A blue sign with the following text.
[Car park symbol]; at ½ mile; intervals for; 21 miles.
Back in the 60s, when much of the A1 was dualled, County Councils along the road were given contracts to design and build the upgrades, and if you look carefully you can still see how each one did things a little bit differently. This ancient sign is a reminder that Lincolnshire put a lay-by every half mile, and wanted you to know it.