14 CFR § 121.95

Approved routes and route segments over U.S. Federal airways or foreign airways (and advisory routes in the case of certificate holders conducting flag operations) have a width equal to the designated width of those airways or routes. Whenever the Administrator finds it necessary to determine the width of other approved routes, he considers the following:

Terrain clearance.

Minimum en route altitudes.

Ground and airborne navigation aids.

Air traffic density.

ATC procedures.

Any route widths of other approved routes determined by the Administrator are specified in the certificate holder's operations specifications.