No, the real problem is that the excise tax barely covers the maintenance on the roads we already have. Another problem is that fuel taxes do nothing to allocate the use of roads. You pay the same fuel tax whether you drive during rush hour or during off peak hours. Toll roads can and do charge more to drive during peak times. Ever try to catch a plane at IAH starting near downtown Houston during rush hour? The Hardy Toll Road is much faster and more reliable than either I-45 or US-59.
The toll roads around Houston have all been built ahead of schedule and well below budget. Compare that to publicly funded roads that are always behind schedule and over budget. Building new roads on new rights of way will be much faster and cheaper than piecemeal expansion of existing roads. Look at I-35. It has been in the process of being expanded from 2 lanes each direction to 3 lanes from south of San Antonio to Hillsboro for over ten years. In order to build the expansion, the whole road is being completely rebuilt, because the original layout doesn't meet modern specifications. There's the expense not just of building the expansion, but rerouting exiting traffic while the old highway is torn out, the right of way regraded, and each new section is built. It might be finished in another ten years. Unfortunately that extra capacity was needed 20 years ago. By the time the I-35 expansion is finished, it will be 30 years behind the need. After that expansion is finished, there isn't any room to build any more lanes except if they're elevated. You can build lots of lanes on new rights of way for what it costs to build each elevated lane.
The TTC corridors don't just have highway lanes, they also have double tracked freight rail. It will be possible to operate trains from border to border across Texas at 85 mph, because there won't be any grade level crossings. This will allow railroads to abandon lots of old single tracked rail that goes through cities and towns and have lots of dangerous grade level crossings. The old abandoned rail rights of way could be used to expand highways or be converted to commuter rail or other uses. The rail system in Texas is breaking down, and was never designed for the current population of Texas much less the population in 2040 when we'll have double our current population . The rail lines in Texas were laid out in the nineteenth century when Texas was sparsely inhabited, and most of it is just single tracked.
We can stick our heads in the sand and pretend that our current infrastructure with a few upgrades will be enough. The currently proposed upgrades to existing highways will be obsolete before they are even started.
Have any of the rail owners signed on to this project? Last time I heard, there were many conflicts of interest.