Actually the confusion is not caused by you but how I read your post.
Anywho.... the actual text of the legislation is as follows;
From section 756 of the Energy Policy Act of 2005 (Enrolled as Agreed to or Passed by Both House and Senate)
(c) Vehicle Weight Exemption- Section 127(a) of title 23, United States Code, is amended--
- (1) by designating the first through eleventh sentences as paragraphs (1) through (11), respectively; and
- (2) by adding at the end the following:
- `(12) HEAVY DUTY VEHICLES-
- `(A) IN GENERAL- Subject to subparagraphs (B) and (C), in order to promote reduction of fuel use and emissions because of engine idling, the maximum gross vehicle weight limit and the axle weight limit for any heavy-duty vehicle equipped with an idle reduction technology shall be increased by a quantity necessary to compensate for the additional weight of the idle reduction system.
- `(B) MAXIMUM WEIGHT INCREASE- The weight increase under subparagraph (A) shall be not greater than 400 pounds.
- `(C) PROOF- On request by a regulatory agency or law enforcement agency, the vehicle operator shall provide proof (through demonstration or certification) that--
- `(i) the idle reduction technology is fully functional at all times; and
- `(ii) the 400-pound gross weight increase is not used for any purpose other than the use of idle reduction technology described in subparagraph (A).'.
You are so right, there is no state decision clause in the legislation and the DOT has interpret it as a state decision. However the EPA was one of the groups who push for this exemption to be in the bill and it was their hope to force this on the states - no exceptions.
AND somewhere, I got to find it, there is a detailed explanation of what a heavy vehicle is, 8500 lb GVW with a diesel engine are the primary qualifications if I remember right