NTA - It seems crazy that people here are saying the opposite. Whilst I agree it could come off as rather controlling and the method is not the best option to take, it's 100 times better than letting her be a huge danger to herself and others.

Seeing as you've said in comments (which you may want to add in the post) that you have talked about this with her before, and she's ignored you, this is a last resort thing.

Lots of people are saying that, since the car is shared equally, you should just not be in the car with her, but if she really is as bad a driver as you say, then it doesn't really matter if you each own half of it if she crashes; owning half a crashed car is pretty worthless.

It's a pretty tough line to not come off as controlling or (according to other commenters here) abusive in order to protect yourself, her and innocent bystanders. I'd reccomend maybe trying to make the car solely hers and you get yourself one (if possible) so that you won't be negatively affected financially if the car is ruined, or failing that to come to some agreement about what will happen if she does crash.

Regardless, I can definetly see that your intentions are completely in the right here, but it's a situation where it's difficult to come up with a happily accepted course of action. Other than breaking up, but there's not enough info to properly suggest that.