I haven't heard of any legal guidelines on driving while hyperglycaemic but I know when my BG is exceptionally high, I am not fit to drive. Shaky, problems with focussing vision and probably judgement affected are not unusual in that situation, for me. But I need to be up in the mid teens before I can feel any effect, although others might be able to see something amiss. I have certainly driven with BG of something around 10 or 12 - and not come to any harm. I suppose everyone reacts differently to BG levels, either high or low.