After reviewing this further, I do appologize for not clarifying these values in greater detail earlier on. The type of design vehicle determines the offset value. You can find tables within the AASHTO - Geometric Design for Highways and Streets (pg. 589). I've attached the document from the TAC manual, table isn't attached.
The easist way to define this "symetrical offset" value would be to prompt the user for a value, if no default value is chosen (enter) than default to 3.0 meters (value for a WB 20 @ 90degress). Hope this helps.
