Since aasc, tasc, and basc are dummy variables of air, bus, train and bus, and the omitted variable is car, we can interpret these logit coefficient in this way. First, exponentiate these logit coefficient and get the odds. Then, you can say, on the first level of the model, after controlling for the effects of the cost and time (gc and ttme) the odds of selecting air versuse travelling by car are ..., the odds of selecting train versus selecting care are ...., and the odds of selecting bus versus car are........
Since the effect of time is negative, you can interpret it like this way: when you increase the time of travel by one minute for any give mode of transportation, the odds of using that mode of travel will decrease by ...., after controlling for the effects of other variables in the model.
You'd better double check with some books, such as Long & Freese (2003), and Powers & Xie (2000). Stata reference does not give too much information, but it will also help you to understand.
I am not an expert in analyzing nlogit modols. If there is any mistake, it is mine.