The Interstate 75/Interstate 24 interchange in Chattanooga has received an "F" from the state in terms of traffic congestion.

But despite plans to spend $132.5 million in an improvement project now underway, the state expects the interchange's congestion score to only rise to a "D" by 2040.

Some transportation executives who took part in a Thrive Regional Partnership meeting on Wednesday said that's not good enough.

"That's not acceptable," said Andy Vanzant, senior vice president of operations for Chattanooga-based trucker Covenant Transport. "I'd ask to get a 50% improvement. An 'F' to a 'D' is a 20 percent improvement."

(Read more: Leaders look for ways to get Chattanoogans out of their cars and off the highways)

Dave Huneryager, president of the Tennessee Trucking Association, said that "obviously" the anticipated improvement in the interchange's congestion isn't enough.

But, he said, the Tennessee Department of Transportation does continue to work to make improvements.

Work began earlier this year on the $132.5 million project to rebuild the interchange just beyond the Tennessee-Georgia state line known as "The Split."

An estimated 124,000 vehicles a day travel the interchange which is known for its daily backups, outdated merge lanes and frequent crashes.

Jennifer Flynn, a TDOT spokeswoman, said that heavy construction will start in late fall and continue until the project completion in the summer of 2021.

Amy Kosanovic of TDOT's long-range planning division said the state is conducting a study of the I-75 corridor along with interstate segments for I-55 and I-26. She said it has been at least 10 years since such a study involved I-75, which runs from Chattanooga to the Kentucky state line.

The 18-month study which began about a year ago will offer recommendations in early 2020 and identify short-and long-term solutions for improving problem spots along the entire corridors.

"We want to recommend solutions," said Kosanovic.

She said it's expected that some 2,000 more jobs will be created in the Ooltewah area alone over the next 20 years.

Terry Hart, president of Chattanooga Metropolitan Airport, asked if someone is looking at the I-75/I-24 interchange and finding that a 'D' is acceptable and shouldn't the state be shooting for a 'B,' or that a 'B' equals 10 times the cost?

"I just don't know to me if a 'D' is acceptable," he said.

Kosanovic said the study looks at factors which the state can currently forecast. She said technology gains such autonomous vehicles might affect congestion scores in the future.

"It could improve that, but that's as it currently stands with the data that's there," Kosanovic said.

Officials cited the impact of transportation work on I-75 in Georgia as affecting what happens in the Chattanooga area.

Bridgett Massengill, executive director of the Thrive Regional Partnership, cited the "critical partnership" between TDOT and the Georgia Department of Transportation.

As the interchange rebuilding project speeds up, motorists will see more orange barrels, lane closures and backups, according to TDOT officials. They say there will be nighttime lane closures throughout the life of the project, and the contractor is allowed four weekend closures on interstate ramps and six weekend closures of Spring Creek Road during the project.

The transportation department will notify the public ahead of the closures, officials said.