Are you starting from scratch? There are several commercial packages on the market.
If you're in the US, your roads should already have designated functional classes (arterial, collector, etc). Ask your MPO if you have one, or state DOT planning department if you're in a rural area. Our DOT sends us an inventory of our roads annually so we can verify our mileage.
Then check the functional classes to make sure they make sense. For example, we have an intersection where the "local" road carries twice as much traffic as the "collector." The functional classes in your PM system should match reality, not what some bureaucrat says the road is.
The next step is braking the roads into segments, based on pavement type and condition, traffic types and volumes, etc.