Sound Transit said it will sue to block Mercer Island’s threatened lawsuit over the planned closure of access ramps to Interstate 90 when light-rail construction begins in June.

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Sound Transit’s CEO Tuesday vowed to take “all legal actions necessary” to block Mercer Island’s threatened lawsuit over the planned closure of Interstate 90’s center lanes when light-rail construction to the Eastside begins in June.

Peter Rogoff said the agency will continue to negotiate with the city over potential traffic mitigation but warned that delays to the East Link project pose “significant risks of increased costs to regional taxpayers and significant delays to opening the project in 2023.”

The Mercer Island City Council on Monday night voted unanimously to sue Sound Transit and the Washington State Department of Transportation (WSDOT) over the scheduled lane closure and the closure of the city’s access ramps at Island Crest Way. Island drivers currently enjoy direct access from Island Crest Way to the freeway’s high-occupancy-vehicle lanes, even for solo drivers.

City leaders say they aren’t trying to block light-rail construction across the island, but object to the planned closure of the island’s busiest entrance ramp without any plans to mitigate the expected traffic congestion.

“After two years of negotiation, we still have not reached a satisfactory agreement with Sound Transit and WSDOT that would avoid the diversion of heavy commuter traffic to local neighborhood streets and school zones. No community would accept that,” said Mercer Island Mayor Bruce Bassett.

The City Council agreed to authorize legal action to temporarily halt the shutdown of the center roadway and Island Crest Way onramps. The council also imposed a six-month moratorium on any permits needed for the construction of light rail across the island and a planned Sound Transit station.

City leaders say that longstanding agreements with the two agencies assured Mercer Island residents that any impacts from the center roadway closure would be mitigated to preserve their mobility.

In addition to congestion, city leaders say they’re concerned about the potential economic impacts to the island if traffic in the Town Center comes to a standstill and people from off-island can’t get there to shop or visit restaurants.

Bassett expressed frustration that Sound Transit and WSDOT had yet to come up with concrete proposals to ease congestion, either short- or long-term.

“Even if you accept the premise that you’re going to push traffic onto city streets, it’s not realistic to design, bid and construct these mitigation projects in three months,” Bassett said.

The state notified Mercer Island this month that federal regulations prohibited giving solo drivers access to the new I-90 HOV lanes that will be added by restriping the existing general-purpose lanes before the center lanes are closed.

Bassett pointed to 2006 and 2007 agreements with WSDOT and Sound Transit that assure residents access to the new HOV lanes under the same terms as their current access to the transit lanes.

The state has identified three long-term projects to improve access to I-90, including a new ramp at Island Crest Way to the new HOV lane, but noted the cost could exceed $60 million.

The state also said the new HOV lanes could be converted to high-occupancy toll (HOT) lanes, but that plan would require legislative authorization, funding and regional concurrence.

Finally, the state said the new HOV lanes could be converted to general-purpose lanes between Island Crest Way and Seattle, but noted that would also require policy discussions about regional mobility and the management of the interstate system.