i've always been skeptical about taxi advertising..
1) the product must have broad appeal, taxi advertising is not targeted marketing
2) unless you advertise on a large fleet of taxis, you are only 'connecting' with a potential customer once, making it difficult for people to remember your specific ad again
3) very difficult to measure effectiveness
just my thoughts
1) As you say, This is one of taxi advertisings unique selling points...The ability to advertise to wide target market on a non restricted route. However, there are target markets that can be reached within taxi advertising, an example would be commuters, executives etc. Also depending on how the taxi is designed means that it can attract the attention of your desired target market on the street.
2) Again this can be dependent on how unique the design is and which city you are advertising in. A client of ours has just 1 taxi working in Liverpool, and as a resident of the city myself I have seen it several times when out and about during the week/weekend, day and night.
3) There is progress being made with regards to this. Cabvert has a track-a-cab system in place which can measure certain things such as
°° Unique hourly operating times.
°° Waiting times at key ranks i.e. Euston Station, Heathrow Airport and Piccadilly Circus.
°° Average Journey times in each city.
°° Rank designated cabs.
°° Real time viewing on your own desktop of the whole campaign.
There are both positives and negatives to all advertising mediums but I think taxi advertising is emerging above all the noise as quite a unique medium over which to communicate commercial messages.