Traffic signals are an evolving thing.  That's hard to believe for Westerners (especially Americans) who have never known anything but the red-yellow-green three-light stack that was mounted to the wall in Mr. Rogers' home.  However, as cities have grown and as intersecting roads have had to carry ever increasing amounts of traffic, traffic signals have become more complicated.  At many intersections, signal arrays control more than one direction of traffic: through and left-turning, and through and right-turning.  And they have two problems.

One problem is with protected/permissive left turn signals (or right turn signals in left-hand-drive countries), called the Yellow Light Trap.  If oncoming cars are waiting to turn against oncoming traffic, and if the intersection's computer decides to offer one of them a protected left turn but not the other one, then as one of the through-traffic directions loses right of way, the car will rightly see he's losing right-of-way to turn and wrongly deduce that, in order to clear the intersection, oncoming traffic is as well.  If the driver then completes his turn under that wrong assumption, then he'll cause his car to be T-boned and block the intersection.  The trouble is that a left-turning driver has no way to tell the difference between oncoming through-traffic losing right-of-way and oncoming left-turning traffic gaining a protected turn.

The solution that looks like it's being rolled out is a dedicated signal with a green arrow, a flashing yellow arrow, a steady yellow arrow, and a red arrow.  Also, it's more readily understood because a flashing yellow arrow is just like a flashing yellow ball, but only for traffic turning in the arrow's direction.

Here's a page with animations explaining the problem and proposed solution.

The other problem is that of drivers with red-green color blindness.  When the signals are in a vertical stack, it's easy to tell which is red and which is green.  At least, when a color-blind driver is at the intersection.  At night and in low-contrast conditions, the color becomes the only indicator, which means color-blind drivers have only subtle differences in apparent hue to tell red and green apart, thus whether to stop or go.  Complicating things further for color-blind drivers are horizontal signal arrays: the only indication is the light's position along the array, and many color-blind drivers still aren't sure unless they can see a corresponding vertical array.

The most obvious solution is to change the shapes of the lights while preserving their colors.  One proposal, Uni-Signal, proposes triangle-circle-square for red-yellow-green.  Another proposal is a single light that changes shape and color.  One that's bouncing around in my head is a circle for green, a diamond for yellow, and two half-circles (a circle bisected horizontally, like a "do not enter" sign) for red.