[computomatic]

Is ultrasound less expensive than a moisture sensor?

The problem with both is they effectively require the vehicle to be in the water already. They need something that can tell depth before the vehicle has to slow down.

[amluto]

What kind of moisture sensor are you thinking of?

[lazide]

Probably an exposed set of electrical contacts and a resistance meter.

Unfortunately, those get screwed up in climates where there is salt used on the roads.