Photo: The intersection at Cushing Square.

As of May 31, driving efficiency will be coming to Cushing Square, whether you’re ready or not.

While it was apparent that many residents were happy with the old pattern of traffic lights at the busy intersection of Trapelo Road and Common Street, one entity that wasn’t were the engineers from the Massachusetts Department of Transportation which has been working on the $17 million Trapelo Road/Belmont Street Corridor Reconstruction project for the past two years.

According to State Sen. Will Brownsberger in a note to his constituents, the engineers will introduce a new traffic light phasing that will allow a greater number of vehicles to move through the square than under the current system.

This is the state’s second attempt to alter the long-time pattern of red and green lights at the corner – last fall it tried unsuccessfully to change the light phasing only to go back to the pre-project configuration.

The current system allows vehicles on Common Street headed towards Watertown get a green light to either travel straight or take a left while cars headed down Common into Belmont have to stop at a red light. Then, that reverses happens.

“This is very inefficient because most of the traffic in both directions would like to go straight. One wants the two straight movements to share green time,” noted Brownsberger. “After a lot of debate, a new approach has emerged which should be clear for drivers and is more efficient than any of the previously attempted configurations.”

The engineer’s explanation of this new approach appears below (with some additions from me):

• For vehicles traveling on Trapelo Road, there will be no change.
• The south bound Common Street (towards Watertown) lights will have a left arrow and a green ball light that still turn green simultaneously.
• But, the south-bound green arrow will turn to red while the south bound straight ball light remains green, allowing vehicles on Common Street heading toward Watertown to continue towards Watertown.  But vehicles in the left turn lane which are turning left onto Trapelo Road will have to stop.
• When that south-bound left arrow goes red, the north-Belmont bound Common Street lights will go green for both north lanes.  For south bound vehicles on Common Street in the right lane and heading toward Watertown, they will find that, during the latter part of their green phase, traffic will start coming from the opposite direction of Common Street and some of that traffic will be wanting to turn left in front of them.
• The North Belmont bound vehicles will see green balls only (no left arrow), as is common at many intersections around the state. There will be a “left-turn-yield-on-green ball” sign, again as is common at many intersections. For vehicles on Common Street coming from Watertown, they will now find that traffic will be coming from the opposite direction of Common Street when they have the green indication and the vehicles turning left into Trapelo Road toward Waverley Square will need to yield to traffic coming from the other direction of Common Street.

“The beauty of this approach is that (a) the signage will be simpler — the complicated signs with three-headed arrows will be less critical; and (b) if north-bound traffic makes the mistake that it tends to make — thinking that the straight move to continue on Common Street is a left turn, they will actually not conflict with the straight movement from the other direction,” said Brownsberger.

The only time they will need to think is when they are making the hard left onto Trapelo and on that movement, it is reasonable to expect the drivers will exercise the caution that they generally should on a left turn with no arrow, he noted.