Your target audience will comprise those who are on the road, commuting on the bus, train or riding a taxi cabs, driving their cars, people stuck at traffic or stopped at a traffic light, people walking-by and those busy shopping inside the mall. Question yourself, will they be able to see your message or just pass by it? Can they get your message in one glimpse? These things will help you in setting the display. Decide if you want to display just the text or images or a combination of both. Also, if you want to add animation, scrolling images, videos or text and want it in monochrome or full color display, and the like.
