From WSDOT:
http://www.wsdot.wa.gov/projects/sr520/pontoons/
This is a very interesting project, that now serves two purposes. The 520 bridge needs to be replaced and a plan is in the works for this replacement. However, in the interim the region is at great risk if the current bridge fails, for example in a major windstorm.
The problem is this: due to the geography of the region, expensive real estate and underplanning, there are few if any alternate routes. And while advances are being made with public transportation, there is still no train to the Eastside and limited direct access for buses. Furthermore, many major employers, including one of the biggest in the country, are located on the Eastside.
What this means is that, while 520 is neither a big nor a long highway, it is very critical to the region and its continued function, even as congested as the area is.
Losing 520 would be catastrophic. Whatever alternates that exist are already packed and they would be overloaded in this event. While there are plans to increase public transportation, they would involve buses, which have to travel on the same congested roads. The best hope would be for employees to telecommute, very possible with all the tech employers, but that does generate costs and policy issues.
That is why this project is so innovative. The longest part of building a floating bridge is making the pontoons. By having the pontoons staged the bridge could potentially be reopened within a month or 2, versus the several years it would otherwise take. However, the pontoons can also be used for the new bridge, therefore not wasting them.
Ben