The SR 520 construction program is replacing half-century-old bridges at risk of failure in a severe earthquake or windstorm with stronger structures built to modern safety standards. Moreover, we are making significant improvements to this key urban corridor that will enhance the public’s transportation options and mobility for generations to come.

More growth = more traffic

When SR 520 and its four-lane floating bridge across Lake Washington opened in 1963, King County had fewer than 1 million residents. Bellevue, the Eastside’s largest community, had only about 13,000 people. Since then, the county’s population has more than doubled, while Bellevue’s has swelled tenfold. The area’s robust growth is expected to continue. The Puget Sound Regional Council predicts that, between 2000 and 2040, the Central Puget Sound region could add 1.2 million more workers and another 1.8 million residents.

The region’s growing population and growing traffic has long overtaxed SR 520’s half-century-old design. Although traffic volumes eased some with the 2011 start of tolling on the SR 520 floating bridge, the highway remains heavily congested during peak commute times, with average speeds below 20 mph.

Because the old, unimproved highway is largely without shoulders, a disabled vehicle causes lengthy backups. And with no dedicated HOV lanes (yet) on much of the highway's western segment in Seattle, SR 520 forces buses and carpools to trudge through the often-choked general-purpose lanes.

Greater mobility for SR 520 users

The SR 520 Bridge Replacement and HOV Program is constructing new highway features that provide greater mobility options for all users, including drivers, transit riders, bicyclists and pedestrians. When SR 520 is fully built out, the highway’s corridor improvements will give the public:

• A dedicated transit/HOV lane, in both directions, from I-5 to Redmond, providing bus riders and carpoolers safer, quicker and more reliable travel.
• Better operations on the Eastside, with median transit stops already open at Evergreen Point Road and 92nd Avenue Northeast, and direct-access ramps open at 108th Avenue Northeast for buses and carpools to make entering and exiting the highway safer, quicker and more reliable.
• Transit improvements in Seattle, with HOV direct-access ramps at the Montlake interchange for buses and carpools traveling to or from the Eastside, and HOV lanes on Montlake Boulevard.
• A regional, cross-lake bicycle and pedestrian path along SR 520, from I-5 to I-405, with connections or close proximity to local bike trails, local and regional bus stops, and light-rail stops.
• A floating bridge design that will allow for the addition of light rail on SR 520 if the region chooses that option in the future.
• Community-connecting highway lids that provide better connections to transit stops, bicycle-pedestrian paths, local streets and SR 520 itself.

Regional congestion relief

The improved SR 520 corridor, when fully built out from I-5 to I-405, will enhance highway safety, reduce congestion and lower travel times for bus riders, carpoolers and drivers. When all phases of the program are funded and complete, regional benefits will include: 

• More reliable and quicker trips between Seattle and the Eastside. When compared to a no-build scenario with no improvements to SR 520, for example, reduced bottlenecks from our highway enhancements will decrease HOV travel time from Seattle to Bellevue by up to 25 minutes during peak periods, and by up to 31 minutes for drivers in the general-purpose lanes.
• Improved on-ramps and merge conditions, and reduced traffic queues onto local arterials after the six-lane corridor is extended to Seattle and a westbound auxiliary lane for I-5 merging is completed on the Portage Bay Bridge.
• Wide shoulders on the new floating bridge and connecting highway for disabled vehicles to pull off and not block traffic as they do today on the shoulderless roadway.
• Significant economic savings thanks to less time spent in traffic slowdowns and backups. A study of the rebuilt Eastside segment (pdf, 182 kb), for example, finds that drivers there are expected to save 1.4 million hours of travel time every year – and $467 million in travel-time savings.
• A 39 percent increase by 2030 in the number of people using SR 520's HOV lanes daily (that's 19,000 more than the number using the Eastside’s new HOV lanes).
• A 15 to 17 percent increase in the total number of people SR 520 carries during the morning and evening commutes as more people use buses and carpools.  This increase in person trips occurs with only a 5 percent to 10 percent increase in vehicles on the highway.
• A 5 to 10 percent reduction in vehicle miles traveled on SR 520 and a nearly 10 percent reduction in vehicles’ greenhouse gas emissions when compared to a "no-build" option with no highway improvements.