Scott:
Can you put in a print statement to determine what citiesL is defined as?
I do not see you setting the workspace in your script (e.g. file geodatabase, SDE database, shapefile), so I do not know how the script knows how to define citiesL for the SelectLayerByLocation_management(citiesL, "WITHIN_A_DISTANCE", interstates, maxKmInterstate, "NEW_SELECTION").