I can kind of see this possible scenario, whether Diane was an alcoholic, or not:
She has tooth pain. She goes for analgesics. Whether she gets them or not, she decides, "Hey, I've got the vodka packed back there. I'll just take a few swigs of that and I'll be pain-free, able to get these kids back home."
Now, at this point, the cunning nature of alcohol takes over.* She ends up swigging more and more, and driving aggressively after a while. Next, pulls over and calls her brother because she knows she's messed up, and smokes some pot she also has packed in there, thinking maybe it will offset the alcohol. (That's how you can get to thinking on alcohol.)
Then she says, "oh, hell, I'm fine, I can just drive. He doesn't have to come up here and that would be humiliating anyway." In alcohol-and-pot-induced confusion, she goes up the wrong ramp. When other cars start honking and swerving, she knows something is wrong but maybe doesn't know what is wrong or can't reason enough to pull over. Instead, she "hunkers down" in [her] slow lane and drives as straight and true as she can until....
* Example of how this has happened to me: I used to tell myself that when I had three limes in the bottom of my glass that I would stop drinking those gin and tonics because I knew that three was my "limit." But more often than not, I'd look down in that glass and suddenly there were like 8 or 9 limes in there! Gee, how'd that happen??