Just to put one point to rest.
I, personally, have witnessed traffic lights showing 'green both directions'.
*TWICE*. One was in the mid-1960s, with what was undoubtedly relay-based
control logic; the second was in the late 1990s, *probably* with solid-state
'management' controls , but I don't know for certain. (The 'relatively
recent' unit's I've seen the insides of have solid-state logic driving final
'output' relays that provide power to the actual signal head.)
In the first case, the pedestal-mounted control unit had been subjected to
excessive impact forces, and some of the 'output' wires had shorted together.