The driver's identity is unclear, but it appears that person is shooting the video, which is focused on the speedometer. The video also shows the vehicle flying by other cars on the highway.
TMZ Sports reached out to Williams for comment but had not heard back as of the article's publication at 11:35 a.m. ET on Friday.
The Washington Football Team traded the left tackle to the 49ers in April. Williams is expected to replace longtime blindside protector Joe Staley, who retired this offseason.
He has one year left on a deal worth $12.5 million in base salary with a $250,000 roster bonus, according to Over The Cap.
The 32-year-old made seven straight Pro Bowls from 2012 to 2018 and earned a second-team All-Pro nod in 2015. He has been in the NFL since 2010 after Washington drafted the ex-Oklahoma star fourth overall.