Question by ursolstsmr: which number of a three digit interstate is most important?
also: If the first digit is odd, the interstate goes in or out of a city?
and if the first digit is even, the interstate goes in or out of a city?
Answer by moonjumper
The direction of an interstate is determined not by its local direction, but by the starting and ending points of the entire interstate. It is possible, even likely, that a north/south interstate may go east or west for awhile to go around a mountains, rivers, cities, or other large objects. You ignore such wiggles when stating the direction of the interstate…. keep true to its overall direction.
The exceptions to these numbers are highways with three-digit numbers (101 to 999). These are “bypass” highways that connect to, or are associated with, a main interstate (e.g., 405 is a bypass for interstate 5). Bypasses can be perpendicular or parallel to their associated interstate. Bypasses can even meet their counterpart head on (in New Jersey, 295 SOUTH meets 95 NORTH, while traveling east/west at the time… causes endless confusion to the unwary). The same bypass number can be used in different states for different bypasses An “I405” bypass is found in both California and the state of Washington.
If the hundreds digit of a bypass (the “4” in “405”) is even, then it is likely (at least planned) that after the bypass splits off from its parent highway, the bypass will join up with its parent again. Somewhere. If you are going in the right direction and construction is finished. If the hundreds digit is odd, then the bypass was not expected to reconnect with the parent highway.
If you must guess at the direction of a bypass from its number first look if the number is even or odd then look at the 100’s digit. If the 100’s digit is even, guess at the same direction as the main highway. Else when the 100’s digit is odd, guess a perpendicular direction, or something close to perpendicular. Examples: 405 may be north/south, just like the 5. 305 may go east/west, angled to the 5.
For one and two digit interstates whose numbers are odd, the lower the number the further west the highway is. For even numbered interstates, the lower the number the further south the highway is (this follows the same direction rules exit numbers and mileposts do). Further, interstate numbers ending in 5 or 0 tend to cross the entire available span of the United States, or nearly so.
What do you think? Answer below!