inattentive, changes lanes without looking at cars in that lane, sits on green lights for 5-10 seconds or until prompted “It’s green,” confuses left with right, drives 15 MPH in a 35 or 45 MPH zone, gets pissed off when I advise paying attention to traffic. Not sure what to do. When I offer advice, we just get into arguments, sometimes apologies, “Everybody does that from time to time.” I’d take over the wheel, though I’m not a big fan of driving myself. Still, I’m A.J. Foyt and Richard Petty in comparison.
How old is this person?
This can be an aspect of handedness. Is SO left-handed or cross-dominant?
Saying “your side” and “my side” can help, if you’re giving directions.
70…
There is a tremendous difference between cautious to the point of timid, and oblivious to the point of dangerous.
My soon-to-be-ex was the timid flavor and has been since her youth. Perfectly safe, aware of surrounding traffic, and courteous to other drivers. If rather challenging for more-aggressive me to ride with. So while she never set a speed record getting anyplace, there was never any concern for our safety while I rode with her.
The OP’s SO sounds more like creeping senility to me, regardless of their age. 15 in a 35 or 45 is wanton disregard for conditions. As is sitting indefinitely at green lights until you, or the driver behind them, reminds them to think about going. Either wanton disregard, or simple obliviousness.
This person may have always been a small town timid driver now suddenly in a more complex environment. Or they’ve simply lost the ability to pay organized attention to anything. Although if that I’d expect the symptoms to extend past mere driving.
In the OP’s opinion, how does the SO’s driving behavior compare to their other behaviors? If you go to the grocery store are they often lost in thought (or zombiehood) in the canned good aisle? Can they have a TV on, be looking at the show and also converse with you about something else? Etc.
I’m 65 and what I’ve noticed about my own decline is an increasing tendency to find my mind has “slipped into neutral” unnoticed by me. Usually in times of low stimulation. The problem of course is that driving really isn’t low stimulation and if the SO is slipping into mental neutral with the car in motion, that’s very dangerous.
ETA: I Now see the SO is age 70. Which can be old enough to be still be a fully capable driver, or old enough to be a menace to all and sundry.
Without editing what I already wrote I’ll add this addendum …
Do you observe other behaviors unrelated to driving where you point out something odd with their behavior and the response is along the lines of "Everybody does that from time to time?”.
e.g. Everyone forgets why they went into another room to do/get [I forget what]. But a teen does it once a month, a healthy 70yo does it once a day, and a becoming-senile person does it once an hour. It’s the frequency that makes the disease. And it’s the vehemence of denial that powers dangerous drivers to keep driving dangerously until they hurt somebody.
Here is a recent thread of Dopers’ self-reported experiences of memory and cognitive decline. The OP here might usefully read that and see how what they know of their SO’s current condition lines up with any of this stuff.
The reason I asked (and I’m 72 myself) is possible diminished physical and mental skills. Maybe time for a doctor’s evaluation and advice.
ETA: I would NOT ride with this person.
Here is another thread on the topic of Doper’s experiences of age-related mental decline. I’d forgotten about this one when I posted earlier :ouch: 
Definitely memory issues, such as we all (older folks) have from time to time. Perhaps more so. My SO doesn’t remember having conversations with me from a day ago sometimes, which until now I was willing to label “forgetfulness” or “everything isn’t equally memorable to everyone.”
I’m going in for a colonoscopy next week, and I had arranged NOT to do the driving, but now I’m having second thoughts. Maybe find someone else to drive me home?
IMO … The incremental risk from two rides with this person is not material to your lifetime hazard. OTOH a policy that when you’re going out together routinely, you’ll always be the one driving would be a very smart move. If you did have an alternate source for rides, do that. Even ubering to the appointment and SO driving you home would halve your risk.
The rest of your post highlights the awful nature of this problem. This isn’t a “disease” with notions of “sickness” or “health”. It happens I have a cold right now as I’m typing. It feels markedly obviously different from 2 days ago when my head & throat didn’t hurt, my nose wasn’t running, and I wasn’t using up kleenex like a bonfire. I also know I’ll easily be able to recognize normalcy when it returns in a couple days. I can hardly wait.
Instead senility is a slow inexorable loss of capability accompanied by a loss of capability to recognize the loss of capability. Eventually they get to oblivious, while being oblivious to their own obliviousness.
Along with an ever-increasing ability to rationalize away the obvious residual evidence, like unwashed dishes piling up, or unexplained minor crunches in their car fenders. If pointed out to them, they blink, offer no answer, and move on, with no “dent” having been made in their internal model of their own competence.
Your SO is evidently sliding down that scale to the point it’s interfering with their life at least some. You need to decide how much you want to be involved with this problem. Including notifying family or the authorities about their actively dangerous incapacity at driving.
Once you really start paying attention to all the gaps and gaffes both you and they have been hand-waving away I think you’ll have a real eye-opening surprise.
Maybe you should take your SO to see a neurologist as well and have him evaluated for cognitive decline.
My father was 72 when he was evaluated and diagnosed with Alzheimers. About a year or so later, we (mom, myself and two siblings) took away my Dad’s car and told him he wasn’t permitted to drive anymore.
A superb item of advice. If he’ll go along with it.
The denial seems strong in that one. And the persuasive “pull” of one BF/GF is a lot less than that of a longtime spouse and assembled adult kids.
Here’s hoping.