One witness to the accident said the driver appeared to be drunk.

【witness】事故的一名目击者说司机似乎喝醉了。

Although, it can also be, if you see a crime or an accident, you are a witness to the accident, a witness to the crime.

虽然，也可以是，如果你看到犯罪或事故，你就是事故的目击者， 犯罪的目击者。