There's Certainly a High Way, But I've Never Seen A Free Way.

“Is there a rhyme or reason to the numbering of Interstates?” asked Angelique.

“Yes, there is. But first you need to know a little about the interstate highway system,” replied the internet.  So here it is:

Known officially as the Dwight D. Eisenhower System of Interstate and Defense Highways, this massive federal road-building project began in the late 1930s. But it wasn't until 1952 that Congress authorized spending and construction began. In 1956, uniform construction standards were adopted, governing such things as access, speeds, number of lanes, width of lanes and width of shoulders.

Standards were also established for numbering the routes:

• Routes with odd numbers run north-south.
• Routes with even numbers run east-west.
• For north-south routes, the lowest numbers are in the west.
• For east-west routes, the lowest numbers are in the south.

So, I-5 runs north-south along the west coast, while I-10 runs east-west in the south.

The major transcontinental routes are:

East-west Transcontinental Routes
Interstate #	From	To	Distance
I-10	Los Angeles, CA	Jacksonville, FL	2,459.96 mi (3,959 km)
I-80	San Francisco, CA	Teaneck, NJ	2,906.77 mi (4,678 km)
I-90	Seattle, WA	Boston, MA	3,085.27 mi (4,965 km)

North-south Transcontinental Routes
Interstate #	From	To	Distance
I-5	San Diego, CA	Blaine, WA	1,382.04 mi (2,224 km)
I-15	San Diego, CA	Sweetgrass, MT	1,436.89 mi (2,312 km)
I-35	Laredo, TX	Duluth, MN	1,831.43 mi (2,947 km)
I-55	New Orleans, LA	Chicago, IL	943.69 mi (1519 km)
I-65	Mobile, AL	Gary, IN	888.08 mi (1,429 km)
I-75	Miami, FL	Sault Ste Marie, MI	1,787.49 mi (2,877 km)
I-95	Miami, FL	Houlton, ME	1,892.76 mi (3,046 km)

When an interstate hits a major urban area, beltways around the city carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. To prevent duplication within a state, prefixes go up. For example, if I-80 runs through three cities in a state, routes around those cities would be I-280, I-480 and I-680. This system is not carried across state lines, so several cities in different states can have a beltway called I-280.

--All words and effort courtesy of A. Thomas