The US Interstate Highway Numbering System

About This Map

The purpose of this map is to demonstrate the unseen logic of the numbering system that underlies the United States' network of high-speed Interstate highways.

This map shows the Interstate highways in the contiguous US, color-coded by route number and type. (Alaska, Hawaii, and Puerto Rico also have Interstate highways, but use separate numbering systems.)

This map uses publicly available data from OpenStreetMap displayed using Mapbox GL JS.

Created by Curt Arledge

Additional sources: Wikipedia.com, highways.dot.gov, Interstate-Guide.com
Overview
Origins & Funding

The US Interstate Highway System, known officially as the Dwight D. Eisenhower National System of Interstate and Defense Highways, is a network of high-speed highways in the United States of America. The system was first authorized by the Federal-Aid Highway Act of 1956 and supported by President Eisenhower, for whom it is named. The Interstate route marker is a blue, red, and white shield with the word “Interstate” at the top.

Interstate highways are owned, built, and operated by the US states, with funding aid from the federal government. State highway departments work through the American Association of State Highway and Transportation Officials (AASHTO) to develop highway design standards and assign numbers to new Interstate highways.
Numbering System

Primary Interstate routes are designated by one- or two-digit numbers. Routes with numbers ending in 0 or 5 are intended to be major arteries, extending from coast to coast or border to border (though many do not).

Geographically, Interstate highway route numbers increase from southwest to northeast, with the lowest numbers in the south and west of the country and the highest numbers in the north and east.

Odd route numbers are used for primary routes running generally north-south, while even route numbers are used for primary routes running generally east-west.

Auxiliary Interstate routes are designated by three-digit numbers, each comprising a parent primary route plus a prefix. Odd-numbered prefixes (e.g. I 540) are used for spur routes, which connect to the parent route at only one point. Even-numbered prefixes (e.g. I 495) are used for radial or bypass routes, which connect to a parent route more than once. Auxiliary route numbers are sometimes repeated in other states along the same parent route, though each state can have only one instance.

Business routes, which use a green version of a parent route’s route marker, commonly serve central business districts and may not comply with Interstate highway construction standards.
Interstate Route Facts

Lowest numbered primary Interstate: I 2

Highest numbered primary Interstate: I 99

Lowest numbered auxiliary Interstates: I 105 (CA) and I 105 (OR)

Highest numbered auxiliary Interstate: I 990 (NY)

Longest Interstates: 1. I 90 (3,020.5 miles), 2. I 80 (2,899.5 miles), 3. I 40 (2,555.4 miles), 4. I 10 (2,460.3 miles), 5. I 70 (2,153.1 miles)

Shortest primary Interstates without planned extensions: 1. I 97 (17.6 miles), 2. I 2 (46.8 miles), 3. I 86 (Western) (62.9 miles), 4. I 19 (63.6 miles), 5. I 66 (76.28 miles)

Primary Interstates that serve only one state: I 2, I 4, I 11, I 12, I 14, I 16, I 17, I 19, I 27, I 37, I 43, I 45, I 73, I 86 (Western), I 87 (Northern), I 87 (Southern), I 88 (Western), I 88 (Eastern), I 96, I 97

Unsigned Interstates: I 305 (CA), I 910 (LA), I 315 (MT), I 124 (TN), I 444 (OK), I 345 (TX), I 478 (NY), I 878 (NY), I 194 (ND), I 495 (ME), I 595 (MD), I 296 (MI)

Interstate numbers shared by two distinct routes, with no plans to connect: I 76 (Western) & I 76 (Eastern), I 84 (Western) & I 84 (Eastern), I 86 (Western) & I 86 (Eastern), I 87 (Northern) & I 87 (Southern), I 88 (Western) & I 88 (Eastern)

States with the most Interstate mileage: 1. Texas (3,233.45 miles), 2. California (2,455.74 miles), 3. Illinois (2,169.53 miles), 4. Pennsylvania (1,759.34 miles), 5. New York (1674.73 miles)

States with the most Interstate highways: 1. New York (29 routes), 2. California (25 routes), 3. Illinois (23), 4. Pennsylvania (22), 5. Ohio (21)

Interstates that travel through the most states: 1. I 95 (16 states), 2. I 90 (13 states), 3. I 80 (11 states), 4. I 70 (10 states), 5. I 10 (8 states)

Only four state capitals are not served by an Interstate: Juneau, AK; Dover, DE; Jefferson City, MO; Pierre, SD

The only auxiliary Interstate without an existing parent Interstate: I 238 (CA)

The shortest auxiliary Interstates: 1. I 878 (NY) (unsigned) (0.7 mile), 2. I 315 (MT) (unsigned) (0.8 mile), 3. I 110 (TX) (0.9 mile)

The only interstates with three of the same digits: I 444 (OK) (unsigned), I 555 (AR)

Interstate loops that do not share any portion of their route with another Interstate: I 610 (TX), I 270 (OH), I 485 (NC), I 295 (FL)

The longest concurrency of three Interstates: I 39, I 90, and I 94 in Wisconsin (29 miles)
Filters
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By Route Number
Color Key

	Major Primary	Other Primary	Auxiliary	Business	Future
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