State Route 520 was built in the early 1960s where it is because so much of it could be on state (Arboretum) land that did not have to be purchased or condemned. If the decision on siting were being made today, SR520 would have been built elsewhere or not at all. Federal laws and the public now discourage the taking of park lands for highways. The National Environmental Policy Act, the Washington State Environmental Policy Act and the Washington State Public Disclosure Act were not passed until years later, and in their absence, the original SR520 planning grossly neglected the environment and kept the public in the dark.

Daniel Evans, a future Governor who was then a Seattle state legislator, was a prominent opponent of the SR520 project. The final decision to build was made between the Washington State Department of Transportation (then more honestly called the Highway Department) and non-Seattle state legislators. The late Bill Burns, a Seattle legislator who lived in Montlake, was completely left out of the decision.

Ever since the four-lane SR520 was completed in 1963, WSDOT has repeatedly tried to expand it to six or more lanes, but has so far been thwarted by elected officials working closely with grassroots groups. One of the most dramatic episodes was debate over the R.H. Thompson Expressway, whose proposed interchange in Union Bay channeling traffic into north and south Seattle through the Arboretum and the University District closely matches the so-called Pacific Street Interchange of the current era.

By bringing huge amounts of traffic from south and north to SR520, the R.H. Thompson Expressway would have necessitated the addition of lanes to SR520. However, Seattle voters would have none of it. More than a thousand stood up for the Arboretum in a colorful march. Seattle voters stopped the project dead by withdrawing the City's cooperation in 1972.

Maynard Arsove, who had been chair of the UW Mathematics Department, President of the Montlake Community Club, and chair of Citizens Against the R.H. Thompson, founded the No Expansion of SR520 Citizens Coalition in 1994 with many others, served as its first chair, and remains as a vice chair.

Those in current control of the Montlake Community Club wish to expand SR520 to six lanes and largely move the increased SR520 traffic into other neighborhoods and the Arboretum via the Pacific Street Interchange. In contrast, the Montlake Community Club was for decades an important ally and leader in successful efforts to keep SR520 from being expanded beyond the existing four lanes, and to protect the Arboretum from SR520 expansion. Only time will tell whether this switch in allegiances causes an expansion that Montlake could have helped prevent.