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See protomaps/basemaps#72 for discussion

HighRoadPlus layers (8 total):

roads_tunnels_2_minus
roads_tunnels_1_casing
roads_tunnels_1
roads_0_casing
roads_0
roads_bridges_1_casing
roads_bridges_1
roads_bridges_2_plus

Each layer visualizes 5 road classes (highway,major_road,medium_road,minor_road,other)

Alternatively,

5 classes * 2 (casing/no casing) * each level (-2,-1,0,1,2,3,4,5) = 80 layers

Concept

This is the combination of two techniques to massively reduce the # of layers.

• Firstly, each road class is visualized using data-driven styling in a single layer. This needs to work around limitations in the MapLibre styling language, namely that zoom-based interpolations must be at the top level of an expression.

• Secondly, casings are drawn using two separate methods. For layer values -1,0 and 1 (97% of all values), the traditional casing layer is used. For values <= -2 and >= 2 (3% of cases) a duplicate geometry is created at tileset creation time, with only the addition of one tag casing=true. This layer is then drawn in sorted order with different paint values depending on the casing tag.

Drawbacks

• duplication in tile data (3% of cases), minor impact
• 2 different ways to draw casings, no impact
• No data-driven line-dash array or line ends (can be worked around)
• Very small visual artifacts at seams between butt line endings

External references

• OSM TagInfo layer= values
• Oliver's Single Highway Layer
• migurski/HighRoad