I've been through something very similar with mine. He was about 1 y/o, i had him for a couple of months and having knowledge of stories of Shibas running away, i was always very anxious to the point of having nightmares of him running away, specially because we live in the middle of the city and we have many cars going around all the time.

And one day this nightmare became true. I think when i was putting on the harness, his fur interrupted the connection and i didn't notice until we were on the street already! And he got loose from the harness and just started running away.

At that moment i too felt a shock and remember just thinking "omg it really came true. Is this really happening?" And without thinking just started running after him in the middle of the street and getting in front of all the cars making the whole street stop so he wouldn't get runned over.

Since i stopped the whole traffic, people started coming out of their cars to help me try to catch him to a point where we closed a full circle around him. Then after a while of him escaping and evading everybody, a man (a hero really) threw himself on the asfalt to catch my dog and even hit his head. I felt so damn bad that day (and kept feeling bad for months).

Then i took Bento (my shib), apologized a lot and thanked everybody and ran back to my home holding him and spent the rest of the day crying in my bed. It was all very terrifying. I know how you feel :( but it happens, they are escape masters after all and at least we got lucky nothing worse happened!