Sentence b is incorrect for the following reasons:
1. In BrE, we do not use "If you wouldn't have gone ..." but it is used in AmE. In BrE, it's "If you hadn't gone yesterday ..."
2. The second "you" should not be capitalised because it comes after a comma, not after a full stop, question mark or exclamation mark.
3. The second clause is ungrammatical. "To be" does not collocate with "escape". You need to say either "You would have escaped from the accident" or, more naturally, "You would not have been in the accident".
To clarify, this is how it should look:
If you hadn't gone yesterday, you wouldn't have been in an/the accident.
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