Component / Join selected
The operation of Join selected command depends on the type of selected components.
Key-in: road join segments
Join circular arcs
If the selected components include circular arcs, the elements are joined into a single circular arc. In this case, the command opens the Road join components dialog:

The Radius value shown in the dialog is an estimate made by the application. The sign of the radius is determined automatically. You may set the new radius of the circular arc to a specific value by typing the value into the Radius field.
The change is attempted and if a solution is found, a preview of the modified component(s) is shown in the CAD view. If you accept the modification by a data click, the component list is updated.
Small radius values are more likely to produce an acceptable solution.
Join clothoids
If the selection only includes clothoids, there is no option to set the radius and the dialog does not appear. You have only the option of accepting or rejecting the result.