Filtering domain list based on field in attribute table

I have two fields in my attribute table with domains and subtypes - SignID and SignType. What I would like to do is when I pick a value for SignID it automatically filters the SignType drop down for the selections.

This is especially important for users in the field so they don't have a long list to scroll through.

So if I pick SignID=RA the drop down should have all of the SignTypes associated with RA.

I have a table with the SignID and SignTypes ready. I am just not sure if this is something I have to program at the domain level or at the attribute level.