[Tagging] Expressway=yes/no versus new tags "dual_carriageway=yes/no", "limited_access=", "grade_separated"=?

[Joseph Eisenberg]

I updated the Key:expressway page to have a global focus, rather than
being United-States-specific.

But I noticed that the key:expressway is currently used in 2 different ways.

While all expressways are major roads designed for high-speed motor
vehicle traffic, they are not often "motorroad=yes", because most
States allow motorcycles and pedestrians on all roads that are not
motorways (and western States allow bicycles on rural motorways even).

The defining characteristics are:
1) The max speed and design speed are relatively high (usually 60 kmh
in urban areas or greater in rural areas)
2) A) They have limited access to adjacent properties
OR
B) They have grade-separated interchanges with major junctions.
3) They are dual carriageway (divided) highways

While many "expressways" in the United States fit all of these
characteristics, many only fit 2)A) or 2)B) but not both.

We already have "max_speed=", but it would be better if there were
more specific tags to describe these three properties:

A) Limited access: there are no or very few service roads or driveways
B) Grade separation: there are no stop lights or stop signs (or
roundabouts) at junctions, instead the roads cross at different
levels, with link roads / ramps to connect them.
C) Dual carriagway: the two directions of travel are divided by a
barrier or median area.

So initial ideas;

A) a new tag "limited_access=yes/no" - or is there a better term in
British English for a road which doesn't have service roads like
driveways, or only very rare intersections with service roads? This
needs a proposal.

B) Use the tag "grade_separated=yes/no" for roads that generally do
not have any at-grade intersections with stop lights, stop signs or
roundabouts or similar. Used only 97 times:
https://taginfo.openstreetmap.org/keys/grade_separated so should need
a new proposal.

This property can be seen by looking at all crossing or intersecting
roads and interpreting layer= and bridge= and tunnel= tags, but that
is quite hard to get right, so having mappers add this tag would help
with quality control and make it much easier for database users.

C): I noticed that the tags "dual_carriageway=yes/no" and
"type=dual_carriageway" have been used in a few limited areas. These
have been used several thousand times
(https://taginfo.openstreetmap.org/keys/grade_separated) but only in a
few places, so a new proposal is still a good idea.

This property is also mapped by drawing two separate ways and adding
oneway=yes, but it is not easy for a computer algorithim to properly
interpret these geometries as "two parts of a single road" without a
tag like "dual_carriageway=yes", especially if there the name= and
ref= are not identical for each direction.

- Joseph Eisenberg