Except where the posted speed limit is less than 65 miles per hour, no person shall drive any commercial motor vehicle, as defined in § 46.2-341.4, on the left-most lane of any interstate highway having more than two lanes in each direction. Furthermore, within the Eighth Planning District and on Interstate Route 81, no person shall drive any commercial motor vehicle, as defined in § 46.2-341.4, on the left-most lane of any interstate highway having more than two lanes in each direction, regardless of the posted speed limit. Every commercial motor vehicle shall keep to the right-most lane when operating at a speed of 15 miles per hour or more below the posted speed limit on an interstate highway with no more than two lanes in each direction. The provisions of this section shall not apply to (i) buses or school buses or (ii) other commercial vehicles when (a) preparing to exit a highway via a left exit or (b) being used to perform maintenance or construction work on an interstate highway.