14 CFR §121.115

1. (a)Routes and route segments over Federal airways, foreign airways, or advisory routes have a width equal to the designated width of those airways or advisory routes. Whenever the Administrator finds it necessary to determine the width of other routes, he considers the following:
   1. (1)Terrain clearance.
   2. (2)Minimum en route altitudes.
   3. (3)Ground and airborne navigation aids.
   4. (4)Air traffic density.
   5. (5)ATC procedures.
2. (b)Any route widths of other routes determined by the Administrator are specified in the certificate holder's operations specifications.