You stop the car as soon as possible. You help her to walk around a little in the fresh air. When this doesn't work and all she can think of is her need to lie down, you clear the back seat. Where did all this stuff suddenly come from? Two coats (it's a cold sunny day), a camera (unused today, as it turns out), a bag containing a hastily prepared but now half-eaten picnic of grapes, Brie cheese and crisps, bottles of water, a cardigan, a road atlas, a book, a magazine, the unused and uncharged GPS, a pair of shoes, a paper bag. Did they all spring out of the boot (trunk) while you were driving? On the very day when you need an empty back seat? Quickly.
Your navigator lies down. Then you have to contort yourself - and her - to work out how to strap her into a seatbelt. After many variations and much quiet swearing (she really doesn't feel well), you leave her to sort it out herself. You cover her with a blanket (a useful thing left on the back seat) and get back in and drive. Home.
Echoing in your head, while you're trying to remember what she said about which road where and in what direction, are also her words, "Just in case I fall asleep, or die, follow signs for Mount Dora. You can find your way home from there."
You spend the next two hours driving faster than you find comfortable, worrying about getting lost, and trying not to constantly look behind you to see if your navigator is dead.
You really need to learn how to use the GPS/Satnav.