Most American flood deaths involve vehicles, and the reason is a physical mismatch between what a car weighs and what shallow moving water can do to it.
Buoyancy removes the grip before it removes the car
A vehicle displaces water as it enters a flooded road, and the weight of that displaced water is subtracted from the force pressing its tires down.
Modern cars enclose a large volume of air in the body and cabin, so the displacement grows quickly once water reaches the floor pan.
Long before a car floats outright, it has lost much of the traction that steering and braking depend on, which is why control disappears first.
The current pushes on a very large side
Flowing water exerts force on the whole submerged side of a vehicle, which presents a broad flat surface to the flow.
That force rises steeply with current speed, so a modest increase in how fast the water is moving multiplies the push considerably.
Combined with reduced traction, a current that would be easy to stand against becomes sufficient to slide a vehicle sideways off the pavement.
The road beneath may not be there
Moving water scours material from around and beneath pavement, and a road that looked intact when the water rose can be undermined while covered.
Bridge approaches and culvert crossings are the common failure points, because the flow concentrates there and removes the fill supporting the surface.
Muddy floodwater conceals all of this, so depth judged from the surface tells a driver nothing about what remains underneath.
Depth is nearly impossible to judge at night
Water over a road offers few visual cues about depth, and headlights reflect off the surface rather than revealing what lies below it.
Many flash floods peak overnight, when a driver encounters the crossing with no daylight reference for how far the water extends.
Familiarity works against the driver here, since a routinely traveled low crossing invites the assumption that the road is where it always was.
Barricades and closures reflect known failure points
Communities that flood repeatedly know which crossings go first, and closures are placed at those locations before conditions become obvious.
Some jurisdictions attach cost recovery to rescues from barricaded roads, an acknowledgment that these incidents consume responder capacity during a wider emergency.
The underlying message of every such measure is that the depth a driver can see is not the variable that determines the outcome.