State Route 520 has a set of ghost ramps in the marshlands the arboretum. They are often referred to as "ramps to nowhere". However, one ramp is currently used for the on ramp to SR 520 Eastbound. The others are unused. They were originally part of a plan to build the R. H. Thomson Expressway which would have cut through the arboretum and down through Seattle towards the I-90/I-5 interchange. Citizens rallied a freeway revolt against the plan on May 4, 1969. Construction near the Arboretum later continued but citizen protest eventually won out and the plan was dropped in 1971.