There are slogans that tell how devout the driver is. Some lorry slogans are social commentaries. There are warnings to rival drivers. And if other drivers can't take a subtle hint, there is outright intimidation. There are messages for the passengers.
Some thoughts seem to be addressed to the world at large.
And there are slogans that are a bit less universal. Some slogans may taunt envious rivals. Some lorry drivers wish to reassure friends and neighbours who may wonder at their success. A few slogans prove you don't have to be literate to be a slogan writer. There are slogans to make you think.