Relay logic has the potential for programming (i.e. wiring) errors also.
Yes, but the complexity of a computerized controller is 3-6 orders of magnitude higher, *and none of it is visible*
It's not fair to compare "conflict monitor" to "properly programmed relay logic". We either have to include the risk of programming failures (which means "improper wiring" in the case of relay logic) in both cases, or exclude programming failures in both cases.
See above, and note that there are at least a couple orders of magnitude more possible failure modes on a computerized controller as well.
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.
but that's not the only risk. When the traffic signal is failing, even if it's failing with dark or red in every direction, the intersection becomes more dangerous. Not as dangerous as conflicting greens,
By 2 or 3 orders of magnitude, usually; the second thing they teach you in driver ed is "a dark traffic signal is a 4-way stop".
but more dangerous than a properly operating intersection. If we can eliminate 1000 failures without conflicting greens, at the cost of one failure with a conflicting green, it might be a net win in terms of safety.
The underlying issue is trust, as it so often is. People assume (for very good reason) that crossing greens is completely impossible. The cost of a crossing-greens accident is *much* higher than might be imagined; think "new Coke".
Modern intersections are often considerably more complicated than a two phase "allow N/S, then allow E/W, then repeat" system. Wiring relays to completley avoid conflict in that case is very complex, and, therefore, more error prone. Even if a properly configured relay solution is more reliable than a properly configured solid-state conflict-monitor solution, if the relay solution is more likely to be misconfigured, then there's not necessarily a net win.
Sure. But we have no numbers on either side.
Cost is an object. If implementing a solid state controller is less expensive (on CapEx and OpEx basis) than a relay-based controller, then it might be possible to implement traffic signals at four previously uncontrolled intersections, instead of just three. That's a pretty big safety win.
See above about whether people trust green lights to be safe.
And, yes, convenience is also an objective. Most people wouldn't want to live in a city where the throughput benefit of modern traffic signalling weren't available, even if they have to accept a very, very small increase in risk.
Assuming they knew they were accepting it. But if it amounts to "Well, it's going to cost you more if we do it [right]", well, look out for #OccupyMainStreet. "We can fake it cause it's cheaper" is pretty close to a dead approach, I suspect. Cheers, -- jra -- Jay R. Ashworth Baylink jra@baylink.com Designer The Things I Think RFC 2100 Ashworth & Associates http://baylink.pitas.com 2000 Land Rover DII St Petersburg FL USA http://photo.imageinc.us +1 727 647 1274