I will also suggest you to creating a 2D DGN model, reference the MX model into this 2D model then 'Merge into Master' and saving this 2D model as DWG.
Also another suggestion export to Native DGN first and export the Native DGN to DWG and then output properly based on your export settings.
In addition to this, please refer the below links for your reference.
http://communities.bentley.com/products/road___site_design/f/5922/p/107791/327925#327925
communities.bentley.com/.../25575.mxroad-ss4-export-to-autocad-converting-to-polylines-rather-than-ltpolyline
Please let us know if you have any questions on the same.
Regards,
Purva Lakde