In the Boston Metro Area, it's already possible to place an ad on the ceiling of trains and buses. Called "Michelangelos" those ads are an innovation by Titan Advertising whose client is the MBTA. Here is a description of that pilot program by Steve Annear at Boston Magazine.
This is pefect for catchy promotions for B2C Boston-area B2C lawyers. For example, the text could read "Trouble reading this because of an auto accident? ...." In addition, being an advertiser on public transportation gets lawyers branding points for green. After all they are financially supporting sustainability.
Over in Connecticut, personal injury law firm Carter Mario had been an early adopter in purchasing huge bus wraparound ads. One winter the firm lucked out. The bus with its wraparound got stuck in the snow. Local television picked it up. Over and over again that clip was shown. Carler Mario has been able to expand into other parts of New England.
But, "Michelangelos" gets B2C lawyers even beyond that. The one negative is that these ads cost about double when a traditionally placed ad on the Boston-area trains and buses cost.