We've seen some neat mobile billboards on Trend Hunter, from cars and trucks to airplanes and even people and sheep.
They're visually appealing and an effective mode of advertising as they travel to different locations, ensuring it captures the attention of different audiences across different regions. The Red Cross ad featured above is a great example of how effective this sort of medium is, even though it's designed to be stationary.
Top 10 Mobile Billboards
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Free Cars (With Billboards)
License Plate Billboard