Question about HEC-RAS variable

[hacrock]

I am working on my first HEC-RAS model of a single culvert underneath a roadway and have a question about one of the variables used to initally determine the location of the first cross section. How is the average length of the side obstruction calculated? Looking at a couple examples, I understand where a couple of the values come from, but not all of them. Thank you in advance for any help you can give me!