I spotted this interesting exterpt posted by Brian Purcell at
http://texhwyman.home.att.net/tex.htm

“Until a few years ago, Texas law prevented the state from building or operating toll highways. Instead, they could be built and operated by subsidiaries of TxDOT created specifically for that purpose. For instance, the Texas Turnpike Authority built and operates tollways in the Dallas area and the Harris County Toll Authority built and operates several tollways in the Houston area (see sidebar). In the late ’90s, a private toll road was built to connect the Columbia Solidarity Bridge northwest of Laredo to I-35. In recent years, with worsening congestion and insufficient funding, the state has been pushing toll roads as a viable option to get new roads built. Ergo, the first phase of a toll road from Georgetown to Seguin is currently under construction to relieve congestion on I-35 in Central Texas and several toll roads are under construction or planned in the Austin area. Recently, there have been additional toll road proposals across the state, especially in the San Antonio area (see the San Antonio Area Tollway System page for more info), not to mention the statewide Trans-Texas Corridor project.”