The Interstate Commerce Act regulates the rates and fees charged by a common carrier. Generally, a public transportation service is required to publish its fee schedules to be charged for the transportation service to the general public. The transportation service is required to charge the published fare. To change the rate or fare, the common carrier must first change the published fare schedule and then enforce the new fare rate. In contrast, airlines were subject to vast deregulation in the latter part of the 20th Century. Most airlines use differentiated pricing and take into account various factors in offering a price to consumers, such as the day of the week of departure, the number of days until departure, expected demand, the number of seats already booked, and prices offered by competing airlines.