Seeing Double

Double green signal in Sonora, California

Where Highway 49 turns onto Washington Street in downtown Sonora, drivers on Washington encounter not just one, but two green lights. It's the only light I've seen like this. Are drivers in Sonora too reluctant to start on a green light? (In California? You must be kidding!)

While there is a dedicated left-turn lane at this intersection, there is no separate signal for a protected left turn. So the function of the double green light was a mystery to me for nearly 10 years.

In January 2011, I received an e-mail from Chris Cargill, a native of Sonora, who explained the history behind this unusually configured traffic light:

This signal was installed in the early 1970's (1972, I think) when this was the intersection of state routes 108 and 49 in downtown Sonora. In 1986 state route 108 was moved to a bypass alignment just to the south of town, which runs from the intersection of route 108 and Mono Way to the current intersection of 108 and 49 at Stockton Road (between Sonora and Jamestown). Prior to 1986 to travel west on route 108 you would continue down Mono Way to Washington Street in downtown Sonora, where you would encounter the signal you have in your photo. When waiting at the intersection in such a manner that you would be using the lights you see in the photo, you would either travel straight through the light (making a transition from Route 108 west to Route 49 north) or you would turn left at the light to continue on route 108 west.

This extra signal oval is a leftover from the old highway 108 routing. In those days this intersection handled more traffic than it does now, in addition to the dedicated left lane you saw on your visit, the right lane also allowed for left turns in addition to movement straight through the intersection (it now only handles straight through traffic). You had one left only lane and one left/straight combo lane. The extra oval was for a left arrow for the combo lane. When they changed the traffic flow of the intersection, they removed the left arrow from that oval and replaced it with a solid green and wired/programmed the light to show the two greens simultaneously. Basically what you're seeing is Caltrans being lazy and doing a quick fix instead of replacing the signal face completely.

Before the signal was reconfigured, traffic wanting to proceed through the intersection to 49 north would have to wait behind anyone that was in the combo lane wanting to turn left on 108 once the traffic traveling on 49 south (the opposing traffic) got a green light to travel through the intersection. This would create large back-ups for traffic wanting to use 49 north.

So there you have it -- the bottom light was once an arrow for protected left turns. Instead of taking it out, Caltrans just rewired the light. Thank you, Chris, for the full story!