Interstate 80
I-80 is the Interstate Highway that most closely approximates the route of the historic Lincoln Highway, the first road across the United States. The highway roughly traces other historically significant travel routes in the Western United States: the Oregon Trail across Wyoming and Nebraska, the California Trail across most of Nevada and California, the first transcontinental airmail route, and except in the Great Salt Lake area, the entire route of the First Transcontinental Railroad. From near Chicago east to near Youngstown, Ohio, I-80 is a toll road, containing the majority of both the Indiana Toll Road and the Ohio Turnpike. I-80 runs concurrently with I-90 from near Portage, Indiana, to Elyria, Ohio. In Pennsylvania, I-80 is known as the Keystone Shortway, a non-tolled freeway that crosses rural north-central portions of the state on the way to New Jersey and New York City.
Route number
The Interstate Highway System, indicated by a red and blue shield with white numbers, is a system of entirely freeways (unlike the U.S. Highway System, which is mostly undivided surface roads). The Interstate System is also based on a grid, with east–west routes bearing even numbers and north–south routes bearing odd numbers. In order to prevent confusion with the earlier U.S. Highway System, however, the Interstates are numbered in the opposite direction, such that the lowest routes numbers are in the south and west, and the highest numbers in the north and east. Major routes end in either a "0" or a "5"; for example Interstate 10 spans the country from Jacksonville, Florida, to Santa Monica, California, while Interstate 35 goes from the Mexican border to the Great Lakes. Like with U.S. Highways, subsidiary routes are numbered by adding a hundreds digit to the parent route. Because of the large number of these routes, three-digit numbers may be repeated within the system, but unique to each state. Additionally, the parity of the hundreds digit tells the nature of the spur route: odd hundreds digits like Interstate 393 only connect to the system at one end (forming "spurs"), while an even hundreds digit like Interstate 440 indicates that the highway connects to another Interstate at both ends (forming loops).