Proof of principle for extending rendering of access tags

access.sql

/* Adapted from https://github.com/imagico/osm-carto-alternative-colors/tree/591c861112b4e5d44badd108f4cd1409146bca0b/sql/roads.sql */

/* Numerical scale for 'severity' of access restriction, including NULL if null input */
CREATE OR REPLACE FUNCTION carto_restriction_severity(highway_type text, accesstag text)
	RETURNS integer
	LANGUAGE SQL
	IMMUTABLE PARALLEL SAFE
AS $$
SELECT
	CASE
	WHEN accesstag IN ('yes', 'designated', 'permissive', 'customers') THEN 0
	WHEN accesstag IN ('destination',  'delivery') THEN
		CASE WHEN highway_type = 'type_footway' THEN 0 ELSE 1 END
	WHEN accesstag IN ('no', 'private') THEN 2
	ELSE NULL::INTEGER
	END
END
$$;

/* Classify highways into categories. Default is NULL in which case simple access tag will be used */
CREATE OR REPLACE FUNCTION carto_access_type(highway text)
	RETURNS text
	LANGUAGE SQL
	IMMUTABLE PARALLEL SAFE
AS $$
SELECT
	CASE 
    WHEN highway IN ('motorway', 'motorway_link', 'trunk', 'trunk_link', 'primary', 'primary_link', 'secondary',
                 'secondary_link', 'tertiary', 'tertiary_link', 'residential', 'unclassified', 'service',
                 'road') THEN 'type_motorvehicle'
	WHEN highway = 'pedestrian' THEN 'type_pedestrian'
	WHEN highway IN ('footway', 'steps') THEN 'type_footway'
	ELSE NULL
	END
END
$$;

/* Return int_access value which will be used to determine access marking. */
/* Only a restricted number of types can be returned, with NULL corresponding to no access restriction */
CREATE OR REPLACE FUNCTION carto_highway_int_access_internal (highway text, "access" text, foot text, tags hstore, OUT int_access text)
  language plpgsql
  immutable
AS $$
DECLARE
highway_type text DEFAULT NULL;
specific_severity INTEGER DEFAULT NULL;
overall_severity INTEGER DEFAULT NULL;
BEGIN
SELECT INTO highway_type carto_access_type(highway);
IF highway_type = 'type_motorvehicle' THEN
	SELECT INTO specific_severity MAX(severity) FROM (VALUES (carto_restriction_severity(highway_type, tags->'motor_vehicle')),
	(carto_restriction_severity(highway_type, tags->'motorcar')), (carto_restriction_severity(highway_type, tags->'vehicle'))) AS tmp(severity);
ELSIF highway_type = 'type_footway' THEN
	SELECT INTO specific_severity carto_restriction_severity(highway_type, foot);
ELSIF highway_type = 'type_pedestrian' THEN
	SELECT INTO specific_severity 2;
END IF;
SELECT INTO overall_severity
	CASE WHEN specific_severity IS NOT NULL THEN specific_severity ELSE carto_restriction_severity(highway_type, "access") END;
SELECT INTO int_access
	CASE overall_severity
		WHEN 0 THEN NULL
		WHEN NULL THEN NULL
		WHEN 1 THEN 'destination'
		WHEN 2 THEN 'no'
	END;
END;
$$;

CREATE OR REPLACE FUNCTION carto_highway_access_modifier (highway text, tags hstore, OUT access_modifier text)
  language plpgsql
  immutable
AS $$
DECLARE
highway_type text DEFAULT NULL;
BEGIN
SELECT INTO highway_type carto_access_type(highway);
SELECT INTO access_modifier
	CASE WHEN highway_type = 'type_motorvehicle' THEN
		CASE
		WHEN (tags->'psv' IN ('yes', 'designated')) OR (tags->'bus' IN ('yes', 'designated')) THEN 'psv' 
		WHEN tags->'hgv' IN ('yes', 'designated') THEN 'hgv'
		ELSE NULL
		END
	ELSE NULL
	END;
END;
$$;

ALTER TABLE planet_osm_line DROP COLUMN IF EXISTS int_access;
ALTER TABLE planet_osm_line
	ADD int_access text GENERATED ALWAYS AS (CASE WHEN highway IS NOT NULL THEN carto_highway_int_access_internal(highway, "access", foot, tags) ELSE NULL END) STORED;

project.mml (extract)

table: &tunnels_sql |-
        (SELECT
            way,
            (CASE WHEN feature IN ('highway_motorway_link', 'highway_trunk_link', 'highway_primary_link', 'highway_secondary_link', 'highway_tertiary_link') THEN substr(feature, 0, length(feature)-4) ELSE feature END) AS feature,
[...]
            int_access AS access,
            access_modifier,
[...]
          FROM ( -- subselect that contains both roads and rail
            SELECT
                way,
                'highway_' || highway AS feature, --only motorway to tertiary links are accepted later on
[...]
                int_access,
                CASE WHEN int_access IS NULL THEN NULL::text ELSE carto_highway_access_modifier (highway, tags) END AS access_modifier,
[...]

roads.mss (extract)

// Taken from osm-carto-alternative-colors
@access-marking-psv: lighten(@transportation-icon, 28%);
@access-marking-hgv: #888;

[...]

[access = 'no'] {
  [feature = 'highway_service'][service = 'INT-normal'] {
      [zoom >= 15] {
        access/line-color: @access-marking;
		    [access_modifier = 'psv'] { access/line-color: @access-marking-psv; }
		    [access_modifier = 'hgv'] { access/line-color: @access-marking-hgv; }
        access/line-join: round;
        access/line-cap: round;
 [...]
    }
  }
}

Examples

Above is a current Carto rendering. Below new access marking in a Carto-derived personal style (ignore the non-access related differences):

PSV access
Bus only road on bridge. access=no, bicycle=yes, bus=yes, foot=yes.

The presence of bus=yes means the access=no restriction is rendered in pale blue.

Honouring motor_vehicle

Residential highway tagged with motor_vehicle=destination.

Honouring foot

North-south footway tagged with highway=footway, foot=private

More tagging normalisation

Cycleway tagged as highway=path, foot=yes, bicycle=yes

Carto currently only accepts bicycle=designated.