedestrians and vehicles
alike – especially at
intersections
above: The Swiss cheese
model of accident
prevention. Slices of
cheese represent the
types of barriers to failure,
and the holes represent
individual weaknesses or
‘latent conditions’. When
latent conditions align,
there is a ‘trajectory of
accident opportunity’

3. product tags to combine the best of
what each technology has to offer.
Real-time vehicle tracking – with
the addition of real-time datalinks
back to vehicles – opens the door to
many possibilities for mitigating
serious errors and increasing safety.
Serious accidents such as driving
off a dock or driving through a
doorway with the mast raised could
conceivably be addressed with this
technology. A vehicle approaching a
danger zone could be slowed or even
stopped automatically to avert an
accident. This is not entirely
dissimilar to the flight protection
systems on modern airliners that
prevent pilots stalling aircraft or
exceeding their design limits.
Operator interface
Materials handling OEMs have
developed numerous technologies
and techniques to help plug many
of the holes that might contribute
to a trajectory of accident
opportunity at the operator interface
in their vehicles.
Good ergonomics, appropriate
control design and response,
labelling, and overall organisation
of the user’s interface contribute to
overall performance accuracy and
consistency. These may well be
traditional areas of focus but there
are now a growing number of
technologies to mitigate the
frequency and severity of error.
Ohio-based Crown Equipment,
one of the world’s leading lift-truck
manufacturers, offers OnTrac Anti-
Slip Traction Control as standard
equipment on the RR5700 series of
narrow-aisle reach trucks (see www.
ivtinternational.com/crown_rr_video.
php for a demonstration video).
Polished, wet or otherwise slippery
floors are a common occurrence in
certain warehouse applications,
especially in freezer or produce
operations. Slippery floors can lead
to loss of traction and considerable
degradation in steering control for
the operator.
OnTrac helps to address these
problems by reducing drive-wheel
slip during acceleration and braking.
Sensors monitor the operator’s
control input, truck speed, and
revolutions of the drive wheel, and
controllers automatically adjust the
power to the drive wheels to reduce
slippage and help the operator
maintain accurate speed and
steering control of the truck.
This is an excellent example of
how clever use of a technology can
improve vehicle performance and
reduce the likelihood of problems
associated with slick, wet, or frozen
floors. It is also an interesting
application of assistive technology
on a vehicle used to mitigate a
physical condition – in this case a
wet or slippery floor – in the
operating environment.
The next example is actually a
single materials handling machine
that embodies many technologies
and techniques to protect the
operator and reduce the chances of
error. The Crown Wave (Work Assist
Vehicle) is designed to transport,
retrieve, and put away multiple
small loads that might otherwise be
transported and moved through the
use of push carts and rolling ladders.
Controlled field research on the
Wave has shown that it can reduce
labour costs by up to 50%.
A number of safety features help
to protect the operator, loads and
even bystanders. The Wave has foot-
presence sensors that must be
depressed in order to move the
machine, ensuring that the
operator’s feet and legs are within
the confines of the vehicle.
Similarly, a hand-presence sensor
further contributes to ensuring the
stability of the operator during
vehicle travel. Travel speed is linked
to both the height of the operator
platform and the position of the
entrance gates. The top speed of
4mph is possible only when the
gates are open and the work
platform is lower than 20in.
Operators wishing to travel quickly
and/or over a long distance can
lower the platform and hence the
centre of gravity of the machine,
operator and load. The gates must
be closed to lift above 20in, and
forward travel speed is restricted to
2.5mph when the operator platform
exceeds 20in. All these features
contribute to the safety of the
machine and help plug the holes in
the theoretical slices of Swiss cheese
or, more technically, block the
trajectory of accident opportunity.
There are, of course, many other
examples of technology being put to
effective use on vehicles to reduce
the probability of unwanted
behaviours and outcomes. Control
interlocks, load sensing, tilt sensing,
cornering and speed sensing, and
other operator presence sensing
systems have all been used by
numerous manufacturers to shape
the actions of operators and reduce
the likelihood of accidents involving
materials handling vehicles.
Safety culture
Vehicle design, vehicle
maintenance, the design and
condition of the physical operating
environment, smart sensing and
information processing and training
are all key parts of any programme
to reduce error and accident rates
associated with materials handling
equipment. If forced to identify the
single key to reducing ‘accident
opportunities’ it is the existence of
a total safety culture that seeks to
plug the holes in the slices of Swiss
cheese. iVT
References:
Reason, J., Managing the risks of
organizational accidents (Ashgate
Publishing Limited, Aldershot, 1997)
Steven Casey PhD is president of Ergonomic
Systems Design Inc in Santa Barbara,
California, USA. He has worked with many
clients in North America, Europe, Asia and
Australia. He is the author of Set Phasers on
Stun: And Other True Tales of Design,
Technology, and Human Error (Aegean,
1998) and The Atomic Chef: And Other
True Tales of Design, Technology, and
Human Error (Aegean, 2006)
Training Day
Many in the industry believe that training of forklift operators is the most
effective avenue for reducing error and accident rates, and research tends
to support this position. In 1999 the US Department of Labor issued new
standards requiring all employers to develop training programs addressing
powered industrial equipment safety and that all current and future
operators of powered industrial vehicles or forklifts undergo training
and acquire certification. Periodic refresher training is also required
to maintain certification.
Recent US government labour statistics show a reduction in
serious forklift accident rates over the past few years, a trend
that is probably due to the new training/certification standards
as well as the introduction of some of the stability, traction and
presence-sensing systems on vehicles.
Below AND RIGHT:
Crown’s RR5700 series
feature an anti-slip traction
control to provide an
effective barrier to one
‘latent condition’ – or an
accident waiting to happen
above AND BELOW: Travel
speed in Crown’s Wave
is linked to the height of
the operator platform and
position of the entrance
gates, helping plug the
holes in the cheese slices
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ACCIDENT PREVENTIONLIFT-TRUCK SPECIAL: