Example: Heston and Finchley

The slip roads are highway links (motorway_link and trunk_link respectively). The problem is that in Mapnik they flow over trunk and primary (proper) respectively instead of under, causing them to “bleed” over (sic). This causes presentation issues, especially when zoomed out.

Most maps in the world tuck slip roads under the main carriageway, and Mapnik used to do that.