custom dynamic text expression

[Anonymous User]

I would like to display dynamic text from feature class attributes in my map layout. I have 2 feature classes that are a subset of a roads feature class (red route & blue route). I have successfully displayed the sum of the line lengths for each route but would like to dig down further and create an expression that calculates the total length times the number of road lanes for each feature without creating a new attribute in the table. Is this possible?

As an alternative, I wouldn't mind doing the math myself but the statistics option in the attribute table shows a total sum - not the sum of the selected features (i.e. 2 lane roads).