Police have accused of driver of traveling 109 miles an hour on Interstate 89 in Williston, Vermont.    Police said 37-year-old Tyran Mohown of Holyoke, Massachusetts, was stopped Saturday on U.S. 2A at exit 12 by a Shelburne officer who had measured Mohown's speed. He said Mohown exceeded the speed limit by more than 40 miles an hour.    The Burlington Free Press reports Mohown said he had not realized he had been traveling that fast. He's scheduled to be in court Monday on a charge of negligent operation.    It could not be verified whether Mohown had an attorney.