Safety
The concept behind low water crossings is that they are convenient and safe to use in normal conditions. Once the water level rises to the point where it crosses the bridge surface the bridge is generally unsafe to use, and in developed countries this will usually be indicated by warning signs. An additional hazard is that the bridge surface may become completely obscured by the water, making it relatively easy to fall off the bridge surface into the deeper and more hazardous water on either side. As a consequence the line of the submerged bridge is often marked with poles or other structures to indicate its course to unwary travelers and emergency users even when submerged.
The force of the moving water may be strong enough to physically push the vehicle off the bridge (partially or totally, the higher percentage of the vehicle in the water, the more likely the water's force will take the vehicle off the bridge and send it down river with substantial damage). One additional risk for trying cross a bridge under water may include the vehicle stalling out (failing to work). Most times, when a vehicle has stalled, people will try to get out of the vehicle. When they leave the vehicle, they step into water currents that can cause them to fall or be pulled down into the water. They are exposed to a strong hydro force, which may sweep them down river and the water may jam their body into debris in the waterway's path which may cause injury and/or death.
Despite the obvious dangers and warnings given there are still a significant number of emergencies and even deaths due to unwary use of low-water bridges during flood conditions.
One attempted solution/deterrent is the Stupid Motorist Law in Arizona, USA.
Read more about this topic: Low Water Crossing
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