The Federal Highway Administration has the ultimate responsibility for determining whether Sound Transit can replace the two center roadway lanes on I-90 Bridge with light rail. Their decision would presumably be predicated on Sound Transit demonstrating light rail operation won’t damage the I-90 Bridge and that the fourth lanes they add to the outer roadways will provide adequate capacity to accommodate all cross-lake vehicles.
In September 2005 the WSDOT attempted to verify “floating-bridge/light rail compatibility” using flat bed trucks to simulate light rail cars. They concluded the bridge can be structurally retrofitted to carry the loads associated with the light rail system. The Sound Transit 2008 DEIS dealt with the capacity issue with promises “Travel times across I-90 for vehicles and trucks would also improve or remain similar with East Link.
The 10/1/13 post details how both the FHWA and the Legislative Joint Transportation Committee concluded additional testing was needed to demonstrate “floating bridge/light rail” compatibility. Five years after being advised of these concerns WSDO/ST is finally having additional testing done at the Transportation Technology Center in Pueblo, Colo. Apparently it will take until the end of the year to complete. The FHWA will presumably use its “Oversight Responsibility” to determine whether the results verify the bridge can be “retrofitted” to withstand the loads associated with the four 74-ton light rail cars ST has promised for East Link operation.
The FHWA document, “ I-90 Two-Way Transit and HOV Operations Project, Record of Decision, September 2004” dealt with the bridge capacity issue. It included the following “Decision” regarding Federal Highway Administration’s approval of the R-8A Alternative;
The Federal Highway Administration (FHWA) concurs with the Washington State Department of Transportation (WSDOT) and Sound Transit in the designation of Alternative R8-A as the selected alternative for the I-90 Two-way Transit and HOV Operations Project in Bellevue, Mercer Island and Seattle King County Washington.
However, the Alternative R8-A Alternative the FHWA concurred with in that document is not the R8-A Alternative WSDOT/ST has chosen for East Link. The FHWA R-8A Alternative is defined on page 9 of the document.
Alternative R-8A will provide HOV lanes on the outer roadways. It will retain the existing reversible operations on the center roadway, with both lanes operating in the same direction, westbound in the AM and eastbound in the PM. SOVs will only be allowed to use the center roadway between Rainier Avenue in Seattle and Island Crest Way on Mercer Island. The center and outer roadway HOV lanes will likely operate with a 2 + occupants per vehicle restriction
Thus, contrary to WSDOT/ST claims the center roadway could be used for light rail, it’s clear the R8-A configuration the FHWA approved required the center roadway be used for vehicles. The FHWA surely has an “Oversight Responsibility” to insist ST demonstrate their modified outer roadway has the needed capacity. Doing so would require ST to expedite adding the 4th lane and temporarily close off the center roadway.
In conclusion, assuring a floating bridge can support light rail operation is a "necessary" but not "sufficient" reason for the FHWA to allow light rail on I-90. Responsible "Approval" requires they insist ST demonstrate the modified outer roadway has the needed capacity. ST current plans to delay completing the 4th lane until 2016 undoubtedly reflect concerns tests will show it won’t provide needed capacity. (Just one of the reasons ST continues to delay a simple bridge modification that would reduce congestion for all cross-lake commuters.) Whether the FHWA will force ST to conduct tests they "fear" would prevent them from installing light rail on center roadway remains to be seen.