That's a tough one. You use 'on the street' to talk about something that's travelling by foot, or is on the footpath. 'I saw him on the street' indicates that you saw someone walking somewhere. 'In the street' indicates something that is on the middle part of the street, which might be the only part of the street if the street isn't divided into a road and a footpath. 'He was lying in the street' indicates that he was lying in the road, probably in the gutter. Confusing the issue is that you can also use 'on the street' if the road is not divided into a road and a footpath, or with something that would not be expected to be travelling. 'I saw a box on the street' indicates that you saw a box on any part of the street.

We use both. 'In the street' implies in the middle of the street. For example: I found some money lying in the street. 'On the street' is used when referring to the flat surface of the road. For example: My house is on Park Street. In South Africa we drive on the left side. P.S. When we refer to lanes of a street we also use 'in'.

On the street