A day after Seattle and King County suggested a partnership with the Port of Seattle to pay for road improvements in Sodo, mostly for freight trucks, the Port repeated its opposition to building a sports arena in the neighborhood.

Port CEO Tay Yoshitani emphasized the Port has spent $1 billion on dock and machinery construction to modernize container terminals, with hopes of growing the container trade more than 50 percent over the long term. Local governments should study non-Sodo sites for an arena before signing a deal, he said.

"Seattle and King County should first fully understand the risks of jeopardizing this investment, and the potential cost to the family-wage jobs and tax revenues supported by the seaport," Yoshitani wrote in a letter Wednesday to the city and county councils.

Yoshitani pointed to arena-related traffic on I-5 and in industrial areas as a threat to competitiveness.

Several of the road projects talked about this week already have been on the drawing board, partly or fully funded, for several years — and all sides agree they're desirable even without an arena.

Senior staff for Mayor Mike McGinn and County Executive Dow Constantine on Tuesday issued a list of 13 improvements, worth roughly $25 million, and called for an agreement to set schedules and funding shares. The city and county didn't say where they would get the money.

"We hope the Port will accept this proposal as a way to move past the debate on the Arena and toward a commitment to an improved transportation system," the city-county letter said, hitting the political ball into the Port's court, in advance of a public hearing Thursday on the arena proposal.

The largest two projects are a new $5 million to $10 million road to the Argo rail yard to help trucks avoid Highway 99 south of the West Seattle Bridge; and a traffic-control system to direct sports fans or commuters away from congested Sodo spots. Port staffers said Wednesday they set aside $2.7 million toward the Argo road and its surroundings.

ArenaCo, led by hedge-fund manager and Seattle native Chris Hansen, proposes an 18,500-seat arena at South Holgate Street and First Avenue South, for pro basketball and hockey. Safeco Field and CenturyLink Field are just to the north.

Yoshitani did not address the letter to arena supporters McGinn and Constantine, but to council members, some of whom have yet to state a position.

McGinn's spokesman, Aaron Pickus, responded: "We're looking for a response to the (13) projects, and right now, we don't see that in this letter."

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