Starts & Stops

An experimental fix: the yellow flashing arrow

 A Somerville resident reported that a left-turn arrow at a Cambridge intersection is confusing because opposing traffic proceeds at the same time.
A Somerville resident reported that a left-turn arrow at a Cambridge intersection is confusing because opposing traffic proceeds at the same time.(Dina Rudick/Globe Staff)

Confusing intersections abound in the Boston region, with ill-timed lights and nonsensical traffic patterns routinely prompting driver ire.

But one Cambridge intersection, spotted recently by Somerville resident Rajiv Ramaiah, may warrant a little-known traffic signal operation rarely seen in Massachusetts.

The scene was near the CambridgeSide Galleria. Ramaiah and his wife idled on Cambridgeside Place, a two-way street, waiting to turn left onto First Street.

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Ahead of them, Cambridgeside Place turns into Charles Street — a one-way street with traffic approaching in the opposite direction.

Along with “DO NOT ENTER” signs, the intersection provides a clear indication that drivers in Ramaiah’s spot must not proceed straight: The traffic light has only a left-turn arrow and a right-turn arrow. After a short wait, both arrows turned green, and Ramaiah’s wife, in the driver’s seat, assumed she was free to make her turn.

“She started to turn left on First, then stopped because she noticed oncoming traffic moving forward,” Ramaiah wrote. She didn’t have an unfettered right to turn left in the intersection, after all.

“I understand that the green arrows are to indicate ‘don’t go straight into the Do Not Enter,’ ” Ramaiah continued. “But isn’t the current signage sufficient without the confusing green arrows?”

It’s a good question — and one that, it seems, has been asked for a while. A cursory poke into the bowels of the Internet reveals a long-dormant message board where commenters debated the same issue in 2005.

“Since when are you yielding on a green arrow!?!?!?!?” read one of the message board comments. “That’s insane.”

As those commenters pointed out, the intersection poses a gripping traffic conundrum: Direct drivers with a green left arrow, giving them a false sense of entitlement as they move into the path of oncoming traffic? Or switch to a regular green light, and run the risk of hapless drivers proceeding onto a one-way street of vehicles coming in the opposite direction?

One thing’s for sure: The green arrow at the intersection is ill-suited.

“A green arrow means you can make a ‘protected’ turn in the direction of the arrow,” reads the Registry of Motor Vehicles driver’s manual. “When a green arrow displays for your turn, pedestrians and oncoming vehicles should be stopped for red lights.”

So, we’re left with the regular green light. An imperfect solution, to be sure.

But there might be another way.

Enter: the flashing yellow arrow.

Not familiar with it? That’s because it may not have appeared on your driver’s ed exam.

The flashing yellow arrow is a relatively new feature in the world of traffic engineering, and it was only in 2006 that the experimental measure was granted “interim approval” by the Federal Highway Administration. The Michigan Department of Transportation premiered the yellow flashing arrow a few years ago, complete with explanatory YouTube video.

Tulsa just premiered its first flashing yellow arrow this month, as did the bustling metropolis of Puyallup, Wash.

An official definition comes from Manual on Uniform Traffic Control Devices — the handbook for all traffic signals, street signs, and road markings maintained by the Federal Highway Administration, used by transportation agencies around the country. It’s like the bible for traffic engineers.

The manual says that cars faced with a flashing yellow arrow are “permitted to cautiously enter the intersection only to make the movement indicated by such arrow . . . [and] shall yield the right of way to: (a) Pedestrians lawfully within an associated crosswalk, and (b) Other vehicles lawfully within the intersection.”

(That’s page 452 of the handbook’s 2009 edition, if you’re looking for some light Sunday afternoon reading.)

And if that’s not enough clarity for you: “Vehicular traffic turning left or making a U-turn to the left shall yield the right-of-way to other vehicles approaching from the opposite direction so closely as to constitute an immediate hazard during the time when such turning vehicle is moving across or within the intersection.”

So, how about it, Cambridge? Will we see the flashing yellow arrow at this spot?

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