I have a 3D bridge model generated in AutoCAD that I need to convert into an Inroads 3D model. How can I do that?

Maybe you don't need to do anything?  It may be sufficient to just reference the DWG into the Default 3D model of your road model.  If the bridge shows up in your cross-sections then you may be done. (I'm not sure what your end goals might be)

If that doesn't work then I would try the following:

First, in AutoCAD open the bridge model and Save As > DGN.  Then reference the DGN file to the road 3D models.

Lastly, If the bridge model is specialized objects only recognized by AutoDesk products then you might have to explode the bridge objects before Save As DGN and reference to the road model.

Keep in mind:  I have never had occasion to try the above suggestions but in theory one of the above should work for you since OpenRoads will recognize anything 3D even it OpenRoads did not create them.

Maybe you don't need to do anything?  It may be sufficient to just reference the DWG into the Default 3D model of your road model.  If the bridge shows up in your cross-sections then you may be done. (I'm not sure what your end goals might be)

If that doesn't work then I would try the following:

First, in AutoCAD open the bridge model and Save As > DGN.  Then reference the DGN file to the road 3D models.

Lastly, If the bridge model is specialized objects only recognized by AutoDesk products then you might have to explode the bridge objects before Save As DGN and reference to the road model.

Keep in mind:  I have never had occasion to try the above suggestions but in theory one of the above should work for you since OpenRoads will recognize anything 3D even it OpenRoads did not create them.