1.   Air travel was growing rapidly, while the role of mass transit on the ground was shrinking almost everywhere.

2.   Air travel was growing rapidly.

3.   Because of this, air travel to deal with the social aspects of human interactions will be more frequent.

4.   Cheap air travel has considerable environmental disadvantages.

5.   Consumers indicate the extent to which they value air travel by the price they are willing to pay.

6.   Delays and cancellations are a regular feature of air travel.

7.   For instance, the private costs of providing air travel are indicated by the price of air tickets.

8.   How much does air travel damage the environment?

9.   In the thirties, air travel really began to take off.