The video has since been deleted from Instagram, where it initially appeared.
Naturally, head coach Kyle Shanahan was asked about the video while speaking with reporters on Saturday after the team's first practice of training camp.
"I haven't addressed it with him yet," Shanahan said. "I didn't really find out about it until I prepared to come in here. I think my concerns about that would be the same as everyone else who understands what can happen when you drive that fast. It puts everyone out there [at] risk and puts yourself at risk, so I'll ask Trent about it.
"I've been told that he said it was his brother and not him. I'll ask Trent for myself and find out. But regardless of it, that's nothing anyone should do on this planet unless you're in a NASCAR race."
The 49ers traded for Williams in April, sending a fifth-round draft pick in 2020 and a third-round pick in 2021 to Washington in exchange for the seven-time Pro Bowler.
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