Before a foot of concrete is poured on a new Highway 520 floating bridge, Washington state taxpayers have spent $221 million on planning, environmental studies, public meetings, and engineering.

While much of this work will bear fruit in the ultimate bridge design, millions of dollars were certainly spent researching ideas that dead-ended, as political momentum shifted over the years.

The state Department of Transportation last month signed a contract for pontoon construction to start late this year in Grays Harbor County, the first step in a $4.65 billion project. But a Montlake Interchange design is far from settled, and there's no final decision yet on how wide to make the floating bridge's deck.

State Department of Transportation released the $221 million figure Wednesday, in response to requests by The SeattleTimes and another news service.

Lawmakers in 2007 settled on two general lanes and one carpool lane each direction, but didn't dictate how or when to add future light rail or bus-rapid transit. There's a new push by Seattle Mayor Mike McGinn, some lawmakers, and a neighborhood coalition to make the bridge rail-ready from day one, using two of the six lanes.

The 520 spending is in addition to $308 million spent preparing for a Highway 99 replacement -- from the 2001 Nisqually Earthquake until early 2009, when state lawmakers decided to build a bored tunnel.

Put those figures together and you've spent a half-billion dollars, comparable to three miles of above-ground light rail or one sports stadium.

In the case of Highway 99, the DOT studied at least eight options for highway construction, before engineers and politicians agreed to attempt the nation's widest bored tunnel.

"Some of the designs for an elevated, we'll never use, a lot of the cut-and-cover -- that's history," said Ron Paananen, Highway 99 project director, in an interview last spring. But the engineering in Sodo is being used, as are soil samples, he emphasized. And the nature of modern megaprojects is to explore all ideas, as due diligence to reassure the public and to meet environmental-study requirements, he said.

One lesson from Highway 520, said state DOT Deputy Director David Dye, is that high-ranking politicians should be involved early in the planning, to set clear directions that guide the technical work. A Translake committee studied 520 from 1997 to 2003, while throughout 2008 a nonbinding "mediation" group of 33 stakeholders, neighbors and officials disagreed, some backing billion-dollar variations such as tunneled Montlake exits.

"I believe we have received good value on all the projects we paid for to date. That said, it is an awful lot of money," said Dye.

Nationally, megaprojects spend 2 to 7 percent of budget on preliminary work, and the two Seattle highways fit the profile, he said.

Whether a nine-figure planning budget is well spent will finally be judged by the public, when they see a completed bridge, Dye surmises.