The most famous example of campaigning through bus adverts in recent years has to be the Atheist Bus Campaign by the British Humanist Association and the assorted opposing messages it provoked.
The adverts do not touch on the polarising subject of the existence or non-existence of a supreme being, but they do provide an alternative way to reach the public.
So far this year, FirstGroup has donated advertising space worth £410,000 on the outside and inside of its buses. The space has been used to promote the ITV1 programme Born to Shine, which raises awareness and funds for Save the Children, and its UK anti-poverty messages.
"For any charity, spending money on advertising is a serious proposition, so to have a partner who's been able to offer this stuff for nothing and forsake earning a serious revenue stream is quite remarkable," says Douglas Rouse, corporate partnerships director at Save the Children.
Rouse says the adverts provide invaluable exposure and are comparable to TV adverts. But their reach is probably greater, given that they have appeared on 7,000 buses around the country.
Avril Gill, marketing manager at FirstGroup, says the adverts can easily be absorbed by people at their own pace while sitting on a bus.
Since its inception in 2007, the partnership has provided support worth £4.5m, most of which has come through gifts in kind such as the advertising space.