The main mode of transportation, however, relies on Seattle's streets, which are laid out in a cardinal directions grid pattern, except in the central business district where early city leaders Arthur Denny and Carson Boren insisted on orienting their plats relative to the shoreline rather than to true North. Only two roads, Interstate 5 and State Route 99 (both limited-access highways), run uninterrupted through the city from north to south. State Route 99 runs through downtown Seattle on the Alaskan Way Viaduct, which was built in 1953. However, due to damage sustained during the 2001 Nisqually earthquake the viaduct will be replaced by a tunnel. The 2-mile (3.2 km) Alaskan Way Viaduct replacement tunnel was originally scheduled to be completed in December 2015 at a cost of US$4.25 billion. Unfortunately, due to issues with the worlds largest tunnel boring machine (TBM), which is nicknamed "Bertha" and is 57 feet (17 m) in diameter, the projected date of completion has been pushed back to 2017. Seattle has the 8th worst traffic congestion of all American cities, and is 10th among all North American cities.
Try to answer this question if possible (otherwise reply "unanswerable"): What is mainly used for transportation in Seattle?
streets