AUSTIN — Transportation funding could take a hit under new revenue projections by State Comptroller Glenn Hegar, who on Tuesday lowered his forecast of state tax collections by $4.6 billion in the face of a volatile oil and gas industry.

Hegar’s new forecast still leaves more than enough money to pay for the overall spending plan approved by lawmakers for the two-year budget period that began Sept. 1. Legislators left funds on the table and ended up with a bigger-than-expected balance when the state closed out the last fiscal year.

The new projection, however, affects the funding expected to be available for the high priority of addressing Texas’ congested roads. It lowers the oil and gas revenues anticipated to be funneled into transportation by $685 million.

“The reality is oil prices have continued to stay lower than what they were projected back in January,” Hegar said in an interview, adding that volatility in the industry makes forecasting difficult.

“We’ve sat down with different major companies and we’ve said, ‘OK, what’s the oil price?’ And boy, they are like a bunch of crawfish,” Hegar said. “It’s impossible to truly predict what the future holds, especially in this volatile commodity.”

In January, Hegar had anticipated the price per barrel of oil would be $64.52 in the 2016 fiscal year that started Sept. 1 and $69.27 in 2017. His Tuesday estimate lowers that to $49.48 and $56.52, respectively.

Hegar has the job of predicting revenue at the beginning of the legislative session to set the parameters for leaders and lawmakers crafting a spending plan for state government.

The comptroller does a second revenue estimate after the budget is completed to show how the spending is certified. The updated estimate also takes into account new laws, changing economic conditions and the closing of the books on the previous budget period.

His most recent forecast predicts that the oil price slump will have a negative impact on the amount of money generated by Proposition 1, a constitutional amendment approved in November that will add some severance tax revenue to the State Highway Fund each year for the next decade. That fund received a $1.74 billion boost from Prop. 1 in the 2015 fiscal year.

Hegar predicted Prop. 1 will add about $1.1 billion to the state highway fund during the 2016 fiscal year, an amount roughly in line with past estimates. But in the next fiscal year, he predicted that amount will drop to about $600 million, about half as much as initially expected.

State Rep. Joe Pickett, D-El Paso, who chairs the House Transportation Committee, said the fiscal year 2017 estimate was lower than he anticipated. But this year’s Prop. 1 revenue — more than twice the amount the committee expected — could cushion the effects of a lower payout two years from now, he said.

“It still averages out to just a little more than what we anticipated,” he said.

Prop. 1 has already boosted local transportation funding. Last week, the Texas Department of Transportation broke ground on building direct connectors between Loop 410 and Texas 151 on the San Antonio’s Northwest Side. The $82 million project was funded entirely with Prop. 1 revenue, which generated about $147 million for San Antonio this year.

But the revised projections could mean less money for local projects. Earlier estimates predicted Prop. 1 would generate nearly $210 million for San Antonio in the next two years, an amount that will decrease if the most recent forecasts hold true.

San Antonio City Councilman Ray Lopez, who chairs the Alamo Area Metropolitan Organization, said he thinks any reduction in the amount of Prop. 1 funding available through 2017 won’t affect the number of projects the organization will be able to complete over a longer period of time.

“There will be a negative impact, but it won’t really destroy the plan,” he said. “What will wind up happening, I think, is perhaps some projects planned for years two and three will get pushed into years three and four. It will be just a delay.”

And the unpredictability of oil prices could mean more money for the highway fund in later years, said Vic Boyer, president and CEO of the San Antonio Mobility Coalition.

“I would say that these kinds of swings are expected in the oil industry,” he said. “Prices are going to vacillate way up and down, and folks are aware of that.”

To prepare for lower-than-expected revenue projections in the short term, Lt. Gov. Dan Patrick said the Senate “worked diligently to prepare for this circumstance by passing a conservative budget while leaving substantial money in reserve.” He said he is “in close communications” with TxDOT officials “to address the revised revenue estimate announced today.”

“Texas voters will have an opportunity to prevent this sort of thing from happening in the future by voting on Nov. 3,” said Patrick referring to a ballot proposition that would dedicate a portion of sales tax revenue to transportation.

The budget approved by lawmakers this year totals about $209.4 billion in state and federal funds, of which more than $106 billion is state general-purpose spending.

In January, Hegar predicted that lawmakers would have $113 billion in state revenue available for general-purpose spending for the 2016-2017 budget period that began Sept. 1, and that tax collections would total about $97.8 billion.

On Tuesday, Hegar lowered his forecast of the revenue available for general spending to $110.4 billion and projected that tax collections would total about $93.1 billion.

The reduced projection of tax collections is separate from lawmakers’ decision to slash the state business tax, Hegar said.

House Speaker Joe Straus, R-San Antonio, said the Legislature’s budget approach prepared the state for such changing projections.

“The Legislature wrote a conservative budget this year so that Texas would be prepared for changing economic conditions and forecasts. Because we kept spending in check, we remain on sound fiscal footing, and we will continue to exercise prudence as the next legislative session nears,” Straus said.

Hegar had projected that oil production and regulation tax revenues would dip by 14.3 percent, but his new forecast anticipates a 42.1 percent drop.

He had forecast that natural gas production tax revenue would fall by 8 percent but his new prediction is that it will plummet by 39.8 percent.

The “subdued spending” in the energy sector has a broad effect, dampening projected sales tax collections as well, according to the report from Hegar’s office to state leaders and lawmakers.

Hegar scaled back projected growth in the sales tax by $1.5 billion, to $59.7 billion for 6.5 percent growth rather than 8.9 percent.

The rainy day fund, which is fueled by oil and gas revenues, also is expected to get less money than earlier projected. The balance of the state savings account is projected to be $10.4 billion at the end of the current budget period rather than $11.1 billion as initially projected.

The report on the certification revenue estimate from Hegar’s office said that “the recent fall in oil and natural gas prices and the absence of significant recovery in those prices is expected to result in reduced economic activity that will slow the rate of growth of the Texas economy during the next two years.”

Employment growth is expected to slow in the budget period, rising by 1.7 percent in 2016 and 1.8 percent in 2017, the report said. In January, Hegar had projected nonfarm employment would grow by 2.2 percent in 2016 and 2.3 percent in 2017.

Hegar projected that the gross state product would increase by 2.4 percent in fiscal year 2016 and 2.3 percent in 2017. In January, he had predicted that economic growth as measured by the real gross state product would average 3.7 percent.

At the same time, Hegar found good news in the projections Tuesday, saying it’s “amazing that this economy has absorbed that contraction in oil and gas … I think that’s just how diverse this economy has become.”

As for the swing in his projection, Hegar said he is keeping on top of the numbers and that they could change again. He said Texas’ economy “is the 12th largest economy in the world, and it’s a lot of numbers and a lot of assumptions.”

“This is the best data we have today, and we’re going to keep looking at it every single week,” he said.

Katherine Blunt reported from San Antonio.