They mark where a pipe runs in the fields parallel to the road. Its probably a high pressure gas pipe but it could be oil. It far easier for the pipe layers to follow the line of a road (then cut across open country) because they can get access to bring the pipes in during installation.
The north side of the A4 also was the site of installation during the 1930s of part of the UK's underground network of Government oil pipeline. This section runs from Aldermaston to Bristol. Next time you do the run, watch out for an anomymous site of about 0.25 acres adjacent to the road, screened with trees. Its a remote pumping station for this oil-pipe - close to Bedwyn.