You may find this useful:
Code:
SELECT array_to_string(array(SELECT column_name FROM table_name), ':');
Change the inner SELECT to any query that returns only one column.
This will concatenate all values into one string, with a ':' between each value.

Code:
SELECT array_to_string(array(SELECT roadnum FROM table_name WHERE ID=4), ':');
would return "N-3:E-2:CA-1"...