This is the USA interstate system (how does the UK system work?)
In the USA, the two-digit interstate highways start at even numbers of the 10 in the south, then 20 further north, then 30, 40.... 80, and finally 90 when you get to the northernmost route (not all of which actually exist but all that do exist are unique).
Notice all interstate mile markers increment similarly south-to-north on odd-numbered two-digit interstates (and the exit numbers follow suit).
Likewise, the odd two-digit interstates start at the 5 on the west coast, and move to 15, 25, 35 ... to 95 (not all of which exist but all that do exist are unique) as you move east.
Similarly, all Interstate mile markers increment in the same direction, west-to-east, on the even-numbered two-digit interstates (and the exit numbers follow suit).
When it comes to three-digit interstates, a *different* numbering system is used, which makes them not always unique and which makes the general direction not always clear.
Also, the three-digit (secondary) interstate system is vastly less complete than the two-digit (primary) interstate system.
Essentially, the second two digits of the three-digit secondary interstate indicates which primary two-digit interstate it connects to; hence, 280, 380, 480, 580, 680, 780, 880, & 980 all connect to "80" in some way, fashion or form (or, will connect to, someday, or, were planned to connect, at one point, etc.).
Given that, there could be, say, a 280 in every state, because all it has to do to comply with the number system is connect to 80.
The significance of the first digit in the three-digit system varies, and I forget the details, but the lower the number, the closer it is to the city center. So, for example, a
480 would be a circle that is closer around a city than an 880, and a 280 would be a spur that is closer to a city center than a 980 spur (although, all bets are off when plans change).Everyone gets confused because they don't understand this system; once you know the system, about 80 to 90% of the roads follow it faithfully (although there are always exceptions that people love to call out).
The beauty is that, without GPS, in just one mile, you always know what direction you're going if you know the road number (in the east coast, you know what direction you're going usually in 1/10th of a mile because of the closer-spaced mile-marker culture compared to the west, which just barely started numbering the exits just a few years ago).
BTW, you also know the exit number, because they have (mostly) been correlated with the mile markers (with a, b, c, etc. used with closely spaced exits).
This is the USA interstate system (how does the UK system work?)