Adam, welcome to the Forum.
The following link from Lorraine - The Dover Historian - may give you the answer.
https://doverhistorian.com/2014/11/29/farthingloe-the-historic-valley-of-legends-and-outstanding-natural-beauty/
You will need to scroll to just over half way down the page to look at the map and a bit further down to where it starts "Prior to building the Channel Tunnel......"
Off the A20 the link road I believe you refer to is gated in two directions. Both were constructed during the building of the Channel Tunnel. The one to the west is a road specifically built for construction traffic and goes over the top of the chalk down (and can be walked). It goes under the A20 and links with the path that leads to Abbot's Cliff.
The one to the left went to the Channel Tunnel Workers Camp, now long demolished.
Hope this helps.