A bus was a large type of automobile ground transportation that carried a large number of passengers. The two primary uses of buses included a mass-transit system within larger cities or as transportation between various settlements. Because of the large size, some buses carried advertising on the sides of the vehicle.
Traveling through crowded streets in Delhi India, Roberta Lincoln was amazed by the number of passengers that had stuffed themselves on the public buses in 1974. (TOS novel: The Eugenics Wars: The Rise and Fall of Khan Noonien Singh, Volume 1)
In 1984, at the Nai Sarak Bazaar in India, a teenage Khan Noonien Singh found himself trapped by a burning and deserted bus blocking the street as an angry mob of rioters approached him with ill intent. (TOS novel: The Eugenics Wars: The Rise and Fall of Khan Noonien Singh, Volume 1)
While on a mission in the year 1986, James T. Kirk and Mr. Spock attempted to use a bus in the city of San Francisco. They were first unsuccessful when they did not have exact change to pay for the bus fare. On a later successful attempt, Kirk and Spock encountered a rowdy Punk on the Bus. The advertisement on the side of a bus provided Kirk with the logical answer of where to find the two Humpback whales, George and Gracie at the Cetacean Institute. (TOS movie, novelization & comic adaptation: The Voyage Home)