The term 'infill service area' has been used on two occasions to describe new motorway services. In the 1970s the Department for Transport/Environment began preparing a number of sites for new service areas, designed to fill in the gaps between existing services. Very few of these were actually went ahead, Sandbach being an example of one.
In 2008 the MSA Policy announced that all existing services should be classed as 'core services', and new services 'infill', again on the basis that they're filling gaps between existing services. Where a new service area is 40 miles or more from existing services it too should be designated as a core site.
Applications for new infill services need to provide a detailed demonstration of the need for their new service area.