Large cab companies have been using gps-enabled electronic dispatching systems since 2004.

I found a video on my old phone that I'd filmed to show how the system works:

http://www.taxiwars.org/p/electronic-taxi-dispatch-v1.html

Usually I'd make it to the address in about 7 minutes. Sometimes I was parked at the pickup address already (convenience store). I think there are two diaries titled 'instant taxi' on my site...