[Talk-us] Freeway directions

Chris Lawrence lordsutch at gmail.com
Fri Oct 18 22:59:58 UTC 2013

On Thu, Oct 17, 2013 at 3:39 PM, Brad Neuhauser <brad.neuhauser at gmail.com>wrote:

> From
> http://en.wikipedia.org/wiki/Interstate_Highway_System#Primary_.28one-_and_two-digit.29_routes_.28contiguous_U.S..29:
> "In the numbering scheme, east-west highways are assigned even numbers and
> north-south highways are assigned odd numbers. Odd route numbers increase
> from west to east, and even-numbered routes increase from south to north
> (to avoid confusion with the U.S. Highways, which increase from east to
> west and north to south), though there are exceptions to both principles in
> several locations."

Field signage is sometimes inconsistent with the official rules; for
example, US 68 is mostly (entirely?) signed north-south, and I-69 becomes
east-west between Lansing and Port Huron.  States may have their own rules;
some states (MS, FL) follow the even-odd rules that the national routes do,
some use an opposite pattern (even N-S, odd E-W), and some have no pattern
at all (GA, TN).

There are also cases of signage by loop nesting ("inner" clockwise/"outer"
counterclockwise for RHD) - I-495 around Washington and I-440 around
Raleigh NC are examples, along with GA 10 Loop around Athens GA. And in
Canada the QEW is "directionally" signed by destination (Toronto on the
clockwise carriageway, Niagara and then Fort Erie on the counterclockwise
one). There may be a few more oddballs I've missed.

IMO preferred practice should be a relation for each continuous cardinal
direction, to keep validation simple; undivided roads should use
forward/backward roles to distinguish which relation applies to the
underlying way's forward/backward traversal. It shouldn't be too terribly
hard to come up with an algorithm to fixup the existing single-relation
cases, particularly for the ones where the routes are entirely dual
carriageway, although occasionally the heuristics will be wrong and need a
manual edit.

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