In the second page of the Plan view, you can see the post-WWII "Mothball fleet" stationed off of Priest Point Park. (top right hand corner)

It's apparent that this was a study of placing the Route 99 expressway along the corridor occupied (in part) by the railroad grade which passes the Capitol and goes through the downtown rail tunnel today... so this would have looked rather like the Eisenhower Expressway in Chicago (a slot trench which divides neighborhoods and forms a kind of "suicide" corridor.)  In those days, all of the downtown traffic was on 99 and went right down through the middle of town.  Friday evening and Sunday evening were impossible for locals, who avoided the main strip.  This traffic was also a large 'boost' to the economy, though - since drivers frustrated with traffic could stop and wait it out at the Spar and similar locales. In one picture you can see the Texaco station right on 99.

Instead, they later decided that smashing downtown Tumwater would be a better economic move.