They call them lagging lefts. Cars don't get a left-turn arrow until after the straight-through traffic.
It's the opposite of how most lights at intersections work. And they've been a mainstay of driving in Gilbert and Scottsdale for years.
But now, Gilbert is ditching them. Well, sort of.
The town is reprogramming its traffic signals along Cooper and McQueen roads to go with leading lefts -- where the left-turn arrow comes before the green light -- to mirror those in Chandler and Mesa. When the project is finished, drivers can travel the length of either road without wondering where in the cycle the turn arrow will pop up.

No city is an island in metro Phoenix

I've been arguing for this change for roughly a decade (I know, I need a better hobby). My point has always been that Gilbert isn't an island. People regularly travel between cities here -- and let's face it: Not everyone pays attention. I always said it was a bad idea to introduce confusion for drivers, particularly those who don't live here, when most of the country and all of our immediate neighbors use the leading left.
Chandler briefly switched to the lagging left at lights that bordered Gilbert years ago but found the change mucked with traffic flow miles away and caused accidents on McQueen Road. To me, that was evidence enough that the lagging left needed to go.
But some people really loved the lagging left (I know, we Gilbert folks really need new hobbies). They argued that the lagging left is safer (though Gilbert traffic engineer Leslie Bubke told me Tuesday that the Federal Highway Administration considers both of equal safety value). And, besides, they were used to how Gilbert's traffic lights worked.

The signal couldn't fix traffic backups

So, nothing changed -- until a recent signal-timing review. Traffic engineers found it impossible to alleviate significant traffic backups along Cooper and McQueen while sticking with the lagging left setup. They looked at several options, including flashing yellow turn arrows, but found that leading lefts did the best job of moving traffic.
Ha ha! Vindication!
Well, sort of. There are no immediate plans to expand the leading left beyond Cooper and McQueen, but Gilbert is keeping its mind open. It is tracking travel times on Cooper before and after the change to ensure everything is working as planned.
I fully expect to be proved right on leading lefts (me? Right for once?). But even if I'm not, it's nice to know Gilbert is gathering data and making decisions accordingly. That's how traffic flow is supposed to work.