That's probably why the signs say "idle speed", not "idle RPMs".

That sounds like an interesting argument, ever tried it on a cop?
I have never seen any enforcement at the Big Carlos bridge but I can imagine an
over zealous deputy trying to enforce "no wake" in a flood tide. (FMP/FWC does
seem smarter than that ... usually)
This bay is draining about 400 square miles of sheet flow through 4 passes.
When you have 17 inches of rain in a month that is a lot of water going under
the bridge. As I said, it was 12 knots the other day on my GPS. That was what
got me thinking about the sign. I was moving at ~3000 RPM, just to keep
headway.