[Tagging] Meaning of access=yes on highway=footway?

[Minh Nguyen]

Vào lúc 13:12 2022-04-24, Niels Elgaard Larsen đã viết:
> Greg Troxel:
>> But, it is also a routing error because highway=footway defines a way on
>> which a human could walk (phsyically, setting aside access rules).
>> Typically, they are too narrow for a passenger car.  A router should not
>> try to use them, even if access is ok, unless there is some width tag of
>>> = 2m (2.5m the US - taking the GB view :-).
> 
> You are asking too much of routers. And then what about motorcycles, 
> mopeds, horse riding, snow scooters, etc?

A car routing profile assumes a typical car. Motorcycles, trucks, etc. 
would ideally use separate profiles that can produce more optimal 
results for those vehicle types, including but not limited to honoring 
more appropriate access tags.

> The widest cycleway in Copenhagen is 10m wide (Dybbelsbro). You could 
> easy drive a car on it.
> 
> If you tag a footway with access=yes and it is not tagged with a narrow 
> width, then why should a router not respect the tagging?
> 
> It is not the job of routers to fix tagging mistakes. We have many tools 
> for finding bad tagging, and when we find bad tagging, we should just 
> fix it in OSM.

If we're talking about the job of a router, it's to find the "best" 
route, however "best" is defined. Users don't want routers to calculate 
a more harrowing, legalistic route when there's a better alternative. 
Even a half-decent routing profile that generally optimizes for travel 
time will still avoid weaving across multiple lanes, divebombing off and 
back on a freeway, or cutting through a residential neighborhood.

If a tag sounds dubious, most routers will try to skirt around the 
issue. For example, OSRM's default car profile refuses to route along 
highway=steps even if tagged access=yes. [1] However, I'd contend that 
this particular combination is valid tagging, because we don't tag 
universal rules such as "cars shouldn't climb steps" anyways.

More qualitative heuristics can be implemented as boosts or penalties, 
rather than binary filters, reducing the likelihood of a false positive 
but allowing for an inconvenient route to be returned as a last resort.

[1] 
https://github.com/Project-OSRM/osrm-backend/blob/79d4363d5972ea4d69abd92b26cda80de7941365/features/car/access.feature#L272-L277

-- 
minh at nguyen.cincinnati.oh.us