Work began this week on the Lagoon Pond drawbridge — a project estimated to cost a total of $31.5 million and reach completion in 2013.

State highway officials plan to first build a temporary bridge for $9 million on the pond side of the current drawbridge, then demolish the existing bridge and build a permanent replacement in its place.

The drawbridge, which was built in 1935 and has been in poor condition for years, is a key link in the Vineyard road system. The bridge also provides crucial access to the Martha’s Vineyard Hospital in Oak Bluffs.

The bridge straddles the boundary between Oak Bluffs and Vineyard Haven. Its reconstruction has been long awaited and often controversial.

Melinda Loberg, chairman of the Lagoon Pond drawbridge committee, reported this week that signs have been installed around the construction site to reduce the speed limit for the duration of the construction. Traffic on the bridge may be restricted to a single lane once work gets under way.

By mid-November, Ms. Loberg said, a barge will be in place in the lagoon for piling and other water work.

According to the state’s current timetable, the bridge pilings will be installed by next February and the abutments completed late next March.

Ms. Loberg said workers will assemble the temporary bridge between the spring of 2008 and fall of 2009. The temporary bridge is scheduled for completion by September 2009.

Parsons Engineering of Canton will design the permanent bridge. The firm will determine the height and location of the boating channel into the pond. Parsons also will determine whether the new drawbridge should be concrete or steel, and have a single or double lift section.

The drawbridge committee, which was created by the Oak Bluffs and Tisbury selectmen, plans to host meetings for Island residents with Parsons and MassHighway as work on the new bridge goes forward.