The subject of traffic signals has been discussed here before. It would be great if colors of signals were standardized to a specific wavelength in nm but they still follow a standard based on a chromaticity diagram first published in 1931. You can read more about that here:
Standard stoplights are not hard to conceptualize and model. Except in rare instances, red is on the top, amber in the middle and green on the bottom. The sequencing is always the same. Red then green then amber and finally red again. Green arrows add to the complexity, but they generally apply to a specific travel lane, which is often defined in the map data. More recently, certain jurisdictions have added flashing amber arrows indicating an unprotected turn. Canada also has a flashing green light but it is becoming rare except in BC and Yukon. It indicates a pedestrian controlled crossing in those provinces.
But the flashing red-amber and red-red signals are the ones that cause issues with FSD. Their recognition by FSD seems to be a mix of mapping data and vision. A further complication is that standard signals may revert to flashing in times of low traffic density.
The first challenge in modeling flashing signals is the angle of the intersection, since the lighting on the cross street can often bleed through to a forward facing vehicle. This seems to be the main problem if the signal is not properly mapped. Of course, flashing signals just possess a repetitive on/off signal so there are no hints from sequencing like there is with a standard signal. Some flashing amber lights are not overhead and may alert the driver to a hazard like a tight curve or a school zone.
On the red flash street, there is no issues. It is no different that a two way stop sign. It's the amber flash side that's a problem. It means proceed with caution. The easiest solution for modeling is to completely ignore flashing amber lights. The problem is the bleed through mentioned above. Red and green are far apart in the light spectrum, but red and amber are not. This is why FSD presently does a hesitation dance at flashing ambers. It's amber, no wait it's red. Oh forget it, someone get me a cheeseburger. (apologies to Gary Larson). It's also possible that the signal is a full stoplight that is flashing amber during a low traffic period.
Anyway, that more than anyone wanted to know about this but not everything that could be said. I didn't even mention overhead lane signals for bridges and tunnels and other alternating traffic flow situations.
