On Tue, Nov 22, 2011 at 02:26:34PM -0500, Jay Ashworth wrote: > > > Some other things to consider. > > > > Relays are more likely to fail. Yes, the relay architecture was > > carefully designed such that the most failures would not result in > > conflicting greens, > > My understanding was that it was completely impossible. You could > fail dark, but you *could not* fail crossing-green.
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.
