single-decker 
noun chiefly British
A bus having only one floor or level: [as modifier]: single-decker buses...
- The accident left a scene of devastation as the single-decker bus, lorry and two cars were strewn across the road amid debris and broken glass.
- A woman was killed when her car collided with a single-decker bus on the A19 between York and Selby yesterday.
- This also poses a problem with safety, as children have been found to be sitting on the back window shelf on a single-decker bus, sitting on each others knees, and also standing up in the aisles.