Physics of Traffic

Can signals be timed in progression in both directions?

A recent letter to Gridlock Sam asked why signals were synchronized in one direction (in traffic parlance a "progression" where signals change to green as you approach them, traveling at the speed limit) but weren't in the opposite direction.  If you follow the Gridlock Sam Traffic signal lesson plan you'll see that while it's easy to time signals in a one-way progression, two-way progressions (at reasonable speeds)requires spacing of intersections at about 1800 feet.  In the suburbs or new communities one could set the spacing for two-way progression.  In NYC our block are mostly 225' to 800' long, way too short for two-way progression.  (Note: at about 90 mph two-way progression would work in NYC!)