About the only data that can be switched to InRoads is the TIN file. You would have to use LandXML to perform this task. Export out then import in to a InRoads DTM. There are more variables with InRoads DTM's such as features. Features are the 3d lines that comprise your DTM, only features are very powerful as they can be assigned levels, line weights, linestyles within the DTM. You can even annotate your cross-sections based on the feature. The feature can be assigned a cell (block) to be displayed in your cross-sections.
I am not familiar with LDD if that is what you are using for AutoCad as a Highway Design package. InRoads is a add-on to Microstation. They work in conjunction with each other. All Autocad files will have to be translated to Microstation. I think what you are up against the most is the translating between Autocad and Mircostation. After that, InRoads can be used to develop DTM's and linework.
Good Luck, I would suggest a class by Bentley to introduce you to InRoads. But first, you will need to know Microstation.