In previous versions of InRoads, there is a command for computing the Surface Area of a DTM (Evaluation > Surface Area). What options are available for OpenRoads terrains?
One option is to use the MicroStation > Measure Area command to measure the surface area of the terrain element. Select the Measure Area command and you can select the terrain as the element. The results will appear in the message center in MicroStation.
Alternatively, the Surface Area command for the native InRoads DTM is still located under Evaluation > Surface Area. Export the Terrain Model to a native DTM, open the DTM in InRoads and use the Evaluation > Surface Area command to compute the Area.