A police post was a key feature of service areas developed in the 1960s. They were usually simple buildings, separate from the rest of the complex, providing refreshments for police officers as well as a safe and dry place to carry out paperwork and interviews after leaving the scene of an incident.
Police posts were combined with motorway service areas because doing so reduced the need to build additional motorway exits, and made it easy to provide the building with electricity and water. It also helped the planning process to keep all the buildings together. There was no real benefit for the service station, indeed the whole point was that the police wouldn't need to use the public facilities.
As policing became more centralised, and traffic police became a less common sight, these buildings received less and less use. New police posts stopped being built, and existing ones became abandoned, where they would then attract vandalism and detract from the area. When the motorway service areas were sold in the 1990s, this was an opportunity for some of the operators to take control of some of the old police posts, many of which have now been demolished.
National Highways Traffic Officers now have some of the roles of traffic police, but they just use the ordinary public facilities when waiting at a service area.
When developing the M1, it was unanimously agreed and recommended that a room should be set aside at each service area for the sole use of the police. This practise continued, where every time a motorway service area was proposed, the Ministry of Transport would offer the County Constabulary the opportunity to lease a police post.
If they accepted, about 400 square yards of land would be identified and removed from the land that would eventually become part of the service area.
Some forces would decline. At Aust, for example, they were happy with their facilities in nearby Bristol. Other forces would push for more, wanting a full police station with vehicle maintenance facilities, or a control room for the new motorway equipment, which the Ministry would try to resist but would sometimes need to give in.
The Police Building Development Group carried out a study in 1967, which found that the different police posts were being used very differently. Recommendations were made in Part 9 of Memorandum on The Planning of Police Buildings with a full design guide for motorway police posts being published in 1967.