[OSM-dev] Map files for entire US (for download)

[Alex Mauer]

Andy Robinson wrote:
> What's everyone's feelings on this?
> 

My feeling is that separate ways for each direction is quite painful to
work with in general (especially where two divided roads meet at an
intersection: the reality is a big square of pavement, while when drawn
as separate ways there's the effect of an "island" of barrier that's not
really there.

Places where a road transitions from divided to undivided can also be
awkward. This is especially so when there's also an intersection at the
same place.  Crappy ascii art (fixed width font may be required)

      F
      |
     D+<--<--<-- C
A----E+-->-->--> B
      |
      G

Lines AE, DC, and EB are all logically the same road (say, "Foo Road").
 Line FDEG is another road, say "Bar Road"
DE is one way when following CDEA, but not when following GEDF.  I would
also expect this to be bad for both rendering and routing ("Turn left
onto Bar road, proceed 1 metre, turn right onto Foo Road")

5 lane roads, that is, roads with a turning lane ("suicide lane" as
they're sometimes called.  For clarification, the 5th lane is not to be
used for normal traffic and passing, only for turning).  These generally
have a center lane the same width as the divider in a divided highway
would be (they sometimes transition from divided highway to turning lane
without angling the road).  They also often have a short (5-10 metres)
barrier next to any intersections that have a traffic light (for the
light to stand on, as well as pedestrians crossing).  This means that if
the 5-lane road is not considered a divided road, you have to divide the
road just before the intersection, and join it again just after the
intersection.  Since the lanes go straight rather than being angled,
there's no good way to make that transition.

Splitting divided roads into two ways does have the advantage of
simplifying the case where the divider blocks off another road though.
Crappy ascii art:

A----------------B
C------F+--------D
        |
        E

AB and CFD are one logical road, FE are another.

All these problems just "go away" when divided roads are treated as a
single way; the last advantage of splitting becomes a simple turn
restriction.

All these exist within 2 miles of me.  If the ascii art is unclear I can
provide photos of road sections giving examples.  (some people have
trouble with the 5-lane road in particular, so I might photograph that
one anyway to provide an example on the wiki)

-Alex Mauer "hawke"

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 252 bytes
Desc: OpenPGP digital signature
URL: <http://lists.openstreetmap.org/pipermail/dev/attachments/20070727/1d4ab6f2/attachment.pgp>