Exit #s on USA systems are Collaborator discrestion. Not required last time I checked. (Outside Interstates)
What did you check?
Project contributors refer to
the manual.
Choose the first type that applies.
...
2. 🔗 Exit numbers: If the highway has interchanges with exit numbers for itself.
This is what it's been since the CHM days.
What you're thinking of may be the situation with United States Numbered Highways.
In CHM, US Highways did not use (their own) exit numbers.
When development of the state systems began, the policy switched, to allow exit numbers on these new state systems. The exception for US routes remained.
In the early days, some state routes used numbers, and others followed the old way of doing things and continued using crossroad labels (or, exits weren't posted at the time, were later). Some were fixed early on, others lingered longer, still others remain now.
The US Highways are more where discretion comes in. Some people won't be down for the busywork of changing over long-established labels that were OK at the time they were created, and that's fine.
