Quick question about the Australian National Highway System...

[Quillz]

How did they determine the numbers for each highway?

I know Highway 1 is a gigantic loop around the continent, but then the interior of the continent has National Highways 31, 85, 94, 95 among several others. From what I have seen on maps, the numbers don't seem to follow any sort of numerical pattern, although, with the exception of 1, odd numbers do travel south-north and even numbers do travel west-east.

EDIT: Looking at the map again, it seems to follow the US Highways grid somewhat, in that odd numbers increase from east to west and even numbers increase from south to north. But that's not really what I'm curious about. It's why, for example, there is a Highway 15, then it jumps to Highway 39 and then to Highway 85. I figured the nation would have adopted a system where highways that end in a particular number, like 5, would be considered significant.

[national highway 1]

Dear Quillz,
As an Aussie who has lived in Sydney all my life, I wouldn't mind giving you some info.
The National Route System was set up in 1955 to distinguish the main regional routes between major cities and towns, with a black on white shield. Routes did not have to cross state borders to be a valid route.
In 1974 the National Highway system was set up so that the most direct NRs btween capital cities were promoted into a NH, which meant that they could receive federal funding by changing the shield into a gold on green shield. (Similar to the US's Interstate System, but they didn't need to be constructed as freeways)

However in the late 1990s Victoria and South Australia decide to change over to the British Alphanumeric scheme with a letter prefix (M,A,B,C,D) denoting the quality of road. Tasmania already converted in 1979, but metro Melbourne keeps its NR and SR numbers rather than converting into alphanumeric. Queensland began converting routes since 2000 and NSW is planning on conversion, with the first route, M7 being proclaimed in Dec 2005 (however it was a newly constructed road).
Check out these sites and see what helps you...

http://www.ozroads.com.au/NationalSystem/natsystem.htm
http://www.ozroads.com.au/NationalSystem/natroutes.htm
http://www.ozroads.com.au/NationalSystem/nathwys.htm

[treichard]

In the newer system of M, A, B, C, and D routes, are the designations intended to be repeatable in each state/territory but matching numbers only on the roads crossing state/terr. boundaries, or are the designations intended to be used for at most one highway in the whole country? Does the answer vary by the letter prefix used?

[national highway 1]

The more regional routes, not the interstate routes can get the same number as a totally unrelated route. Eg, A3 exists in Tasmania & also Adelaide.