Home' Australian Govlink : Issue 1 2015 Contents During periods of flooding the safety
message issued by road agencies, media and
emergency services personnel to motorists
is simply ‘never drive, ride, walk or play in
This is a very safe message, given the
uncertainty of whether the road pavement
is still there, the actual depth and velocity of
the water and their combined effect. While
this message is most appropriate (litigation
concerns) it is not seen to be a practical
approach given that too often drivers have
satisfactorily negotiated a flooded roadway
due to the pressures to get home, to work or
to medical services etc.
Anecdotal evidence indicates that most
floodways, particularly on low-volume roads,
do not comply with the desired geometric
guidelines nor have flood warning signs as
required in AS 1742.2 (2009) causing drivers
to take greater risks when deciding to cross
a waterway and possibly exposing road
agencies to future litigation.
When vehicles attempt to cross floodways
that are under water, there is a possibility
that they can become buoyant, and are at
risk of being swept from the roadway even
at low flow velocities. Many drivers are
unaware of, or under-estimate the dangers
that floodways present, and take the risk of
crossing when they should not.
Existing floodway signs, even if provided,
do not adequately alert a driver to the risks
involved in crossing a flooded roadway in
relation to the depth of water, flow velocity
and in particular the combined effect of
both on a given vehicle. This is the intrinsic
weakness with the current road signage at floodways which leaves a driver with incomplete
information in deciding when not to cross.
The literature review highlighted the need to complement the use of existing static warning signs
by providing active warning treatments, particularly at potentially high-risk sites. The intention is to
provide motorists with additional information to base a decision on whether to cross.
Automated warning systems can take various forms with a variety of sensors, communication
systems and activation devices. Such systems provide real-time information alerting drivers
of actual floodway conditions by triggering flashing beacons, message signs and/or lowering
of barriers when water depth reach pre-set values. This removes much of the risk-taking
confronting drivers when crossing major flows at floodways.
A trial of various automated warning systems could be undertaken to assess their
effectiveness, cost and benefits in an Australasian context. The results of this work should
lead over time to the establishment of selection criteria or warrants as to which treatment or
device should be installed for given site conditions.
If automated warning systems can be proven to be effective, it is most unlikely that
widespread installations would be possible for the many thousands of significant floodway
crossings in Australasia, due primarily to costs.
Existing depth gauges should be reviewed to find a more appropriate sign that ensures that
a driver crossing a flooded roadway has information on water depth and velocity relating to a
vehicle type (i.e. car, 4WD and truck) for a specific crossing. The basis for such an indicator is
that each floodway has a particular channel profile and water-flow characteristics, hence the
depth and velocity relationship can be determined. The calculated value can be placed on the
indicator for a given vehicle type and would be unique for a specific floodway crossing. This new
indicator offers greater promise as it would be low cost, have no mechanical parts and would
be much more effective in advising a driver when not to cross a floodway for a particular vehicle
type (based on water depth and velocity).
The approach to improving the operation of floodways should follow the Safe System
principles which relate to safer roads, safer vehicles and safer road user behaviour. Under the
Safe System principles, further consideration would also be given to finding solutions outside
the engineering requirements. This may involve developing better ways to educate drivers on
the dangers and risks associated with crossing floodways and explaining the adverse effects
floodwaters can have on different vehicle types, taking account of water depth and flow
velocity. Vehicle manufacturers should establish for a given vehicle type the depth of water (at
low flow velocity) at which the vehicle can become buoyant and make this information more
readily known to drivers.
The full report may be viewed at www.austroads.com.au
GOVLINK » ISSUE 1 2015
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