So.. on several occasions I have found people that have gotten stuck in an express tube. It sucks. So, now, express tubes have a hail verb which will allow you to hail a cab.
It could be set up by level.. but differentiating between outside a hotel and some other random street is not something easily done.
So it's pretty much all or nothing.. or done on a level by level basis.. IE: it's possible on gold green blue but not on Red.. which makes it some what worthless since most of the people that play are on RED most of the time..
I say partially because I doubt ambpop has 100% coverage of street locations as it's a prop set per location, but I did set most of red and gold streets. But yea, you're talking about setting a value per location and there's one already there which kinda fits the bill.