Infraworks 360 is a great tool for preliminary design in the context of the existing site. It should NOT be used for a "final" design because the terrain that it imports is based on USGS/SRTM data and is not 100% accurate - I.E. the terrain won't show vertical changes in elevation due to retaining walls. The terrain is more rolling and averaged. The imagery is based on BING maps and the roads, buildings, etc that it can find is based on OpenStreetMaps data.
You can sketch in roads using design speeds and it will set the curve automatically based on that speed. You can also modify the vertical PIs in a profile editor (based on the USGS terrain). Infraworks 360 will also allow you to do some schematic grading based on slope, distance, etc. and do a rough cut/fill analysis. Once you get an accurate field survey file, you can continue to work on your design with alignments, polylines or lines and then export them to LandXML or SHP files and import them into Infraworks 360 for presentation purposes.
You can import ESRI SHP data for preliminary planning purposes prior to getting an actual field survey which is readily available online.
You can also import Revit and Civil 3D models.
Infraworks 360 wouldn't be used for clash detection like Navisworks. With Navisworks you would import your final Civil 3D model, Revit models, etc. and THEN check for clashes.
The bottom line is this... Infraworks 360 is not intended to used to generate a finished design or construction drawings.