In England billboards tend to be in urban areas and not, so much, on major roads. There is legislation which prevents this, on safety grounds, [https://www.gov.uk/guidance/advertisements#considerations-af...] which has led to a few innovative workarounds such as large vans parked in fields near to motorways with signs on the side.

One person used a redundant rocket transporter as an innovative alternative and got a fair amount of publicity in return. https://www.lingscars.com/missile-truck