How do they determine freeway names?

Question

My daughter asked me how do they choose numbers on freeways such as the 405 or 91?

Answer 1

Odd numbers are North and South

East and West are Even numbers.

Answer 2

Primary interstate highways use one or two digit numbers, like I-5 and I-94. Odd numbers run north and south; even numbers run east and west. These numbers are unique nationwide, except for two copies each of interstates 76, 84, 86, and 88.

Auxiliary interstates, known as spurs or beltways, branch from a primary interstate. They have three digits, hence the nickname 3di for "3 digit interstate". Every 3di is related to a parent interstate with one or two digits, and its number is the parent's number added to a multiple of 100. This way we know I-180 is related to Interstate 80.

An even starting digit (such as I-210 or I-465) means the 3di meets another interstate at both ends (or is a loop). These are usually bypasses or beltway routes.

An odd starting digit (such as I-195) means the 3di meets an interstate highway at only one end. These are usually spurs from a main interstate to a location some short distance away.

The rules for 3di numbers differ from 2di's. First, the "even is east/west and odd is north/south" rule does not apply, because a parent interstate and its 3di often go in different directions. Second, a 3di number is not unique nationwide -- only within the same state.

Official road naming and marking conventions don't vary much throughout the state; unofficially, though, the way people refer to the various roads do vary a bit.

Officially, Federal freeways and highways are identified with white and black shields, the route number in black on a white shield bordered by black. If you've ever seen the old Route 66 television show, that Route 66 sign is a classic example of this. Federal / US routes are usually identified on maps and in publications in the form "US 101", or the shield with the number in it, or similar. All Interstates are identified with a red, white, and blue shield: white numbering on a blue background, with a red top to the shield (also, the shield is slightly differently-shaped to the US route shields). On maps or in official publications Interstate freeways are usually referred to in the form "I-880" or "Interstate 880", or the number-on-a-shield form. State Routes are identified with a green and white, rounded-triangular shield: white numbering on a green background with white borders. State routes and highways are referred to in a wide variety of ways in maps and publications; many do not use the California state green shield markings but use ovals or squares. County roads and highways are identified in various ways, but virtually all identifications include a letter, e.g. "A17" or "J14". These letters do not seem to correspond to the County name, unfortunately.

With one well-known exception that I'm aware of (Interstate 110 and State Route 110 in the LA region), all US, Interstate and State route numbers are unique within the state -- there is no US 4 or Interstate 4 in California because there is already a State Route 4 (and in the case of the routes 110, they're actually two ends of the same road). This is not true for County routes (there's both an A17 and a J17 in California, in Shasta County and Merced County respectively), nor is it true across the US as a whole -- there are separate Interstate 280s in at least California and New Jersey.

Freeways and highways are usually marked periodically with signs giving the freeway number and general direction; this is particularly common after major interchanges or freeway entrances. The direction given ("North", for example), is the logical direction of the freeway as a whole, not necessarily the direction the freeway seems to be traveling in at that particular location. This can be very confusing if you don't know the ultimate destination of the freeway -- there's a classic spot on Interstate 80 in Berkeley where the signs say that this particular stretch of road is both "I-80 East" and "I-580 West" -- but you're actually driving north at this point... (I-80 is heading north here as the first step on its journey all the way east to New York, while 580 has joined I-80 on its way north for a few miles before splitting again from I-80 and heading west towards the Richmond Bridge and its ultimate destination, San Quentin). This same point holds true for freeway and highway destination and exit signs in general -- if the area you're going to isn't explicitly on the sign, you will need to know which direction the freeway or highway you need to exit to is ultimately going, or the ultimate destination city. This can be very difficult to determine.

Answer 3

It's really intricate, actually. 405, for example, is a freeway tributary to I-5.  We know that it is a N-S traveling freeway because it's front number is an even number, meaning the route follows the "main" route and is a bypass becuase of it's 3-digit Interstate number.  I-10 is actually a better example, because the 210 travels E-W like the 10, but the 110 travels N-S, intersecting the main route.  As far as the california highways go, I'm not too sure of the rhyme or reason as the 91 travels E-W, but so does the 118 and the 60, so go figure.