TL;DR: Buy a blind spot mirror for your car. They are $2, and can keep you from getting in an accident. Not a lot of people have them, though they're awesome.
I spend an inordinate amount of time thinking about how to make driving safer.
Step 1 to making driving safer is "don't drive". (See also "increasing safety while wrestling with alligators" and "increasing safety with home-made parachutes").
Assuming you'll drive, the next advice is "don't be human". The most common cause of car accidents is human error.
Since Tesla has not taken over the world yet, we'll still be in charge of our own vehicles for at least a little while longer.
A significant sub-set of human-error accidents is "Unsafe Lane Changes". Of these accidents, many are related to not using a turn signal. (I couldn't find a free version of the study, but everyone and their mother has cited this SAE report.)
So, it's estimated that failure to use turn signals contributes to about 2 million roadway collisions every year.
The most obvious place where a turn signal can cause a problem is changing lanes on a highway, where a car might be in your blind spot. Even if you use your turn signal, the other driver might not see you there, or, worse, you and the other driver might try to change into the same lane, from opposite sides.
Blind spot mirrors are an embarrassingly cheap partial solution.
I've used these $2.11 blind spot mirrors for years.
These rimless mirrors look even nicer, but cost MORE THAN TWICE AS MUCH at $7.16.
Order one or the other. The price is insignificant, peace of mind and not getting in an expensive and/or dangerous and/or time-consuming car accident is worth it.
BTW, I don't do affiliate links. I just thought of this as I drove from Denver to Seattle, and started trying to find cars with blind spot mirrors. There are not as many out there as there should be.