Lani from Alaska has co-pilot syndrome (also known as ‘passenger monkey stress’ from psychology experiments in the '60’s). For example, on a space mission, all the astronauts are just as well trained as the pilot. What keeps them from wrenching controls from the pilot’s hands? They all have their own jobs to do. One of my sisters scans trees & wires for raptors. (Probably an easy job in Alaska.) I focus on the history – how old are these houses, is this highway an old railroad bed, etc. Lani could assign herself a ‘job’ that takes some focus – and takes her eyes away from the road in front of her.
I think Ray and Tom really missed the boat on this one. I, too have the same problem, except I am the one who is a distracted driver and my wife is the one with white knuckles. Instead of suggesting that Lani “take a chill pill”, why on earth would you not suggest the simple solution we’ve come up with, to wit: let Lani drive whenever they are in the car together. Since we’ve started doing this, everyone is happy. We’re both more relaxed and stress- free on the highway.
You could buy the new Chevy Cruze, I hear it has a detachable steering wheel right from the factory. 
I have the perfect solution. When my husband and I were first married he was also a bad driver and I had an irresistable urge to nag him. About 5 years ago I took up knitting and have became an obsessive knitter. I have a project with me everywhere I go and I am thrilled to have a car ride anywhere because it gives me knitting time. It takes enough concentration that her focus will be on the knitting instead of her husband’s driving. Tom and Ray suggested yoga and I have heard knitting called the new yoga. It is very calming and relaxing. Plus, it is much easier to knit a sock in the car than it is to do a downward dog! And since she lives in Alaska she needs all the handknits she can get. Bring on the wool!
I also had to ‘learn to be a passenger’, but as I get motion sickness when I focus anywhere inside the vehicle (yes, I drive whenever I can manage, I love to drive, even on long road trips) I had to do the same thing as Seattlesal, look out the side window, or at the front horizon, anything to keep my eyes from where they would be when I drive. The other solution is to get a set of headphones & listen to music, audiobooks, podcasts of Car Talk, etc. while they are on the road. Of course, that means the driver and passenger can’t talk for that period, but somethimes THAT’S not such a bad thing either 
Once in a while we drivers finally learn to actually listen to the “co-pilot”.
About the same time the co-pilot learns to be quiet about the little things and speak up when it matters.
Married for 35 years, my wife and I have refined the process to a fine art.
Both of us initially being the same ignoramus’ as most people until a series of incidents taught us both important lessons about listening AND choosing when to speak up or not.
I’ve learned her driving style and she’s learned mine.
Yet sometimes that second set of eyes is invaluable.
I could go into greater detail about the teaching/learning incidents but I don’t think youall want to nap just yet.
But, suffice it to say that , as a passenger we’ve each learned a few very important things to watch out for knowing where the driver’s focus and attention is and can truly co-pilot and not backseat drive. ( like the boy who cried wolf we’ve learned, the hard way, not to say too much so that what we do say will be heard. )
We’ve had the discussion to the point of naming those things that the driver NEEDS to know from a second set of eyes. This gives the ‘‘backseat driver’’ true specifics to focus on and can become a true co-pilot
It takes a while to re-learn the habbit of co-piloting and UN-learn tha bad habbit of backseat driving.
A couple of years ago my wife lost the vision of her right eye and it suddenly became ultra important to be an astute and observant co-pilot on my part.
Yet, as time passed and she adapted to her new driving parameters , my co-piloting points of mention needed to adapt as well. There wasn’t as much to point out the more she learned.