As far back as I can recall most bus companies allocated specific vehicle types to routes, either based on their type, or at least on their capacity (minibus/single deck/double deck).
This system of vehicle allocation requires discipline on the part of the vehicle allocation staff at the depot. This is a job that can be made more complex where vehicles carry route branding - but with the right mindset even extensive route branding can be made to work – TrentBarton, Nottingham, Reading Go North East are some examples of companies that can manage allocation of branded buses to routes well.
I have spent a considerable time travelling around by bus and also observing operations this year. In certain areas of the country I get the impression that controllers and vehicle allocation staff have ‘thrown in the towel’. Branded buses on the wrong route and frequently single decks are being allocated to double deck routes with the resultant heavy loadings and overcrowding whilst the double decks appear on routes where they carry fresh air around for much of the day.
Then there are routes for which a business case must have been made for investment in new buses but those new buses are used elsewhere.
I accept that in order to provide a service the allocation staff may take the view that ‘a bus is a bus’ but if a route is allocated double decks then that must be for a reason.