i would like to travel around in the world with my friends.. only this !

If we have travel time and travel cost, under what conditions can they be considered as substitues? Under what conditions can they be considered as complements?
Could you please give example in transportation system?
When commuting to work people choose mode of transportation bases on a combination of time and cost, Public transportation or car pooling is cheaper than driving alone but takes more time, so people can substitute one for the other.
Vacation travel requires time so the more time you have for vacations the more money you will spend on transportation for travel, so time and cost will act as compliments.