I believe you could set the value using a calculation in a script with trigger. You could use validation on data entry but allow user overrides. Or not use validation and just change the value manually. Though if you went through the process again the script would reset the value to the calculated result.
I have done this with values in Check boxes.
Maybe someone else has a better idea. I am pretty new to this.
With the correct relationships an auto-enter setting--either a looked up value or an auto-entered calculation can copy the default value from a related table without any scripting or script triggers. This is how an invoice's line item records typically look up a price from a related products table and you can see an example of this in the Invoices Starter solution that comes with FileMaker.
Since this is a text field into which data has been copied when you selected the item, you still have the option of editing the field to select a different route. And a radiou button formatted field will automatically show a selected radio button value if the looked up value exactly matches one of the values specified in the radio button's value list.