For a number of years, Michigan, Indiana, and Wisconsin have modified some traffic signals either to replace the left-on-green-arrow only with either a green arrow giving way to a flashing yellow during the cycle, or to the flashing yellow completely replacing the green turn arrow.  At the time I noted this change, I cited research to the point that "a more permissive aspect expedites traffic flow."

Except, until recently, in Illinois, notorious for its left-on-green-arrow-only intersections and frustratingly long cycle times at some crossings.  In the DeKalb area, there are a number of crossings in which if you're using the inferior route and you don't get through with the cluster of cars waiting for the light to change, you sit until the signal cycles, sometimes 30 to 45 seconds after the last of the cluster on the superior route has cleared the crossing.   So much for those sensors expediting movement.  As I noted, "First you create channels at crossroads, and set up traffic lights that permit left turns only on arrows, and then make people wait for the light to cycle, and then you conclude that all those constraints on motion impede motion. Duh."

Earlier this summer, I noticed the flashing left arrow aspect at work in the Galesburg area.  These were apparently experimental, and the experiment has worked.
According to the study results, accidents did not increase but instead fell over 20 percent.

"It was better for the drivers that didn't have to sit and wait," says [state senator Dave] Syverson. "It was better for reduced emissions 'cause drivers aren't sitting and idling so really it's a win-win to implement this common sense change."

Syverson has now asked the district with the Illinois Department of Transportation that oversees northern Illinois to start putting yellow arrow signals at state road intersections.
That's progress. Perhaps synchronizing the cycles will reduce the speeding that emerges as drivers figure out how fast they must go to beat the light before the sensor senses a gap in traffic and throws a yellow in the face of the law abiding drivers.

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