You need to use your general instincts on top of the sat nav.
In another hilarious in-law satnav incident, I went with FiL about 40 miles, to collect something for MrsLH (like freecycle). Given that FiL would stuggle to find his ass with both hands, you'd think a satnav was invaluable.
So off we set. At this point (2006) we (MrsLH & I) just could not afford a satnav. So in-laws loved showing it off.
On the way there - north on the M6 (so "home" is south) FiL kept telling me how you needed to "be able to follow a satnav", and how difficult it was.
We collect item, head off home. Following the satnav. All well for a few miles, until I noticed we passed an "M6 South" road sign. And another. And another. I also noticed the satnav wasn't complaining. Just before we passed another "M6 South" sign, and were going on a road saying "M6 North", I queried whether we were going the right way. After a lot of bluster, FiL checked, and discovered we were heading to Old Trafford. We went 20 miles out of our way.
(If I knew then what I knew now, I would have stayed schtum, and let the old git drive into Manchester, and explain to all. However, as my previous story suggests, it would only have been "my fault".)