There are two systems of highway exit numbering in this country. One has the exit number the same as the mileage marker post rounded to a whole mile. The other goes in sequential order. My state has the latter system. So a highway will have say 40 exits labelled exit 1, exit 2, etc but they are not usually 1 mile apart.
The Federal government is forcing anyone who wants to get Federal highway dollars to change to the first system. I see no reason to justify mandating this change which I would imagine, when multiple states are impacted, will amount to millions of dollars.