This is probably the answer to my previous question: List Elements or Datasets of a given Layer.
The version of arcpy that I am using does not include GetNAClassNames
naClasses = arcpy.na.GetNAClassNames(serviceAreaLayer)
'module' object has no attribute 'GetClassNames'
According to this help topic, GetNAClassNames is in ArcGIS 10.1. Why is it not in my version?