[OSM-dev] Isochrone generation with mode=CAR starting or ending in private roads

[Rhys Gareth]

When generating Car/drive isochrones, I found that OTP returned empty
GeoJSON for some locations which were on private roads and not near-enough
to make a "jump" to a public road. I then discovered it's not possible to
have a trip of any transport mode starting at that point at all.

Example start point which fails to generate isochrones:
http://www.openstreetmap.org/#map=18/1.24930/103.84794&layers=T

After thinking about it, I think I'd want isochrones to be able to start or
end in private roads, but not use them as "through roads" in case there are
ones which have more than one connection (there are some like this in
Singapore, such as large government complexes). The logic I think could
work is quite simple:
1. It's OK to start in a private road and connect to public
2. It's OK to end in a private road
3. If you have gone from public-private it's not OK to go back to public -
the trip ends there.

I prefer this over attempting to make a large jump to the nearest public
road.

Alternatively, can this be solved by "correcting" the OSM road
classification type, or adjusting OTP's settings for which road types are
allowed? My own assessment was that private was the correct road type, and
enabling OTP to use private roads would just result in problems with it
generating isochrones which drive right through private complexes and out
the other side.

Rhys
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