how to autopopulate Uniq ID field after select by location

[kaww02]

I want to identify which lines share the same right of way. I think what I need to do is be able to edit the line table and add the Uniq Id of the polygon it sits on top of. There may be a few polygon Uniq Id's per line segments. 

I know how to select by location but after that how can I iterate through and have the uniq Id populate in a field I created? Any other suggestion on how to go about this? Picture is attached

[DanPatterson]

A Spatial Join or intersect might be a better candidate