Over the past two years, Bay Area transit agencies have rolled out the Clipper Card system. The single plastic card allows riders to travel across over 20 transit systems throughout the Bay Area, including AC Transit, BART and SF Muni. Though not without it's technical glitches and implementation criticism, the Clipper Card has successfully attracted commuter support and adoption. This success has been, in part, due to the use of critical design elements outlined in McGrath and Murray's "Principles of e-Government":