Neighborhood groups in Seattle dispute Gov. Chris Gregoire's assertion that a new bridge would be delayed 18 to 24 months, if the state were to take their advice and redesign the carpool lanes to be exclusively for high-capacity bus and rail.

The group issued a letter calling the transit-only lanes "a minimal change," because existing state law already requires that a new bridge be able to carry bus-rapid transit and eventually light rail.

Gregoire herself insisted in January 2008 that the bridge be buoyant enough, or could be modified, to support trains someday (four railcars can weigh up to 200 tons). The long-term strategy was left ambiguous. But two years ago, some neighbors declared that rail should replace the HOV lanes, instead of causing the bridge to be widened. In other words "six means six," a similar stance to the one declared by neighbors and area legislators this week.

"Construction of all six lanes [of 520] could start as soon as conceptual agreement is reached. So with goodwill, the project could move ahead promptly," argues the letter by Fran Conley, co-coordinator for Coalition for a Sustainable SR 520, and co-signed by leaders of the Sierra Club and Cascade Bicycle Club.

And in the case of the Alaskan Way tunnel proposal, the state changed course in a few weeks to ditch surface and elevated "hybrid" options and become convinced to seek a bored tunnel, in December 2008.

But it's not engineering so much as legal rules that would cause delays of up to two years for 520, by forcing changes in the environmental impact statement, the governor said in her letter to the City Council this week.

Transit would still use any carpool lanes, as is done by express buses throughout the region.

The neighborhood groups are hoping transit-only lanes would allow for smaller ramps at a Montlake Interchange, or at least, limit the overall car traffic entering Seattle.

A new pontoon construction plan, by the state Department of Transportation says the six-lane bridge can be retrofitted in the future for rail, by adding "supplemental stability pontoons" below deck.