I heard that the convention was to set the duration of the amber stage to something around the time it takes a fast-walking pedestrian to cross, so that you could start crossing near the end of a pedestrian green signal without being on the road during green for drivers. Unfortunately at some crossings these days, the full duration of the green signal isn't long enough for me to fully cross before it turns to amber (and I probably walk about 4x faster than your garden-variety old dear with a stick).