Yes, that's a very difficult situation.  I had the same thing with my own father who insisted on driving well past when it was safe for him to do so.

I think that if it's a serious safety concern, she would be justified and well within her rights to "fight" a little, i.e. forcefully voice her disagreement and disapproval.  Arguably she might even have an obligation to do so.  Submission to your husband doesn't require that you submit to situations that compromise your safety or endanger others.  If the husband were waving a loaded gun around, even in the presence of children, the wife would be absolutely within her rights to forcefully object and even to remove herself and the children from that situation.  In fact, she would be OBLIGATED to break submission.  Vehicles are deadly weapons.  What if some innocent pedestrian gets killed?

So sometimes there's not only a right but even a DUTY to disobey.  Let's not get too absolutist about obedience.

So the difficulty isn't one of principle (the right to disobey or disagree or refuse submission) ... but it's a judgment call about when it gets to a point where his driving poses a positive threat to the lives of others.  In one person's viewpoint he might be just a little sloppy, but in another's it may appear as if he poses a real threat.  That could be a tad subjective.  If a number of objective third parties AGREE that he's gotten to the point of posing a threat, then he really needs to be confronted about it.  Yes, some older men can get EXTREMELY stubborn about this kind of thing as it hurts their pride and won't go down without a fight.

Being a good wife doesn't require taking needless risks with life and limb but, rather, doing so involves at least one sin.

Being a good (x), is contingent upon the good, not the (x).

Just politely, sans attitude, don't go. If he asks, just say "I'm afraid". If he asks why, answer simply, directly, meekly, and humbly. 

Endure him "getting it", and let him sort it out, including determining a course of remedial actions; if he asks you, answer. 

If he doesn't, zip it and be sweet. 

The overwhelming weakness of women and children, provided they're real and not "wielded" like witchery, is not to be underestimated, forbids all "doing", and consists in simply "being".

Catholic chicks are even "worse".

Suggest she  consider whether medication may be the cause of his (seemingly) poor judgement.