[Tagging] Access restrictions and expressway=yes

[Minh Nguyen]

Vào lúc 06:49 2021-02-19, Fernando Trebien đã viết:
> Hi,
> 
> I noticed that the usage of expressway=yes went up from ~2k to ~12k
> elements from mid 2018 to mid 2019, after remaining stable around
> ~1.8k elements between 2012 to 2018, and has grown at a rate of
> approximately +2k elements per year since then. It is mainly used in
> the U.S., but it would also be useful in Brazil and probably in many
> other countries as well. I saw that the article does not mention any
> default access restrictions, but the original proposal does, [1] so
> I'm not sure if they apply or not. They almost completely overlap with
> access restrictions implied by motorroad=*. Before recommending wider
> adoption, I would like to clarify this detail first. [2]
> 
> [1] https://wiki.openstreetmap.org/wiki/Proposed_features/Expressway_indication#Implied_tags
> [2] https://wiki.openstreetmap.org/wiki/Talk:Key:expressway#Implied_access_restrictions

Vid the Kid proposed expressway=* alongside a proposal for motorway=*. 
[1] These proposals seem to have been primarily influenced by local 
cartographic and traffic engineering distinctions, rather than routing 
considerations.

Conventionally, U.S. road maps distinguish between "expressways" 
(limited-access or partially controlled-access roads) and "freeways" 
(controlled-access roads) with a different color and/or line pattern. 
[2] (The term "limited-access" refers to the presence of abutters and 
at-grade intersections, not access keys like foot=*.) Unlike in other 
countries, the distinction between expressways and freeways is not 
consistently expressed by law or signage, and it also doesn't neatly 
align with the global highway=* definitions in OSM, especially when it 
comes to substandard roads.

 From a traffic engineering standpoint, an expressway differs from a 
surface street by meeting most of these criteria:

* Designed for high speeds (doesn't necessarily have a high speed limit)
* A dual carriageway
* Frontage roads consolidate driveway access
* A mix of ramps and at-grade intersections

An expressway doesn't meet all the criteria of a freeway, for example by 
having a few signalized or unsignalized intersections with cross 
streets. An expressway may be a relic of an older time before freeways, 
a political compromise between motorist and environmental interests, or 
a surface street being gradually upgraded to a freeway in stages as 
funding allows.

I think the proposal's mention of implied access tags may have been 
meant to justify the need for new keys apart from the existing highway=* 
scheme, but it doesn't seem core to the proposal. The current key 
description at [3] doesn't mention access restrictions at all.

By contrast, motorroad=* apparently is all about access restrictions and 
legal distinctions. [4][5] If expressway=* gets deprecated in favor of 
motorroad=*, then the latter key's definition becomes even more muddled, 
and it becomes even less likely that a global router would ever use it 
for anything.

[1] 
https://wiki.openstreetmap.org/wiki/Proposed_features/Motorway_indication
[2] 
https://commons.wikimedia.org/wiki/File:Kansas_official_transportation_map_legend.png
[3] https://wiki.openstreetmap.org/wiki/Key:expressway
[4] 
https://lists.openstreetmap.org/pipermail/tagging/2021-January/058960.html
[5] 
https://lists.openstreetmap.org/pipermail/tagging/2021-January/058972.html

-- 
minh at nguyen.cincinnati.oh.us