How do I model an intersection where queue spillbacks (a.k.a. tailbacks) do not occur?
And as I am trying to code a yellow box junction, is it possible to visualize a yellow box?

Priority rules can be used to avoid spillbacks. For details on how to model this, in PTV Vissim, click Help > PTV Vissim Help and refer to the following section:
>> Creating and editing a network > Modeling right-of-way without SC > Modeling priority rules > Examples of priority rules > Priority rule Example 2: Avoiding tailbacks at a junction

Visualization of the yellow box can be done if you include it in a background graphics file. You can also load multiple background files simultaneously.