OT civic law: principle & application

A friend suggested that we distinguish between the principle and application of Old Testament civic law (following Thomas Arnold in this). The Old Testament shows us how principles applied in practice in Israel. We must discern the principles and apply them to our own situation. The close our situation to that of ancient Israel, the closer the application will be to the original instruction. The classic example of this would be the command to fence your roof in ancient Israel  given in Dt 22:8 (roofs being flat busy places from which one could fall and be injured). It would normally be absurd to fence your sloping unused roof in modern Britain but that commandment might lead you to fence your swimming pool.