It’s a Tarrant County milestone that shouldn’t pass without notice: Final approval granted by the Texas Transportation Commission last week means, for the first time, that a portion of a local roadway built with tax money is being converted to toll lanes.Several other toll projects are in the works for Tarrant County, but the common theme on those is use of projected toll revenues from the get-go to help finance construction and expansion of toll-free lanes. The conversion of 18.8 miles of high-occupancy-vehicle lanes to toll lanes on Interstate 30 between Arlington and Dallas will be different. The Arlington HOV lanes were part of $166 million in tax-funded reconstruction known as the “Three Bridges” project, which was completed in 2010.Texas Transportation Department spokesman Tony Hartzel said Monday that the I-30 toll revenue will pay for operating and maintaining the lanes and will help pay for other transportation projects in North Texas.A decade ago, when Gov. Rick Perry proposed the 4,000-mile Trans-Texas Corridor network of toll roads, rail right of way and utility easements, and as more toll-road agreements were made possible by the Legislature, conversion of tax-built roads to toll roads was a big no-no in the state, a line not to be crossed.The change in attitude has been slow and subtle.In 2006, the Federal Highway Administration designated I-30 a “value pricing pilot project.” That authorized tolls for single-occupant vehicles in the HOV lanes. “Value pricing” means tolls vary during the day depending on congestion.When the “Three Bridges” project was completed in time for Super Bowl XLV in Arlington, the HOV lanes were given an “interim” designation. Local, regional, state and national officials have been working out the details of the toll lane plan ever since.Now the HOV lanes are closed while a contractor completes $63 million worth of transition work, including limited entry and exit points for the toll lanes from near downtown Dallas to west of Fielder Road in Arlington. The toll lanes aren’t expected to open until 2015. The timing could change with contemplated reconstruction at the I-30/I-35E intersection in Dallas.