"travel" and "travelling" are similar, but they are not quite the same:
- "travelling" is the present participle form of the (intransitive) verb "to travel", which refers to the act of moving oneself from one place to another (usually in the sense of going to another city/country/etc). You can think of "travelling" as basically "going on a journey".
- "travel", as a noun, refers to the movement itself of something or someone from one place to another.
That is, "travelling" is an activity that people do, while "travel" is something that happens to people. Another way to look at this is that "travel" is the result of "travelling".
Given that "travelling" and "travel" are usually pretty closely tied (when one happens, the other happens), the two can sometimes be used somewhat interchangeably. One difference, though, is that "travelling" usually implies a specific person (or people) going to a specific destination, while "travel" just describes movement in general.
For this reason, when we are talking about overall concepts of many people going different places (for example, "air travel", or "business travel"), we usually talk about the resulting movement ("travel"), not the (individual) activity of going to a particular place ("travelling"). Talking about the activity generally implies you're referring more to a specific instance or a specific person travelling somewhere.
So in the case of your example, either form could technically be used, but since the context seems to be talking more about "people going places in general" and not "a specific trip" or "a specific person's activity", then in this case I think "travel" is probably the more appropriate word.