Twenty metres.
The pavement in front of your door, and a bit either side. That is where it starts — small enough that you can actually promise it.
What it is
Stand at your door and look both ways.
The pavement
In front of your door, and past the next door on either side.
The wall you park against
The strip where things collect and nobody has ever claimed it.
The verge
The end of your street, where the pavement gives up.
Twenty metres is roughly your own frontage plus the stretch either side of it. The ground you already walk past every single day without looking at it.
It is not the neighbourhood. It is not the island. It is the part you can see from where you are standing, and the part you would notice if someone else kept it. That is not a limit on what you care about — it is the piece you can be sure of.
Where it meets next door
You and next door already keep the same few metres.
Doors sit closer together than twenty metres, so your stretch runs into your neighbour’s. The pavement by the wall between you gets looked after twice, by two people who never arranged it.
The overlap is the whole trick. Nobody ever has to agree where one person stops and the next begins, and nothing ends up in the gap between two people who each assumed it belonged to the other.
Why not the whole street
A street that belongs to everyone belongs to no one.
Ask people to look after the street and most of them will agree, and then nothing will happen — because nobody can tell which part was theirs.
Twenty metres is small enough to be certain about. It needs no rota, no meeting and nobody checking. You either kept it this week or you did not, and you are the only one who needs to know.
It costs a few minutes, now and then. And twenty metres is the least you can be certain of, not the most you are allowed — anything you pick up beyond it is simply what you happened to walk past.
If everyone kept twenty metres, there would be nothing left to clean.
That is the whole arithmetic, and it starts at your own front door.