I think the idea is pretty good but i guess it will not be really successful in metro's as people here are very conservative about people around and what will everyone say and all.
In tier-3 cities this can work & the problem will be like the advertisers will be willing to put adds on big cars like honda city etc but those people driving a city will not put adds...& those people will be interested who drive a maruti 800 or something but advertisers will not find much value putting their adds on maruti 800's..
I guess the best way out here will be putting adds on 2 whellers but that will again be only on scoters...
so lets see how well does this business will do.
If it does well den for sure many more companies will jump into it as the idea and the market for it attractive..