Edit Junction Dimensions tool is found on the 
junction tools toolbar.
The Edit Junction Dimensions tool allows you to quickly modify the dimensions of a junction, or add a lane or taper to a junction.
The Edit Junction Dimensions tool is found on the junction tools toolbar.
If you right click over the left or right side of a junction you will see the following menu:
Selecting either Enter Radius will allow you to input the radius of the closest junction fillet.
Selecting either Change to Taper or Change to Lane will change the junction style.
By default, junctions are created with as standard "T-Junction", as per this example:
By selecting Change to Taper or Change to Lane from the right click menu you can apply the new junction style to the entry/exit side of the junction.
The junction style can be applied to either side of the junction independantly, as shown below:
NOTE: It is always possible to change a taper to a lane, lane to taper or revert a lane or taper back to a normal fillet by right clicking over the appropriate side of the junction whilst in the Edit Junction Dimensions tool.
To specify or alter the lane/taper length, width or fillet radius, select the Edit Junction Dimension tool, and select the relevant dimension item on the plan-view with the mouse (left click). You will then see the design stretching as you move the mouse. Left-click the mouse again to confirm the new length, width or radius.
You can specify exact lane/taper length, width, radius or crossfall values by right-clicking the mouse whilst in the Edit Junction Dimensions tool: From the menu, select either the Enter Lane/Taper Length, Enter Lane/Taper Width, Enter Radius or Edit Crossfall option. This will ask you to enter a value, then update the junction instantly.
The example below shows a taper on the left and a lane on the right with their appropriate dimension annotations which can all be edited in the ways described above.