Texas is a big state with a lot of taxpayers. Tolls build most of the urban stuff, so I guess most of the general tax money goes towards maintenance and building of rural roads. That doesn't explain how they are able to keep all the FMs up, though...
Agencies responsible for state highways in Texas (which include not just TxDOT but also cities, counties, RMAs, and regional toll road agencies) issue an awful lot of debt and do a lot of work through Comprehensive Development Agreements which in effect sign away a cashflow in order to avoid paying the entire costs of construction up-front. It is getting increasingly hard to find large projects through TxDOT lettings, not just because a lot of the high-dollar stuff has been farmed out to RMAs and regional toll agencies, but also because the gas tax has not been increased in Texas since the early 1990's and TxDOT is running out of money. Quite a lot of the stuff TxDOT does let is minor construction and major maintenance on FM and RM roads, generally in the sub-$10 million range.
I have been following TxDOT lettings for over eight years. The very first contract I downloaded from TxDOT and archived was a sign replacement contract for I-20 west of Fort Worth, in February 2002. Most of the Katy Freeway contracts (well over $2 billion of work) passed through the TxDOT lettings and I got all of those contracts, over a three-year period beginning in 2003 and ending in 2006. TxDOT has also been running construction for the Marsha Sharp Freeway in Lubbock through its lettings. But that has been pretty much it for the big projects since 2006, with isolated exceptions like the southwestern end of the Crosby Freeway in Houston (modifying the I-10/I-610 east stack to incorporate new ramps for the US 90 connection). At the moment I have 9,437 pattern-accurate sign design sheets from TxDOT, but the growth rate has slowed to 700 sheets annually, and the proportion of sheets I have which are from toll road agency/RMA contracts with TxDOT-assigned CCSJs has gone way up in the last three years. At the present rate I won't break 10,000 sheets until September 2011 at earliest. There are two stacks under construction right now in Texas that I am aware of, and I have the construction plans for both of them, but they are both administered by NTTA, not TxDOT.
I am missing construction plans for the US 271 Mount Pleasant Bypass and SL 79 down near Del Rio because those are both being administered by counties through a new TxDOT financing mechanism--Pass-Through Funding. I would like to obtain the construction plans for them, but to my knowledge they were never put online, and I do not look forward to dealing with the counties.