everyone will have different views on this, as most cats are let out...........if it were me in your situation though with a timid cat and a busy road, I would keep her in as I would worry myself to death while she were out, and couldnt live with myself if anything happened.
My fathers cat hasnt been out of the house on his own for 6 years, it started when he kept fighting with the cats over the road, and as an OAP he came off worse. He goes for a walk around the garden on a flexi lead and a cat harness which some would think is cruel, but at least he is safe, and even if the door is open for a moment, he shows no inclination to escape on his own anymore.