What's the difference?

What's the difference between

交通事故にあった This is grammatical and natural. It means you got involved in a traffic acctident. あう here means 'to encounter'.

and

交通事故にいました This isn't correct. ～にいます means I am somewhere. 交通事故 means just an event, not a place itself. You can say 交通事故の現場にいました instead, which means I was in the scene of the traffic accident.

私の文法は間違っている?  I don't think so. The issue of the second sentence is about vocabulary.

Thanks!

交通事故にあった
This means "i got into the traffic accident".
You are involved in a traffic accident

交通事故にいました
This sentence sounds a little bit unnatural.
It depends what you want to say.

交通事故に[あ]いました
It means same as 1st one but more polite.

交通事故の[現場に]いました
You saw a traffic accident or just there. but you were not involved it.

Hope i could help:)