Thanks for your help Phil. Thanks to Alex Greenland I have found another answer which I prefer, on
https://www.standardsforhighways.co.uk in CG 501 Design of highway drainage systems "Road drainage can be broadly
split into two categories: SURFACE AND SUB-SURFACE
DRAINAGE." and " SURFACE DRAINAGE is achieved through collection of
surface water runoff from the pavement (and potentially
adjacent land) through formal (e.g. kerbs and gullies) or informal (over
the edge) drainage, discharging into longitudinal surface
channels or piped systems. SUB-SURFACE DRAINAGE is achieved by installation of
longitudinal sub-surface drains situated in verges and/or
central reserves at the low edges of road pavements. These drain the pavement layers
and the pavement foundation (pavement and capping layers are
contiguous with the side of the trench) and prevent ingress
of water from verge areas adjacent to the pavement. "