I note that
@Grant1969 's profile says he is diet only. You only need to test blood sugars before driving, if you are using insulin or a drug which stimulates your own insulin production. If you are not taking drugs which force down your blood sugar, a reading below 5, eg in the 4's or high 3's, is perfectly normal and your body will bring it up again at the appropriate time. You can drive at these levels, thousands of people do, all the time, with no idea even what blood sugar means.
But, readings in the high teens are dangerous to your health. They need to be dealt with. The first thing I would do is look very hard at what I had been eating.
Sally