A number of roads, known as control roads, have been chosen to divide the State into logical segments. Within these segments a numbering plan is established. Roads running north to south are 1 or 2 digits and end with 5, i. e 5, 15, 25, 35, 45, 55, 65, 75, 85 and 95. Roads running east to west are 2 digits and end with a 0, i, e 10, 20, 30, 40, 50, 60, 70, 80 and 90. Additionally there are 4 diagonally running control roads namely the 100, 200, 500 and 600.
Connecting roads between the control routes also have a unique numbering system. For example a north and south main connecting route between control routes 25 and 35 would be 27. East/ west connecting routes between the 50 and 60 would be the 52, 54, 56 and 58.
Roads that are considered secondary are assigned 3 digits. The first digit signifies the location relative to the east/ west control routes. These routes are allocated as follows:-
- Between State line and route 10 – secondary numbers 101 to 199
- Between route 10 and route 20 – secondary numbers 201 to 299
- Between route 20 and route 40 – secondary numbers 301 to 399
- Between route 40 and route 50 – secondary numbers 401 to 499
- Between route 50 and route 60 – secondary numbers 501 to 599
- Between route 60 and route 70 – secondary numbers 601 to 699
- Between route 70 and route 80 – secondary numbers 701 to 799
- Between route 80 and route 90 – secondary numbers 801 to 899
- South of route 90 – secondary numbers 701 to 799
The numbering grid was established in 1945 so it is true to say that over the years the numbering system has gone slightly astray as the need to add more routes has become apparent. The 192 is a case in point, and even after researching the logic behind the State roads I am still unsure why it is positioned where it is.