I am in agreement about the emptiness in the scene... what you have is good... but a city in the distance would add a lot... as would some mountain or trees or some cacti in a sandy desert environment. Search the freestuff for Dystopia..... by Mobius. Very nice ready made cities you can incorporate into your image. Good start though. Those taxies are pretty neat. 8)

There are 10 kinds of people: Those who know binary, and those who don't. 

A whiner is about as useful as a one-legged man at an arse kicking contest.