Green will still mean go, and yellow, slow down. But drivers turning left from Fisherville Road onto Bog Road or Sewalls Falls Road will soon see another signal in the mix: a flashing yellow arrow.

Like green, it means go, after yielding to oncoming traffic. But traffic officials say the flashing yellow arrow will do something more: prompt drivers to exercise greater caution when making left turns.

"It's still the same rule of the road," said Robert Mack, the city's traffic engineer. The flashing yellow arrow, he said, will serve as an added reminder to look and yield before turning left - "and it's very self-intuitive. People see the yellow light and think, 'Wow, caution.' "

That's the conclusion the Federal Highway Administration drew after studying the arrow's effect on drivers. "They did a bunch of trials across the country with a bunch of different indications to say what besides a green round bulb might be safer," Mack said.

With the flashing yellow arrow, drivers made fewer mistakes than with a plain green light, he said.

The two new lights, expected to be installed in December, will be the first for Concord, and only the second and third in New Hampshire, Mack said. The state installed one last year on northbound Route 3A in Bow at the intersection of I-89 and Hall Street.

"Motorists seem to understand it very quickly," said Bill Boynton, spokesman for the state Department of Transportation. "Whereas the green circle, you're not quite sure: 'Can I go? Can I beat this guy before he goes? Is it advance green?' "

While the state has yet to install any more of the arrows, "we're putting them more and more into our specifications to bid," Boynton said. "It looks like it's probably going to be a national standard within five to 10 years."

Boynton said he didn't know the cost of installing the arrows.

When building a new stoplight, "putting the infrastructure in, it's really not a cost issue," Mack said. But he said the cost of upgrading lights around town would depend on their age and the existing computer's ability to control different traffic signals.

"The older the light gets, the higher the cost gets," Mack said. "We'll look at each one individually."

At the two Fisherville Road intersections where the signals will be installed, left-turning drivers will either receive a green arrow or the flashing yellow arrow, depending on the time of day. Mack expects drivers turning left onto Bog Road during heavier afternoon traffic will first see a green arrow, allowing several cars the right of way before drivers heading north or south on Fisherville Road get the green light. When that happens, the left turn arrow will start flashing yellow, Mack said.

The options will let the city better regulate traffic flow, he said. Drivers headed straight on Fisherville during nonbusy hours won't have to wait for a green arrow to run out before their light comes on. And that means less idling at stoplights, which means less of a carbon footprint, according to material on the city's website promoting the changes.

Lt. Greg Taylor of the Concord police said he couldn't say exactly how many accidents across the city are caused by drivers making improper left turns, since the department doesn't categorize them that way. But "there are a lot," he said. "Left turns are one of the more troublesome moves that we see."

While he hadn't reviewed the federal study examining the flashing yellow arrows, he said he had been told its conclusions.

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