If you haven't experienced a pothole on a motorway then count yourself lucky, but local roads suffer more from potholes because they are built to a lower standard than motorways. Urban roads aren't generally built to cope with high volumes of traffic and heavy goods vehicles, whereas motorways have to be able to handle 44 tonnes of HGVs day and night.

It's also worth mentioning that local roads are the responsibility of local councils, whereas motorways are looked after by Highways England, who likely have access to a bigger budget.