The limiting factor when it comes to managing diabetes with intensive insulin therapy (that is, usually four [sometimes more, occasionally less] injections per day) is hypoglycemia ("low blood sugar").  Most people on insulin are all too familiar with what that is all about.  Hypoglycemia manifests as hunger, sweating, tremulousness and, if severe, with confusion, disorientation and, on occasion, loss of consciousness. 

Fortunately the great majority of insulin reactions are of the mild type and readily dealt with by consuming a sugar-containing product (preferably as glucose or sucrose tablets rather than orange juice or glucose gel).

Severe hypoglycemia (by definition this is low blood sugar to the point that the patient requires someone else to assist them) occurs, generally speaking, in two circumstances:

1. In very longstanding diabetes sometimes the "warning system" falters and the usual clues that hypoglycemia is occurring  are absent.  This may be irreversible and safety necessitates running the blood sugars higher than what would generally be considered optimal.

2. In diabetes of virtually any duration, if one is on intensive management and trying valiantly to run "tight" blood sugars (i.e., in the 3.8-7.0 range) one has frequent, mild hypoglycemia (usually defined as being under 3.6 to 3.8).  These episodes are usually picked up readily for a time, but if occurring excessively often, the warning system fails.  It is as if repeated insults to the body make it say, "the heck with you-you've made me hypoglycemic so often, I'm not even going to tell you the next time you're low."  Thankfully, this is usually reversible.  The way to reverse this problem and restore hypoglycemia awareness (i.e., making one aware again of when the blood sugar is low) is to let ALL the blood sugars run in a significantly higher range for a couple of weeks.  Specifically, this usually entails cutting back from 20 to 50 per cent on virtually all insulin doses to let the sugars run about 7-12.  After a couple of weeks, the insulin doses can be raised again, the sugar levels brought back down to the aggressive target of 4.0-7.0 and awareness of low blood sugar is generally restored.  I find the main reason this strategy fails is that the poor patient who I've spent ages trying to convince about the hazards of high sugars is just not going to readily accept this sudden request to run what would generally be considered fairly crummy readings.  Please refer to my disclaimer before you follow this advice.  It may not pertain to your specific case.  Speak to your doctor or diabetes educator for recommendations specific to your situation.

Because hypoglycemia can affect driving performance (even mild low blood sugar can do this, by the way), if you are at all prone to hypoglycemia unawareness I would strongly (note the bolding) recommend you test your blood sugar level EVERY time you are about to drive.  Before you put the key in the ignition, test your blood.  If it is low, treat yourself and do not drive until your blood sugar is back to normal.  Always make sure you have something to eat within easy reach in the car (not in the trunk!) and if you are going to be driving for any sort of extended period of time, periodically find an appropriate place to pull over and retest your blood.  When it comes to driving: if in doubt, test & treat!

For a general discussion of hypoglycemia, have a look here.