Depending on your negotiations it can be cost effective & extremely handy. The idea of hoardings/car/bus/train adverts or all adverts that are visible/audible is generally, repeated psychological conditioning or brainwashing if you will.
You are driving from your workplace to your residence, it takes you approximately 1 hour or more to reach home (Mumbai how we love you & your traffic) now during the course of your drive your wife/mother/family member/room mate etc. calls up and asks you to get some detergent. So you continue driving down the highway & come across;
A hoarding displaying Tide
A hoarding advertising Ariel
A hoarding displaying a Honda
A rickshaw passes you with Surf written predominantly at the back
A bus with the latest soap/movie release
A large hoarding as you are pulling into the supermarket displaying “Tide safedi ki jhankaar” (or whatever the ad is these days). Ask yourself this; Which product are you most likely to purchase once you reach the detergent counter? why?
Check rates of car branding here !!
Statistics point out that if a person has no prior connection with any of the brands or is not knowledgeable about the product he wishes to purchase he will most likely purchase the product which he remembers best or is the freshest in his memory. Almost 60-70% of shoppers fall into this category at some point or the other.
You can draw your own conclusions regarding the feasibility of advertising on private cars based on this.
From a business perspective you need to gather stats on how many prospective eyeballs can you gather daily, what kind of areas are you going to ply in, on what scale are you going to ply, how much visibility to be given to each client vis a vis availability of vehicles. What products are being advertised (an add for Toyota on a Honda is an oxymoronic redundancy might even get you sued) how much will you charge? on what basis? what rates? what about zone segregation into premium/standard, the population demographics of zones being plied in, what products would be viable where? all of this information can help you build a strong foundation & will help you gain business early on due to having done your due diligence & making it easier for potential clients to invest in your model.