I think most of us who have done work with Land Planners have received a sketch that looked something like this:
Usually it’s on paper. If you are lucky, you might get some AutoCAD entities. Your mission, if you choose to accept it, is to convert a hopeless spaghetti of splines and compound curves into a collection of legal roads.
The Civil 3D model provides the most value when you think of it as a clay sculpture. Start with a rough shape, then use the tools to refine your elements.
Create a few simple Design Checks for your local roads- like minimum tangent lengths and radii.
I am giving you permission to live dangerously, here. Go ahead and create an alignment from those fit polylines so that you can see for yourself that the world does not explode. The design checks will alert you to the pieces that are completely off track.
Grid view will give you more information on what is wrong with each piece and give you the opportunity to change the curve type (in this case I might want to go from fixed to free). Now I have the intelligence I need to tweak each area to meet standards without starting over or losing the planner’s road skeleton.
I can run down the list and type in new lengths or radii, or I can once again be a complete free spirit and… wait for it… play around with some grip editing! Shocking, I know.
Some related information:
Criteria Based Design for Alignments is a old post over on Civil 3D Rocks, some of the links are dead, but it’s a good quick explanation of the tool.
There are a few good whitepapers on road design over on the Civil 3D Whitepapers page. You’ll definitely want to read Rule-Based Road Design using AutoCAD Civil and AutoCAD Civil 3D (pdf - 1230Kb). It was written for Civil 3D 2009, so it won’t have some of the new nifty curve parameter changes documented, but the criteria and standards information still works the same.
It’s challenging to keep these colored pencil and tracing paper guys happy.