[OSM-talk] Advanced highway tagging

[Andrew MacKinnon]

OpenStreetMap's highway tagging is sorely lacking in breadth.
Currently, it is based on tags like "trunk", "primary", "secondary",
and "tertiary", which are inconsistently applied. Some mappers always
stick to the official designations on the signs - i.e. an A road with
a green sign is always "trunk", an A road with a black and white sign
is "primary", a B road is "secondary", and so on. Others ignore the
signs and designate roads according to their relative importance -
e.g. a major road is "primary" regardless of its official designation.
Neither of these tagging schemes is very useful, as each omits useful
information. It is quite common for a road officially given a higher
classification to be a slower, poorer or less important route than a
road officially given a lower classification. What OpenStreetMap
needs, then, is a new scheme which preserves both kinds of
information: the official designation of a road, and its physical and
other properties. I would add a third kind of information, zoom level,
which is an indicator of what zoom level a road should be rendered at
- so that a narrow dirt road in a remote part of the country
connecting two remote towns is rendered even at low zoom levels, but a
minor residential road in a city is not rendered.

Back in August I proposed a new tagging scheme to address these
concerns (see http://lists.openstreetmap.org/pipermail/talk/2007-August/017480.html).
However, it suffered from various flaws, and nothing happened to it.
Adding this type of information to the OSM database is necessary,
though, given its wide variety of applications. For example, routing
would benefit greatly from it - since it would prevent the router from
automatically suggesting routes along officially designated highways
when routes along municipal roads (currently designated "tertiary" in
many cases) are faster. It would also make the map itself much more
useful - since both major roads (regardless of classification) and
officially designated highways are shown on the map.

It is true that some of the tags for tagging physical properties
listed below already exist. For example, there are tags for surface
type, number of lanes, and speed limit. However, many of them are
rarely used because it takes a lot of effort to collect that much
information while mapping. There is no "summary" tag which summarizes
a road's quality and importance, which can be used directly by a
renderer to colour a road on the map. Adding better tagging for
quality and importance that can directly be used by a renderer would
allow both the quality and importance and official designation of a
road to be mapped.

The two main features of a road that need to be mapped are the road's
official designation, and its quality and importance.

Official designation: What is the designation on the sign? Weakly
correlated with the road's actual properties
- Motorway
- Interstate highway
- Municipal freeway
- Trunk road (A road with green sign)
- A-road (with black & white sign)
- B road
- C road
- Municipal road
- Private road
- Federal highway
- Provincial/state highway
- County road
- Trans-Canada highway
- Scenic/historic route
etc.

Quality and importance: A summary measure of the quality and
importance of a road - determines the colouring of the road on a map
- Based loosely on tags indicating various properties of the road such as:
-- Controlled-access?
-- Dual carriageway?
-- Line in centre?
-- Paved?
-- Posted speed limit
-- Average speed (when not congested)/safe speed
-- Traffic counts (peak/average/off-peak)
- Might be slightly different from the actual physical properties of a
road - e.g. to make a road appear in the same colour continuously on a
map - thus somewhat subjective
- Used for routing if the other tags indicating a road's physical
properties are not present or the router does not support them
- Could have different tags for each type of user (i.e. pedestrian,
cyclist, motorist)

A third property, zoom level, is used to determine at what zoom level
roads are shown - so as to show minor roads connecting isolated towns
in remote areas at low zoom for example, but not show hundreds of
arterial roads around Central London at low zoom and clutter up the
map.

What is most important here is the separation of the tag which
indicates a road's administrative classification, from the tag which
indicates the road's quality and importance. In this example, let's
call the first tag "designation", and the second tag "highway". The
first tag, designation, would describe a road's official designation.
It could be as simple as motorway, trunk, primary, secondary,
tertiary, etc. or it could be more detailed - for example including
details on which jurisdiction maintains the road. The second tag,
highway, would describe a road's quality and importance. It could have
levels like primary, secondary, tertiary, etc. or something more
complicated. See my previous post (linked to above) for more ideas on
the details of these tags. Both the designation and the
quality/importance tags should be allowed to be tagged either using
relations, or by tagging the ways directly. Additional information on
details of a road's physical properties can be added using other tags,
some of which are already in map features and some of which can be
added later.