A Routemaster is 8 feet wide.
London Transport had some rather strange ideas as most noticeably did the Metropolitan Police who had a long history of vetoing various aspects of bus operation and design. The 8 foot wide RTL was not allowed to operate in Central London initially because the Met said no. IIRC there was something similar regarding the RML being too long initially. Going into the mists of time the Met objected to buses having a roof over the top deck, similarly objecting to the driver having a cab door or a windscreen. Service buses pre-war were regarded as Hackney Carriages requiring a (taxi) licence plate to be displayed; the Met was the issuing authority.
Edit: RTW not RTL