Here’s how convoluted funding transportation in Texas has become: just going over the spending needs and sources of money for highways, transit, vehicle registration and public safety took a legislative subcommittee more than six hours Tuesday.

That was just to bring the House Select Committee on Transportation Funding, Expenditures and Finance up to speed on how your taxes and fees go from your pocket to fixing a crack on the freeway. Next they must figure out how to fix things, and maybe raise some of the $4 billion more that transportation experts believe Texas needs to fix its transportation system.

The $4 billion figure needs a disclaimer, however. David Ellis with the Texas A&M Transportation Institute said the figure assumes voters approve a plan to use rainy day fund revenues in November. It’s also based on transit spending and use staying on its current trajectory and certain steps that reduce driving, such as telecommuting, staying on their current course.

“If you make different assumptions about those things, you get a different answer,” Ellis told the committee.

How much money the Texas Department of Transportation has also varies. Currently, the agency can borrow up to $18 billion by selling bonds. TxDOT’s debt varies, but if the agency borrowed the full $18 billion, under the current terms of bonds it issues, it would cost $31.2 billion to repay.

If TxDOT had more money, it could start shaving down that debt more quickly, saving the state money — much like paying off a home mortgage early — but not giving transportation officials money for new projects.

Looking to Washington is unlikely. Texas officials said uncertainty about a new federal transportation bill means they can’t count on any long-term money.

“We know what we don’t know, but we don’t know it,” said James Bass, TxDOT’s chief financial officer.

The outlook from Austin-controlled money also isn’t clear. State money for roads comes from a host of sources, though mainly from the state’s gas tax. Registration fees on vehicles also pay for some improvements, though tracking down each dollar of the $50.75-plus can be tough. The state has a private vendor that adds a fee for allowing online registration renewals, and law enforcement gets certain cuts of some fees and taxes — including the gas tax.

Undoing those diversions to other uses might make transportation funding simpler, but there’s no guarantee it means more money for transportation because some of the diversions also benefit transportation, said Rep. Joe Pickett, D-El Paso, chairman of the select committee. Further, eliminating the diversions open up other complexities.

“We’re just displacing that amount of money and will have to come up with it from somewhere else,” Pickett said.

Tuesday’s session was the first of what officials predict will be many discussions, in hopes of settling on a broad plan for reforming transportation funding for the 2014-15 legislative session.

“You make the best decisions when you have time and not have a gun to your head,” said Rep. Ron Simmons, R-Carrollton.

Pickett started Tuesday’s session noting “there are no sacred cows.”But during the presentations by state transportation and budget officials, Pickett often acknowledged political and practical challenges to new approaches.

“Some of this stuff, there is not the political will to change,” he said.