Hello Masao -
The decision on whether to create a multi-field, single-field, or autogenerated Route ID is truly a business decision relative to the needs of your organization; the system obviously supports any of these so long as the Route ID you choose is unique.
Many organizations embarking on Roads and Highways implementation projects tend to have well defined geodatabase design activities since they are migrating their legacy systems to R&H and need to plan out their projects in detail.This is why I mentioned modeling ahead of time..
However, there is no specific rule that you need to model ahead of time. If you wish to use the out of the box tools to create the LRS container and LRS Network you can always go back to the network feature class after network creation, add the fields you want to concatenate for Route IDs, and then run the Modify LRS Network GP and also the Modify Route ID Padding GPs to configure your concatenation logic.
-amit@esri
Esri LRS Transportation Team