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.