In an attempt to protect the driver's feet and legs, NASCAR will gradually implement changes to strengthen the car by modifying the floorboard, driver's anti-intrusion plating, firewall and footbox areas.

The changes, sparked by the accidents to Kyle Busch and Austin Dillon last year, are optional for 2016, mandatory for the Daytona and Talladega races in 2017 and then all tracks in 2018.

While Dillon was unhurt in his crash, Busch suffered a broken right leg and broken left foot in his February 2015 crash. Danica Patrick also had an accident in May at Talladega that concerned the drivers about the strength of the front end of the car despite her being able to walk away.

NASCAR is hoping alterations made to cars will further protect drivers' legs and feet during crashes like this one which left Kyle Busch with a broken right leg and broken left foot as the result of a his February 2015 crash at Daytona. AP Photo/John Raoux

NASCAR senior vice president Gene Stefanyshyn said teams designed various proposed changes, and NASCAR had done two crash tests to help determine the changes.

"We're replacing existing materials with materials in instances which are thicker or being formed in a way with less welds," Stefanyshyn said in announcing the changes Thursday. "Also the way we attach part of it is we are creating, for lack of a better term, a zipper so we provide a lot more weld surface.

"Basically we are making the structure that encapsulates the driver more robust and susceptible to manage energy in a better way."

Stefanyshyn said he couldn't put a percentage on how much better the driver's lower extremities will be protected but the test results show "very good" improvement.

"It's been well tested and we feel very, very comfortable and very good with the direction here," he said.

The gradual implementation is so teams don't obsolete current chassis, as the new chassis will have to go through the NASCAR certification process. With the new thickness of materials, the weight changes would impact how the car races, which also could deter teams from implementing them before NASCAR makes them mandatory -- teams likely would have to be convinced they aren't at a competitive disadvantage by using the new designs during the optional period.

NASCAR also announced Thursday that it would use the potential 2017 aerodynamic package for the Aug. 28 race at Michigan International Speedway. It had used that package, which includes a smaller spoiler and reduced paneling of the front splitter underneath the car, in June at Michigan and July at Kentucky Speedway.