limits may not meaningfully improve air quality. In roughly a radius of the Houston–Galveston and Dallas–Ft. Worth regions, the Texas Commission on Environmental Quality convinced the Texas Department of Transportation to reduce the speed limit on all roads with speed limits by 5 mph. This was instituted as part of a plan to reduce smog-forming emissions in areas out of compliance with the federal Clean Air Act. Initial studies found that lower speed limits could provide roughly 1.5% of the emissions reductions required for Clean Air Act compliance. However, follow-up studies found that the actual reduction is far less: The emissions modeling software initially used, MOBILE 5a, overestimated the emissions contribution of speed limit reductions. Rerunning the models with the next generation software, MOBILE 6, produced dramatically lower emissions reductions. Speed checks in the Dallas area performed 1 year after implementation of speed limit reductions show that actual speed reductions are only about 1.6 mph, a fraction of the anticipated 10% (5.5 mph) speed reduction. With both of these facts combined, it is possible that the speed limit reductions only provide a thousandth of the total emissions reductions necessary for Clean Air Act compliance. In mid-2002, all speed limits in the Houston–Galveston area were capped at . Facing immense opposition, poor compliance, and the finding that lowered speed limits produced only a fraction of the originally estimated emissions reductions, the TCEQ relented and reverted to the 5 mph reduction scheme. Due to its enormous unpopularity, the Texas Commission on Environmental Quality examined alternatives to the 55 mph speed cap. Analysis suggested that the vast majority of emissions reductions were from reduced heavy truck emissions. A proposed alternative was to restore passenger vehicle limits but retain a 55 mph truck speed limit. Concerns about safety problems and enforceability of such a large differential (up to 15 mph on many roads) scuttled that proposal, and a compromise plan, described above, was enacted that retained uniform, but still reduced, speed limits. In 2003, the Texas Legislature prospectively banned environmental speed limits, effective September 1, 2003. The wording of the bill allows environmental speed limits already in place to remain, but no new miles of roadway may be subjected to environmental speed limits. Elimination of Dallas-Fort Worth region environmental speed limits In 2009, the North Texas Tollway Authority raised the speed limit by 10 mph on two tollways. Several miles of these tollways had