[Talk-us] Road classification

[Chris Lawrence]

I think the place to start in the US (and I'd assume Canada too, which
tends to use US-originating classification) is the idea of "functional
classification" which is used by highway planners.  See e.g.
http://www.fhwa.dot.gov/environment/flex/ch03.htm and
http://www.fhwa.dot.gov/planning/fctoc.htm (the latter is incredibly
long for our purposes).  Unfortunately TIGER's scheme is theoretically
a functional classification but in practice devolves to a
designation-based scheme, which isn't very useful for mapping from
TIGER to something to be used for navigation.

Here's my rule-of-thumb:

- highway=motorway: What is referred to legally in most states as a
"freeway."  A highway with no access (except at termini) except via
grade-separated interchanges - usually called "full control of
access."  Typically does not have any signalized intersections,
although occasional drawbridges (e.g. the Woodrow Wilson Bridge on
I-95/495 near Washington) or railway crossings may not disqualify the
route.  Virtually all of the Interstate system counts, but there are
exceptions.  (I-180 in Wyoming -
http://en.wikipedia.org/wiki/Interstate_180_(Wyoming) - is the most
prominent example of a non-freeway Interstate.)  Routes without
Interstate shields also can qualify (e.g. CA 91 and CA 60 in the LA
area, most of US 78 in Mississippi and Alabama, US 59 through
Houston).  In most US states, freeway is a legal designation not
unlike the "motorway" rules in the UK and Ireland ("freeway
ends/entrance/begins" is usually only signed in a few states, most
notably California, but many states post a sign listing their state's
freeway restrictions when entering a freeway).

- highway=trunk.  Referred to in many states as an "expressway."  A
highway typically with limited access by adjoining property owners,
but access via both at-grade intersections and grade-separated
interchanges.  Systems like the Mississippi Four-Lane Highway Program
are often constructed to these standards (so routes like US 61 and US
82 in Mississippi qualify).  Should not be used for routes with
frequent traffic control devices (signals, four-way stops, etc.)
although an occasional traffic signal would not disqualify a route.
To borrow from Russ' example, parts of US 17 in southern New York that
haven't yet been fully upgraded to I-86 would be "highway=trunk,"
while sections with exclusively interchange access would be
"highway=motorway."  See also US 101 between San Jose and Santa
Barbara, CA, which is a mix of "expressway" and "freeway" segments.
Most commercial maps show this distinction fairly well.

- highway=primary.  Probably the lowest-tier "arterial."  Signed US
and state highways that don't fit in a higher category probably belong
here, although less important highways (state secondary routes, Texas
FM/RM roads and most spurs and loops) could be highway=secondary.
"Primary" routes have more signals and more adjoining property access
than a trunk route.

- highway=secondary.  High-capacity "collectors."  May have a highway
designation, may have a county road number, may just be named.

- highway=tertiary.  Medium and low-capacity through "collectors."
Typically do not have a highway designation (but may have a county
road number).  The lowest level of the "through" street network in an
area.

- highway=unclassified.  Non-through collectors; typically would be
classified as "tertiary" but does not (yet) connect two important
streets.  Since there is no rendering difference between
"unclassified" and "residential" it might make a difference in a
routing application but on a map it won't have undue importance.

- highway=residential.  A street that primarily provides access to
adjoining properties with limited or no through traffic.

This classification has the drawback of requiring a lot of fixing up
from TIGER and a decent level of local knowledge to decide the
corner-cases.


Chris
-- 
Website: http://www.cnlawrence.com/