Driving Speeds Road Type Classifications in Microsoft Streets and Trips

I've used Microsoft Streets & Trips for 5 or 6 years & am still puzzled about which road types MSFT includes in the 3 driving speed breakdowns other than limited access and streets, which seem pretty clear, ie, main highways, other highways, & arterial roads.

I'd sure appreciate any clarifications anyone [especially MSFT] might have about what characteristics distinguish them.

Thanks!

It looks in the map data. It's actually a simple database, not a picture. Each "line" has a set of attributes that define it, whether it's a road or a river, etc. S&T simply checks the attribute that the map developers (in this case, a company named Navteq) have assigned to that line segment and that's what they use.

EDIT: No, I don't know how the map developers decide what attribute to assign.

...ken...