M8 road cleaning and litter collection: EIR release

The amount of littering along the M8 in Glasgow, essentially along its whole length in the city boundaries is shocking. How often is litter cleared from verges and land adjoining the motorway? Do you stipulate a minimum frequency of litter clearing by the companies you employ to maintain the trunk roads? How often are they surveyed for litter? I would appreciate a response so you can treat this as an FOI