Due to Seattle's isthmus-like geography and the concentration of jobs within the city, much of the flow of transportation in the metropolitan area is through the city proper. North-south transportation is highly dependent on the Interstate 5 corridor, which connects the Puget Sound area with southwest Washington cities and the Portland metropolitan area, as well as cities to the north such as Bellingham and Vancouver, Canada. I-5 continues as British Columbia Highway 99 at the U.S.-Canada border's Peace Arch crossing, between Blaine and Surrey. State Route 99 is also a major arterial in the western half of the city; it included the now-defunct Alaskan Way Viaduct along the Seattle waterfront. The Alaskan Way Viaduct replacement tunnel was opened in place of the elevated viaduct in February 2019 on account of seismic instability.