If the Port of Seattle continues growing at its recent pace, the shipping trade will require night shifts that conflict with a proposed basketball arena in Sodo, a Port-hired traffic expert reported Monday.

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If the Port of Seattle continues growing at its recent pace, the shipping trade will require night shifts that conflict with a proposed basketball arena in Sodo, a Port-hired traffic expert reported Monday.

The analysis by transportation engineer Marni Heffron offers little by way of new findings, but she did gather trucking data to bolster the Port’s anti-arena position.

An investor team led by Chris Hansen, Steve Ballmer, and Erik and Peter Nordstrom is proposing an arena for basketball, hockey and other events, south of Safeco Field.

Arena supporters have derided the Port’s growth aspirations as mere hopes.

They argue that the waterfront container docks typically close their gates at 4:30 p.m., leaving time to spare before a 7 p.m. ballgame.

Highlights of Heffron’s analysis include:

• At a growth rate of 3.5 percent per year, the Port would move the equivalent of 3.5 million 20-foot containers a year by 2030, its “Century Agenda” goal, compared with 2 million units now. At only 2 percent growth, the goal would be met in 2050.

• Meeting that goal would translate into 4,200 one-way truck trips added per day, most of those to the Seattle International Gateway rail yard in Sodo, and the Argo rail yard south of Spokane street.

• The trucking firm MacMillan Piper already stops sending trucks to Terminal 46 after 2:30 p.m. on Mariners baseball game days, and some Eastern Washington firms will not send trucks to Seattle on game days. Waterfront truck trips in June were 9 percent lower on a game day compared with nongame days, Heffron said.

“We believe the proposed arena would put our Sodo operations at risk,” John Odland, chairman of MacMillan Piper, told Port commissioners Tuesday.

• Even with a new overpass, to be built above the new Highway 99 into T-46, the intersection of Edgar Martinez Drive and First Avenue South will be severely congested.

Heffron makes no mention of future Highway 99 tunnel tolls, which are likely to aggravate traffic delays for sports fans, commuters and truckers alike, if rates are set high enough to cause massive diversion.

Tim Ceis, Sodo-area consultant for the Seattle Department of Transportation, said the report doesn’t show how the Port might meet those cargo goals.

“There’s no data or analytics behind it to suggest whether it’s achievable,” he said Tuesday.

Port statistics show that container loads have doubled since 1990, to a record 2.2 million units in 2010, Heffron found. However, the Port lost the Grand Alliance shipping consortium, amounting to 20 percent of its container loads, to the Port of Tacoma this summer.

That setback has provided political spin to Port backers who say Seattle can’t afford to degrade its traffic conditions and still compete; and to arena supporters who say the Seattle Port’s growth plans are inflated.

Competition will come from widening of the Panama Canal and projects to boost British Columbia’s import capacity.

Ceis also argues that a potential 18 daily coal trains would cause far more traffic havoc than an arena, but the Port hasn’t complained. Thirdly, he said the Port could squeeze another hour of shipments in before game time, if the docks operate through the noon hour. They now close for the longshore union’s lunch break.

Aaron Pickus, spokesman for pro-arena Mayor Mike McGinn, said the Port hasn’t yet responded to an invitation by the city and King County to partner on 12 spot improvements to Sodo-area traffic bottlenecks. The Port, in fact, has planned and budgeted funds for some already.

Port commissioners voted 5-0 Tuesday to pass a resolution urging the Seattle City Council not to make agreements with Hansen’s ArenaCo, until after an environmental-impact statement is completed.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.