zone of rate flexibility: 
Railroads may raise rates by a percentage increase in the railroad cost index that the ICC determines; the railroads could raise rates by 6 percent per year through 1984 and 4 percent thereafter.

zone of rate freedom: 
Motor carriers may raise or lower rates by 10 percent in one year without ICC interference; if the rate change is within the zone of freedom, the rate is presumed to be reasonable.

zone of reasonableness: 
A zone or limit within which air carriers may change rates without regulatory scrutiny; if the rate change is within the zone, the new rate is presumed to be reasonable.

The constant price of a product at all geographic locations within a zone.