Voters in the region have approved three system plans: Sound Move in 1990, Sound Transit 2 in 2008 and Sound Transit 3 in 2016.

System plans derive from the Regional Transit Long-Range Plan, which is updated periodically. The Long-Range Plan represents a list of projects the agency can choose from when building a system plan to present to voters.

In a system plan, Sound Transit selects projects, describes approximate route and station locations, funding and tax sources, cost and revenue projections, and project timelines. Sound Transit combines these projects into a draft system plan that goes to the community for input.

Based on public input, technical studies, and budget and time constraints, the Sound Transit Board selects a proposed system plan made up of representative projects and financing and puts the system plan before voters. If voters approve the system plan, Sound Transit begins planning and environmental work for each project, followed by design, engineering and finally construction.