[Talk-us] Freeway directions

stevea steveaOSM at softworkers.com
Fri Oct 18 18:27:43 UTC 2013

Saikrishna Arcot wrote:

>Just to add, three-digit routes tend to be either regional or be
>loop-shaped, where the designated direction changes.

>On Thu 17 Oct 2013 03:40:07 PM EDT, Ian Dees wrote:
>  > On Thu, Oct 17, 2013 at 2:31 PM, Martijn van Exel <m at rtijn.org wrote:
>  >
>>      Yea, I realized that as well. There's even a section of I-80 / I-580
>>      in Berkeley, CA where the directionality of I-80 and I-580 is
>>      opposite... http://goo.gl/maps/XROab (The actual compass direction is
>>      more like N/S on that stretch.)
>>      I don't know if there's a definitive reference for the 'official'
>  >     directionality of the freeways?

(Sorry, meant to send this to the list yesterday):
I am not pointing to an authoritative source of Interstate numbering, 
though I believe this to be accurate or at least very close:  it is 
my understanding that in three-digit routes xyz, these are "belt" or 
"spur" routes off of the yz route, where x=even means belt and x=odd 
means spur (single connection "back to" or "out from" the parent yz 
route).  A belt has two connections to the parent route, but I'm not 
positive if that is exactly two or not.

We have an example in California in the South Bay (area of San 
Francisco) of 280 and 680 (together) being a belt that connects to 80 
(twice, well, almost twice).  The split between these two happens 
"sort of conveniently" at US-101.

Yes, E-W even and N-S odd is correct.  Furthermore, routes ending in 
0 or 5 are "major" routes.

Remember, directions are for long-distance motorways (Interstate 
freeways, in local parlance).  A bit of local wander in an "off" 
direction happens to a lot of roads.  Many of these are 
direction-specific routed roads.  That's OK, consider these a "local 
bend in the road."  We have roads here signed north which go south. 
That just means we have geography (coastlines, mountains, et cetera) 
together with "get me from San Francisco to New Jersey on a single 
named road."

80/580 through Berkeley are an especially wild example of this.  It's 
just local geography messing with long-distance cardinal direction 
routing.  It does that!


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