原创 Traffic lights failure

By the time I arrived at the junction in question, there were police cars with their lights flashing all over the place, policemen standing in all four branches of the intersection, and a number of cars (or pieces thereof) piled up at the sides of the road.

That must be a horrible experience -- entering a junction brimming with confidence that you have a green light, and meeting cars coming from your right and left whose drivers also think they have right of way.

I remember them waffling on about this at university -- how to design systems so that they always fail into a known state. As I recall, they used traffic lights as an example, with the design being such that they would always fail to all red; never to all green.

But that was 35+ years ago now and -- like so many other things -- it's faded from my mind. Let's assume the simplest case of two roads crossing at 90 degrees (North-South and East-West).

Do you remember how to create this sort of design such that it will only fail with both directions red (or off)? And can you conceive how the lights I saw yesterday evening had managed to fail with both directions green?

Max Maxfield