I think it was Oregon which has FYA with no sign. maybe Idaho.
Quite possibly Oregon. They were an early adopter of the FYA during/after FHWA allowed experimentation. I believe one of the stipulations during the experimentation phase was that no sign be installed, so that researchers could really evaluate driver understanding of the operation without a sign telling drivers how to act.
You'd think something as counter-intuitive as a FYA would require a sign explaining it's operation.
Obviously, someone who lives in an area where these have become commonplace will no longer need it for themselves, but those from areas without them probably need some explanation.
The National research studies indicate the FYA is more intuitive
than the old MUTCD 5-section display (doghouse, horizontal or vertical) and other methods of displaying PPLT operations, which is why FHWA adopted it as the standard PPLT display in the 2009 MUTCD. The research also indicated drivers understood the meaning fairly intuitively without the need for an explanatory sign...
Honestly, if more states used the doghouse signal instead of having a separate left turn signal phase with the red ball signal the flashing yellow signal probably wouldn't have been concieved up because drivers would know to yield to oncoming traffic unless they have an arrow. In upstate NY the doghouse is everywhere and fully protected left turns are rare, so nobody assumes they can turn left without looking if they don't have an arrow. Most of our signals with a left turn phase use arrows on the signal anyways. The only red ball signal I can recall in the entire state (NY 590/Titus Ave) was replaced with a roundabout a couple of years ago.
Not exactly the point you're making here, as "red ball signal" is a phrase that can mean many things. However, if there is a proliferation of doghouse signals, that implies there is at least some use of protected left turn phasing, since the doghouse signal should be used for both protected and permitted turns--if there were no protected turns, there would be no need for a doghouse and a simple three-section head could be used instead...
In any event, one of the main concerns with the 5-section display is that drivers in a left turn lane seeing the circular green ball overhead during the permissive phase commonly mistake that indication as a signal to go, regardless of oncoming traffic--in other words, drivers would see only the green ball and not automatically assume they have to yield to oncoming traffic. A driver confused by the meaning of the 5-section signal quite often proceeds to cause an accident, a critical failure maneuver. In contrast, a driver confused by the FYA often hesitates to proceed, which is a safer failure scenario.