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.