1. Name given to the bus which picks up retarded people or those with 'special needs' and transports them to any given destination; usually a special school or a day trip to the seaside. The bus is normally decorated with a happy colourful scene, such as a beach, or a row of sunflowers or just very bright colours. The sole purpose of this is to create a happy atmosphere for a happy day out.
2. People who are particularly drunk or wasted on drugs.
Those people over there are definitely on the happy bus.