According to the Washington State Department of Transportation (WSDOT), electronic tolling on the 520 bridge is scheduled to start in December, although the exact date has not been set or released. So, if you haven't already done so, it's probably time to purchase a pass and be ready.
Once tolling starts, when you travel across the SR 520 bridge the system will automatically detect your “Good To Go!” Pass or license plate in both directions. The toll will then be deducted from your Good To Go! prepaid account. Those without a Pass such as visitors or infrequent users will billed and pay by mail.
There are five or six different Good to Go! pass options. WSDOT has a video explaining the options and a website as well. It's nice to see everyone in the video so happy and smiling about paying this new tax.
Good To Go! User Guide Video