Implement a commuter #toll from Williamson County into Travis County in the mornings. Similar to the Manhattan tolls http://www.panynj.gov/bridges-tunnels/pdf/toll-table-2001-2015.pdf.
This will incentivize people to live closer to the workplace, commute less distance, and carpool/use alternative transport more. In addition it will encourage less low density suburban development in the surrounding areas resulting in much less congestion. Most of the congestion is a result of sparsely populated low density housing far away from where people need to be daily.