Have you ever wondered how we name all our highways? It’s fantastically complicated, yet brilliantly explained by Mark Bozanich from one of our Olympia offices.
You’re probably familiar with the three different route-numbering systems that we use: Interstate, US and State Route. Examples of each include: Interstate 5; US 2; and SR 20.
The Interstate and US numbering systems are set up in a grid to cover the 48 contiguous states, plus the District of Columbia. Interstate highway numbers progress from west to east and south to north. US highway numbers progress from north to south and east to west. This minimizes the use of the same route number for an Interstate and a US highway within the same state.
• US 10 is in the north and I-10 is in the south,
• US 5 is on the east coast and I-5 in the west.
One- and two-digit route numbers are used for the longer through-routes, and three-digit numbers are used for branches. Though, US 101 is an anomaly; it’s just the next odd-numbered through-route after 99.
The three-digit branch numbers use the second and third digits to identify the trunk or parent route. Thus, I-405 is a branch of I-5 and US 195 is a branch of US 95.
On the interstate system, branches that begin with even numbers make partial or full circles of an urban area. Branches that begin with odd numbers are spurs that link the parent route with a feature such as a downtown area not directly adjacent to the parent route (e.g., I-705 links I-5 with downtown Tacoma).
Now that we’ve covered the US and Interstate numbering systems, let’s move onto the State Route system.
The State Route system is in a grid that progresses from west to east and south to north. It does not use numbers used by Interstate or US routes within Washington State. Since we have US 2 and I-5, there are no other state routes with these numbers. US 101 is implicitly State Route 1.
The longer state routes tend to have one- or two-digit route numbers, and the shorter routes have three-digit numbers, like the Interstate and US routes.
Unlike the Interstate and US routes, however, the first digit or two digits indicate the trunk or parent route. Thus, SR 500, 501, etc., are considered to be branches of Interstate (implicitly State Route) 5, and SR 160, 161, etc., are branches of SR 16. There are some anomalies; SR 10, SR 99 and SR 410 are portions of former US 10, US 99, and US 410, and SR 92 and SR 96 are branches of SR 9.
There, now you’re a scholar of highway naming systems. Don’t say I didn’t warn you that it was a little complicated.
Here's an online highway map, in case you care to look up any of the highways mentioned.