Transportation planning generally deals with changing populations and the impact on roads in terms of traffic and planning for the future, i.e., where to add new roads/lanes.
ArcGIS Network Analyst deals mostly with routing, logistics, location analysis, not the transportation planning as described above.
Dan's suggestion above of starting with the tutorials is an excellent place to start with learning about NA. You can also watch some of the User Conference presentations. E.g.,
Esri Videos: GIS, Events, ArcGIS Products & Industries
Jay Sandhu
