You should never drive into floodwater – some roads are more deadly than others

The floods that deluged parts of Victoria over the weekend are the latest in the state’s long history of flooding, following on from major floods in 2010, 2011, 2012 and 2016. In all such events, emergency services are on standby to rescue motorists who drive into floodwaters and get stuck or washed away – with potentially fatal consequences.

Most of the 178 flood-related deaths since 2000 have been a consequence of motorists driving into floodwaters.

Although there is a growing body of research on the decision-making of people who choose to enter floodwater, little research has been done before now on the factors that make some stretches of road more dangerous than others.

Read more: Flood deaths are avoidable: don’t go in the water

Why do people drive into floodwater? Often they are trying to get to or from work, or carrying out work-related duties such as surveying farmland or driving trucks. It is hard to judge the depth and speed of water, or to tell if there is debris or damage below the surface. A simple error of judgement can prove fatal, which is why emergency services warn against driving through floods even when the conditions look benign.

Our research, carried out by Risk Frontiers, Macquarie University and the Bushfire and Natural Hazards Cooperative Research Centre (CRC), is among the first to consider the influence of road characteristics on flood deaths.

We analysed the road characteristics at 21 sites in NSW, Victoria, Western Australia and Queensland where flood-related motoring deaths have occurred since 2010. We found that some roads are clearly more dangerous than others.

Several factors increase the danger of a particular stretch of road, including:

• small upstream catchment size
• the presence of roadside barricades
• the depth of flooding next to the roadway
• absence of street lighting
• dipping road grade
• lack of kerbs and guttering
• inability of motorists to easily turn around.

Each of these factors was observed in at least half of the cases we studied. Yet these kinds of risk profiles largely not considered in emergency planning or flood-management approaches, which tend to focus on urban flood risk. The large number of flood-prone road sections means that we need to prioritise those roads that are most in need of safety improvements.

At eight of the sites we studied, emergency services or passers-by were on the scene within several minutes, either to attempt rescue or to call for assistance. This suggests that road conditions in some locations are so dangerous that first responders will be unable to help if a vehicle is submerged or swept away.

At 12 of our study sites, roadside flood signage was probably present at the time of the incident. (As our assessments were done some time after the deaths occurred, we have to use the best available information, which presents a small degree of uncertainty that the signage may have changed.)

Depth markers were the most common, followed by “Road subject to flooding” signs. This suggests that some motorists ignore, misinterpret or fail to notice warning signs, or that signs can be damaged, lost or obscured after installation. We suggest that signage should be improved in areas identified as high-risk.

Read more: Natural hazard risk: is it just going to get worse, or can we do something about it?

The difficulty of turning a vehicle around at many sites underlines the importance of encouraging motorists to plan ahead, rather than make a snap decision to enter floodwater.

When severe weather is forecast, motorists should plan journeys in advance, with the help of up-to-date road information from an authoritative source such as the VicRoads website.

As many motorists enter floodwater for work-related purposes, employers also have a responsibility to promote safe driving behaviour.

Whatever the exact reasons, all of the deaths at our study sites ultimately happened because someone decided to drive into floodwater. Our study showed that roads vary widely in safety. The water might look shallow and easy to negotiate, but you can never be completely sure. For that reason, the safest option is always to stay well clear.