The model: Road categories: through roads, distributor roads and access roads

Three categories, three behaviours (Book p.81): a through road lets traffic flow as fast as possible (often 100 km/h, grade-separated junctions); a distributor road has at-grade junctions (built-up 50/70 km/h, outside built-up 80 km/h); an access road has mixed traffic with no direction separation and usually no separated cycle path.