Buses, also called omnibuses and multibuses, are public, passenger-conveying vehicles with origins dating back to the carriage line launched by Blaise Pascal in 1662. Buses can be single-level, or single-deck, or they can be double-decker buses.
Double-deckers buses contain two decks layered on top of each other, each with the ability to transport a set number of passengers. Double-decker buses, like single-deck buses, are widely used for public transportation, and are most common in some European cities and in parts of Asia. Double-decker buses vary in size and style, from the BVG line of double-decker buses in Berlin that hold 128 passengers to the open-roof 12-meter (39-feet) double-deck tram system in Hong Kong.
Double-decker buses reduce fuel consumption by accommodating a larger number of passengers (approximately 70) compared to an average U.S. city bus capacity of 40 passengers. Double-decker buses are smaller, or more compact, than single-deck buses, and offer more safety by minimising standing.
In the United Kingdom, double-decker buses, which typically span a length of 9.5 to 10.9 meters (31 to 36 feet), have become a common reference point in the description of large objects. For example, a U.K. resident might say, "That whale is the size of a double-decker bus."