In 1902 the bus service as we kind-of-know-it was born. No longer horse drawn, but reliant on motors, they were following the car (automobile) into a more modern age.
By the 1960's passenger numbers began to dwindle, due to the preference for personal cars, whilst today numbers have risen dramatically as we try to support our environment. In the year 2014 - 2015, there were 5.2 billion UK bus journeys, with a staggering 2.4 billion in the capital (London) alone.
This means, they are the perfect platform for advertisements that people will see on a daily basis and will hopefully, drive customers to the chosen product.