Highway 99 Seattle WA

So, why is STATE HIGHWAY 99 considered merely a Local Access Only Road? This is one of two Major routes in and out of this city to the South, the other Being Interstate Highway 5 and proportionately to its lane count sees just as much traffic….. It actually has the same speed limit in places as State Highway 520, and just as many Lanes?

WA State Law Regarding Designation of State Highways as Arterials:
http://apps.leg.wa.gov/rcw/default.aspx?cite=46.61.400