We can see here Flash is moving beyond speed of light and the path is forming before him:
How is he creating paths before him?
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Sign up to join this communityWonder Woman stated that when the three Mother Boxes join together and synchronise, they form a Unity which cleanses the planet they're on with fire.
WONDER WOMAN: [Voice-Over] To conquer, three boxes have to synchronize and join together into The Unity. The Unity cleanses a planet with fire, transforming it into a copy of the enemy's world. All who live become servants of Darkseid, alive but drained of life. Parademons.
In order to prevent this happening to the Earth, the Flash was supposed to help Cyborg access the Unity by running faster than he'd ever attempted before, and then transmitting the electrical charge built up by that to Cyborg through physical contact with him. Once Cyborg had access to the Unity, he could shut it down and separate the Mother Boxes, with Superman's help.
WONDER WOMAN: Once Victor is linked with The Unity, with Barry's help, he'll break through The Unity's defenses.
AQUAMAN: Remind me, how's that, again?
THE FLASH: Uh, with the power of love?
CYBORG: Barry.
THE FLASH: With a catastrophic energy surge.
CYBORG: Good.
BATMAN: You'll have to run faster than you've ever attempted to generate a charge of that size. Then you make physical contact with Victor. That force should propel him inside The Unity. And, Victor, it's all up to you. Take it apart from the inside before it synchronizes.
THE FLASH: We finally, actually have a plan.
BATMAN: We have a plan.
However, Flash was knocked down and injured by an energy blast fired by a Parademon. Consequently, he failed to make it to Cyborg before the Unity synchronised, and sent out an incredibly powerful blastwave, strong enough to wipe out Superman and Cyborg, and seemingly the planet as well.
At that point, Flash realises he has to break his 'rule' and run far faster than the speed of light.
THE FLASH: Wait... Just gotta go faster than the speed of light, far beyond the speed of light, you gotta break the rule, Barry, and you gotta do it now!
This is presumably a reference to his earlier statement that crazy things happen to time when he approaches the speed of light.
THE FLASH: I might be able to do it. I mean, I- I might be able to jumpstart it. I- I don't like to break this rule, but when I approach the speed of light, I-- look, crazy things happen to time, but if I do it, I create massive electrical power.
And based on the visuals, it appears he runs so fast that he's actually turning back time as he goes, or running backwards through time. So it's not so much that he's creating a path beneath his feet, but rather, the ground that used to be there a few seconds earlier is reforming as time moves backwards and its destruction is reversed.
It's somewhat similar to the scene in the 1978 Superman movie, where Superman flies around the world and turns back time in order to reverse Lois Lane's death.