We've done the models, it's time to build the real bridge

The closure of the Highway 520 bridge during the recent windstorm was only the latest reminder of the vulnerability of this structure.

The hard work of local government leaders on both sides of Lake Washington has forged a regional consensus on how to proceed with replacing the bridge, and we must build on that consensus to keep the project moving.

Gov. Christine Gregoire has now endorsed a six-lane alternative for the Highway 520 replacement project, adding safety shoulders, a bicycle/pedestrian path, and a transit/HOV lane in each direction to the current four lanes. This option is supported by cities on both sides of the lake, and it offers us the opportunity to improve mobility and safety, increase transit use and enhance communities and the environment.

Of the two six-lane alternatives that the state Department of Transportation has studied, however, the six-lane "Pacific Interchange" will do a much better job of achieving these goals than the "Montlake Interchange."

The Pacific Interchange proposal was initiated by community members and has been endorsed in approximately two-thirds of the comments to the state on the draft environmental-impact statement.

It is the only option that will allow Metro buses to directly connect to Sound Transit's LINK light-rail stop at the University of Washington's Husky Stadium, providing a vital connection between two multibillion dollar transportation projects and giving UW students and staff and other commuters a significant incentive to use transit.

By allowing through traffic to the north to bypass the neighborhood altogether, the Pacific Interchange will vastly improve the Montlake Bridge and Montlake Boulevard bottlenecks, saving up to 20 minutes in travel time on Montlake Boulevard Northeast between Northeast 45th Street and Highway 520.

It is also the only option that will completely lid Highway 520 in Montlake and create a continuous greenbelt with trails from Portage Bay to the Arboretum.

The Pacific Interchange has a number of other benefits, including reducing the number of severely congested intersections in the area from five today to two in 2030, with no increase in traffic on nearby streets.

It will require the lowest number of property acquisitions of any alternative. It also has the lowest number of residences with noise impacts, and 50 percent fewer lanes over Portage Bay than the other six-lane alternative.

The old idea that transportation corridors like Highway 520 funnel people into the city in the morning and out in the evening is particularly not applicable in this case, where 520 is a two-way street connecting urban centers on both sides of the lake.

It is used for commuter purposes in both directions and is a critical element in implementing the region's growth-management plan, which requires effective transportation linkages connecting increasingly dense urban areas.

By dramatically increasing transit reliability and decreasing travel time between the Eastside and the university area and downtown Seattle, this alternative does not increase single-occupancy vehicle use, but significantly increases the number of people who choose transit.

Traffic models of a six-lane bridge show many more people crossing the lake, but hardly any increase in vehicle numbers. By 2030, a majority of bridge trips, 54 percent, are projected to be by transit or carpool, up from 29 percent today. By stimulating this mode shift, this alternative reduces pollution and greenhouse emissions, and supports growth management by connecting urban centers efficiently.

Both six-lane alternatives have impacts on the natural environment that must be fully mitigated. The good news is that the number of support columns in the Arboretum area will be greatly reduced with either new bridge.

All alternatives are significantly higher than the current highway, allowing more light and air under the highway and improving the environment. All alternatives will improve water quality by capturing and treating the runoff from the bridge, which currently flows directly from the highway into the lake.

While the amount of wetlands lost is relatively low (one-fifth of an acre under either six-lane alternative), regrettably there will be nearly four acres of parkland lost and significant shading from the new higher bridge.

Full replacement of the wetlands and parks spaces is possible and must be part of the project, along with measures that will reduce traffic impacts on the Arboretum.

With appropriate mitigations, the six-lane Pacific Interchange option offers the greatest mobility of all the project alternatives, in a way that will improve livability in Seattle neighborhoods and in the region. We have already built a figurative bridge across the lake in agreeing on an alternative, and now it's time to build the real bridge.

Richard Conlin is a member of the Seattle City Council and chairman of the council's Committee of the Whole on Highway 520.