Generalize roads more at zoom levels 4 -- 6

layers/transportation/transportation.yaml

@@ -64,6 +64,7 @@ layer:
     query: (SELECT geometry, class, oneway, ramp, brunnel, service FROM layer_transportation(!bbox!, z(!scale_denominator!))) AS t
 schema:
   - ./class.sql
+  - ./merge_highways.sql
   - ./layer.sql
 datasources:
   - type: imposm3