Off the top of my head...
A good chunk of AASHTO roads can be found on the DLS, just search 'AASHTO' and you should be able to find plenty.
There are a couple AASHTO roads on USLW, they're in the Surveyor Objects section, and alternatively can be found by searching for AASHTO - http://uslw.net/search.php
Lastly, there is a pack of reskinned 32ft AASHTO roads on TEH, currently under the 'new releases' page but are likely to be moved during the next site update. Also should be searchable - http://theerectinghall.info/