LUFKIN, Texas (AP) — U.S. Highway 59, a major evacuation route from Houston, has been a problem for East Texas for decades. And as flooding rivaling that of Hurricane Harvey inundated the region in April, the highway closed in several places, cutting off a major evacuation route for countless people seeking shelter from the floods.

Plans to upgrade the highway, which stretches more than 600 miles through Texas from Laredo to Texarkana, to interstate standards have been on the books for decades. But the Texas Department of Transportation says it cannot guarantee that the billions of dollars being poured into the project will fix the flooding problem.

“U.S. 59 was one of the issues during (hurricanes) Rita, Ike and Katrina,” Polk County Judge Sydney Murphy said. “So you think by now we would be committed to expanding that roadway.”

Texas has poured millions of dollars over the past 30 years into upgrading parts of the highway to interstate standards — an effort known as the I-69 project — with the goal of relieving traffic congestion, supporting economic development, improving safety for travelers and upgrading a major evacuation route for the state’s most populous city.

Thus far, only the part of U.S. 59 that runs through Houston has been upgraded to interstate standards, with a minimum of two travel lanes in each direction, 12-foot lane widths and paved shoulders of a specific width on both sides.

Steps to upgrade the highway in other areas have focused on larger population centers, such as Lufkin and Nacogdoches.

Portions of U.S. 59 between Cleveland and Shepherd as well as between Shepherd and Livingston saw significant flooding in April. Those stretches of highway were closed multiple times between April 29 and May 4 — then again when more heavy rain came the weekend of May 16 — and are supposed to receive upgrades in the next four years.

Those sections are part of nearly $6 billion the state plans to pour into the highway over the next decade or more to upgrade the highway to interstate standards, address safety issues and cover basic maintenance. TxDOT says it has allocated $1.5 billion for projects already underway or that begin soon on U.S. 59. The agency has another $4.3 billion allocated for future projects scheduled to begin in the next four to 10 years.

But it’s unclear whether those upgrades will prevent the kind of flooding that submerged parts of the highway this spring and during Hurricane Rita in 2005 and Hurricane Ike in 2008.

TxDOT said the upgraded highway will be engineered to avoid flooding during a 100-year flood event. However, 100-year floods — which have a 1% chance of happening in any given year — have become more commonplace, as have 500-year floods — which are more severe and have a .2% chance of occurring in any given year.

For example, Hurricane Harvey in 2017 was the third 500-year flood to hit Houston in three years. Memorial Day floods in 2015 and 2016 were also classified as 500-year floods.

John Nielsen-Gammon, the state’s climatologist at Texas A&M University, warned that floods are becoming more extreme in Texas.

“East Texas in general has experienced a large increase in extreme rainfall compared to last century,” Nielsen-Gammon said. “Part of that is due to climate change. Climate change has increased the intensity of very heavy rainfall across the southern U.S. by nearly 20%.”

TxDOT would not say whether current improvement plans take into account warnings from climatologists of even more severe flooding to come due to climate change.

“The projects being developed along the future I-69 corridor are designed to be serviceable for a 100-year flood event, however TxDOT cannot predict the amount of rain or potential flooding our lakes, rivers and streams could see in the future,” said Rhonda Oaks, the public information officer for TxDOT’s Lufkin District, where plans are currently underway to upgrade around a dozen miles of U.S. 59 to interstate standards.

Laura Butterbrodt, another TxDOT spokesperson, said the agency is currently developing the Statewide Resiliency Plan, “which will specifically target critical routes for the most appropriate design, maintenance and operations to foster resilience.”

The first draft will be available for review by the Resilience Steering Committee in June.

When the federal government authorized building 41,000 miles of interstate highways crisscrossing the nation in the 1950s, the federal government paid 90% of the cost, leaving the remaining 10% to the states.

But the I-69 project was not included in the original plans and didn’t receive federal designation until the early 2000s, state Sen. Robert Nichols said. When completed, the interstate will stretch more than 2,600 miles across multiple states from the Texas-Mexico border to the Michigan-Canada border.

But each state along the proposed interstate highway is expected to cover the cost — not the federal government.

“At present, there is no dedicated federal funding for the entire conversion of U.S. 59 to a future I-69 route through Texas,” the Federal Highway Administration said in an email statement. “It is up to the State (Texas Department of Transportation), to move projects forward.”

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This story was originally published by The Texas Tribune and distributed through a partnership with The Associated Press.