Hossein, perhaps you may also consider the question "is there any difference for travel time between ground and air transportation systems?" For the original question, you can apply point biserial correlation where the dependent variable is time (with ratio/interval measure) and the independent variable is means of transportation (dichotomous, 0=ground, 1=air).

@ Eddie

Thanks for comment. Do you see any function for this relationship in the literature?

As there are different modes of travelling on the ground (such as road or rail; I don't think you'd want to include sea travel), and their speed varies with local circumstances (topography, condition of the road and rail links), it will be difficult to establish a single function. There is also the question whether you include travelling time from and to the airports or rail stations or not, or whether you look at connections between the same places or over similar distances.

For a start, I would suggest looking up travel times between major cities in the online time tables, or the travel time estimates given in road atlases, and try to correlate them. Start with single countries or regions. Instead of the difference in travel times, their ratio may be a more useful quantity.