A traffic study of Edgartown’s seasonally-congested Upper Main street area will be the first step in a public review of a planned expansion of the Shop & Stop store in town.

Last fall Stop & Shop representatives came before the Edgartown planning board to unveil preliminary plans for an 11,320-square-foot addition to the 25,000-square-foot grocery store. Expansion plans for the Edgartown store emerged about a year and a half after the company withdrew plans to expand its Vineyard Haven location.

The subject was back before the Edgartown planning board this week for a scoping discussion about a traffic study for the plan. Commissioned by Stop & Shop, the study will be used by the planning board and the Martha’s Vineyard Commission in regulatory reviews of the project.

The engineering and design firm Vanasse Hangen Brustlin is overseeing the expansion, including the traffic study.

With an outline of the study on hand, commission executive director Adam Turner met with planning board members and staff Tuesday to narrow the scope of the work. A key question was how to gear the study: for a peak-traffic summer day in August or for slower off-season periods.

“If we take a 12-month average that’s a false reading,” planning board chairman Michael McCourt said. “We have eight months that’s nothing and then August, so . . . . I think we take a good hard look at the summer months,” he concluded. Mr. McCourt, who owns a business in the heart of town, said summer traffic congestion deters people from visiting downtown.

But board member Robert Cavallo suggested using off-season traffic as a benchmark.

“My personal opinion here is we want to control traffic on Martha’s Vineyard — that starts in Woods Hole and not here,” Mr. Cavallo said. “There’s nothing we can do about those 10 weeks in front of Stop & Shop — it’s busy, it was busy 20 years ago. Today there was no traffic, in and out.”

Board members said the roundabout, installed two years ago at the intersection of Barnes and Edgartown-Vineyard Haven Roads, appears to have worsened traffic congestion at the Triangle and along Upper Main Street. Another point of concern centers on the Vineyard Transit Authority bus stop near the grocery store, which planning board members said is one of the busiest stops in town, causing frequent traffic backups.

Mr. Turner said he and planning board assistant Georgiana Greenough will convey the ideas and suggestions to VHB traffic planner Randy Hart in the next few weeks. He said the traffic study will help develop models and explore how an expansion would affect traffic, as well as potential mitigation or improvements, including analysis of whether a turning lane should be built on the road. Planning board members welcomed the latter idea.

“I’d like to see a professional planner’s opinion of a third lane,” said Mr. Cavallo, adding that the issue had been discussed for years.

Geoghan Coogan, an attorney representing Stop & Shop in the permitting process who attended the meeting for another matter, said a turning lane would need approval from the state Department of Transportation.

The project will require review by the commission as a development of regional impact (DRI), as well as planning board and board of health review.

During the discussion Tuesday, Mr. Turner outlined a collaborative approach to the public process, involving hearings at the town level at an early stage. He said he hopes the planning board will have an initial hearing on the plan to raise questions and create a staff report before the project is formally referred to the commission.

“The last thing we want, the last thing the applicant wants, is a full review [at the commission] and none of your stuff has been addressed,” Mr. Turner told the planning board. “If you give me the comments, we will raise them over there.”

“Traffic is going to be a big issue on this project,” he added.

The first hearing before the planning board on the plan is tentatively expected sometime in March.