BROOKLYN, Mich. —Tony Stewart was the center of conversation Friday at Michigan International Speedway even though the three-time NASCAR champion was not at the track.

Stewart withdrew from this weekend's race after a fatal incident a week ago when his car struck and killed 20-year-old Kevin Ward Jr. while Ward was on foot during a sprint-car race in upstate New York. Ward had climbed from his wrecked car, walked on the track and apparently was angrily gesturing toward Stewart when he was struck by Stewart's car.

Stewart's race team said Friday he was grieving at an undisclosed location and had not decided when to return. It's the second consecutive NASCAR race he will miss since the incident.

Stewart, 43, has not been charged with any wrongdoing; an investigation by the Ontario County sheriff's office is ongoing.

In the meantime, NASCAR on Friday implemented a new rule sharply curbing drivers' ability to walk on a race track after their car is disabled.

“This is one of those times where we look outside our sport,” Robin Pemberton, NASCAR's vice president of competition, said at a news conference at Michigan International. The incident “was obviously something that everybody paid attention to” and the rule “is on the heels of that.”

NASCAR drivers have been known to climb from their wrecked cars, stand on the track and gesture at rival drivers.

Under the new rule, drivers generally are to stay in their cars until safety crews direct them to an ambulance or elsewhere unless the drivers face an emergency situation

Kyle Larson, a Cup driver who came up through the sprint-car ranks, said, “I'm sure you (will) still see people get out and point and stuff, but I'm sure they won't venture too much away from their own race car from now on.”

Other NASCAR drivers on Friday expressed their condolences to Ward's family and noted the heated public debate about who might have been at fault in Ward's death, a debate fueled by a graphic amateur video of the incident.

“I watched the video many, many times myself trying to understand it,” four-time Cup champion Jeff Gordon said.