A Belgian holiday

 This picks up from A Roman Holiday about aspects of Roman Roads. A friend pointed out this YouTube video, How A Roman Road Changed Belgium Forever, here is their synopsis:

Why does Belgium speak two languages? The answer is a 2,000-year-old Roman road.

In this video, I explain how the Via Belgica, a highway built by the Roman empire around 50 BC, created a language border that still divides Belgium today. The road ran from Tongeren to Bavay, separating Romanized Gaul in the south from Germanic tribes in the north. That linguistic divide has persisted through the Middle Ages, Spanish rule, Austrian rule, French occupation, and Belgian independence.

Today, Dutch speakers live north of the line, French speakers live south of it, and the border is still influenced by that ancient Roman road. Even Belgium's election results are decided by this 2,000-year-old line.

Having mapped Roman Roads, I recreated a focussed map on the region, adding the Belgian Wallonie region in green. As roads were unidentified in the AWNC database, I selected them per video. I found a source (no metadata in arcgis.com) that put Via Belgica right along it, confirming my choice.

click to enlarge, full size

Notice how Roman Via Belgica lies along the N edge of present-date Wallonie. Note also larger the extent of the sea water encroaching in "archaic" (light blue), "medium" (pink) & Roman ("open" tan) times. Current river courses also tan to match the sea. Indirect evidence, less settlements N of Via Belgica. Note also how the Roman Road web overlays by-and-large current cities. 

Note: for those in East Anglia, notice at top left near Norwich, the Norfolk Broads are entirely under sea!

Perplexity AI comparison of current vs. mapped stats are Sheet 1 here and summarize by me as:

Derived estimates	Roman period (Via Belgica corridor)	Modern period (same rectangle on map)
density settlements	0.5 / squ. km.	exceed 1 / squ. km.
density population	tens / squ. km.	350 / squ. km.

While the population density difference isn't surprising, the settlement density difference is - only half the present, when population density is an order of  magnitude smaller - you can see their spread as black dots on the map.

Much to my amazement, Perplexity was able to distinguish between sides of Via Belgica! Details are Sheet 2 here and summarized by me as:

Relative to Via Belgica	Roman	Modern
North / West	Low density, marshes, small rural settlements, widely spaced	v. hi dens., near-continuous, major ports & cities, kms. spaced
South / East	mod. density, provincial capitals, villas, forts, mod. spaced	hi-v. hi density, urban-industrial corridors, 10s km. spaced