The problem with making the opposing red end sooner ("oncoming traffic has longer green," as the signs say) is that the opposing permissive left turns that have

The problem with making the opposing red end sooner ("oncoming traffic has longer green," as the signs say) is that the opposing permissive left turns that haven't cleared by the end of the yellow are then caught in the intersection with their signal red and yours still green. The problem is called the "yellow trap" (http://en.wikipedia.org/wiki/Yellow_trap http://en.wikipedia.org/wiki/Yellow_trap), and some states, including California, outlaw the practice because it is too dangerous. I'm completely with you about wanting everything demand-actuated to be very responsive, though. Traffic signals are all engineering with no user interface design at all.