For a person who recently took herself off the road due to diabetes, here is my take.
Short version: I got out of college, and fought for 6 months to get health insurance in NY. I finally have it, and in the process of find an endcronologist (one that could see me rather soon, I needed new insulin prescriptions..) my primary care doctor stepped in. My A1C was so high, he jacked up my insulin - but only my 24 hour insulin. Slowly increased the dosage every night of my 24 hour insulin. I woke up every morning feeling like I was hit by a truck but my bloodsugar was high - not low. I had issues getting up. My boyfriend tried and failed to wake me up one night when I was panting and grasping his hand so tightly to my body he had to pry it out of my hands, leaving finger marks on my arm the next morning. Needless to say, I didn't go to the full dosage, but driving every morning I felt fuzzy and shaky. I noticed I was having a really hard time paying attention. Eventually, I rolled into a car coming off an on ramp on the way to work. The next day, I found an emergency endocrinologist appointment through the primary care doctor who was acting as my endo.
The Endo helped me fix my insulin problem. He was so pissed that my primary and tried to manage my diabetes by just raising the 24 hour insulin (which, as it turns out, Levemir is not 24 hour insulin. I now take it twice a day and POOF high blood sugars toward the end of the day are normalized.) I now take 15 units of levemir every 12-14 hours - and my bloodsugars are in the 130 range for the last week. (my primary wanted me taking a totally of 40 units before bed... I only took it up to 36.)
Moral of the story, GETTING a tighter control on your diabetes can lead to an increase of car crashes. Like, for example, someone changes your insulin. But if you already have good control of this disease, just test before you drive and make sure you have snacks, and you should be fine.