Should fuel-efficiency standards be collective?

Fuel-efficiency standards are usually talked about in terms of individual vehicles, but should they be made collective?

Could states, municipalities, corporations, fleets, etc. be required to achieve a collective passenger-mpg target that would allow them to offset certain lower fuel-efficiency vehicles by combining passengers in higher-efficiency ones?

So, for example, a state could offset the lower efficiency of certain service vehicles, utility vehicles, etc. by maintaining higher public transit utilization rates.

In this way, a state, municipality, corporation, or other fleet could achieve higher collective fuel efficiency targets than by merely having more personal vehicles that get higher mpg ratings.