Density and the 15-Minute City#
SLOP2418 — Urban Design Studio
Week 4 · Marisol Quaye · 15 March 2027
direct readout · derived measure · worked hypothetical
Where we are#
- Week 1: the site read before a road went down. Three metrics named — fiscal performance, mobility performance, livability.
- Week 2: the block pattern, and what a grid does to a pathfinder.
- Week 3: separation of uses, and the commute it buys you.
- Week 4: the radius. How far a resident will walk, and what therefore has to be inside the circle.
The argument#
A 15-minute city is a claim about a time budget and a balance sheet, not a claim about a distance.
Perry, 1929#
Perry’s radius is geometric. The studio must turn it into elapsed time.
What Perry fixed and what he built in#
Fixed:
- Through traffic on the boundary, not the interior.
- A daily destination at the centre, within a child’s walk.
- Retail at the corners, shared between cells, so it has two catchments. Built in:
- A cell defined by a school catchment is a cell defined by who is admitted to the school.
- Interior streets that discourage through movement also discourage through movement by bus.
- The unit is sized for a population it assumes will not change.
Moreno, 2021: the radius becomes a budget#
- Six functions must be reachable: living, working, supply, care, learning, enjoyment.
- Reachable means fifteen minutes on foot or by bicycle, not fifteen minutes by car.
- Four dimensions carry it: proximity, diversity, density, ubiquity.
- Ubiquity is the one students drop: the model has to hold for every part of the district, not for the part with the good park.
Jacobs on concentration#
There must be a sufficiently dense concentration of people, for whatever purposes they may be there.
— Jane Jacobs, The Death and Life of Great American Cities (1961), the fourth condition for city diversity
Density is a precondition, not a goal#
- A corner shop needs a catchment. Below roughly 4,000 residents inside a 600 m walk, the tenancy on Pelling Street cycles: occupied, struggling, abandoned, occupied.
- You will read this in the model as commercial demand that never clears and buildings that will not hold their level.
- The fix is not another commercial zone. The fix is more people inside the same radius.
- Density added beyond the threshold buys you variety of tenancy, not survival of tenancy. Those are different design problems.
Marchetti’s hour#
| Settlement | Typical daily travel | Time spent |
|---|---|---|
| Walking village, historic | 5 km on foot | ~1 hour |
| Streetcar district | 12 km, tram and foot | ~1 hour |
| Car-based suburb | 45 km, car | ~1 hour |
The hour does not move. Only the radius does.
The amenity radius, measured#
Walk speed 4.8 km/h, that is 80 metres per minute.
| Amenity | Perry’s placement | Distance from cell centre | Minutes on foot | Inside the 15-minute circle |
|---|---|---|---|---|
| Playground | interior, distributed | 300 m | 4 | yes |
| Elementary school | centre | 0–400 m | 0–5 | yes |
| Grocery | corner, shared | 500 m | 6 | yes |
| Clinic | shared between cells | 900 m | 11 | yes, barely |
| Secondary school | one per four to six cells | 1,600 m | 20 | no |
| Hospital | district-wide | 3,200 m | 40 | no — cycle or transit |
The circle over Halstead Reach#
A walking catchment and a service-coverage overlay answer different questions.
What the circle costs#
Table of the Halstead Reach cell at Perry’s dimensions — 0.65 km², approximately 5,000 residents.
| Facility inside the cell | Monthly upkeep | Cell’s share |
|---|---|---|
| Elementary school | 3,200 | 3,200 |
| Clinic | 2,400 | 2,400 |
| Three small parks | 900 | 900 |
| Fire station (shared, three cells) | 1,900 | 633 |
| Total | 7,133 |
Residential rate take at the default rate, 5,000 residents: ≈ 5,800 / month. Monthly shortfall: ≈ 1,300.
Two ways to close 1,300#
| Move | Fiscal effect | Livability effect |
|---|---|---|
| Upzone Pelling Street frontage to mixed housing: +1,400 residents | +1,600 / month; closes the gap | School reaches capacity; the playground catchment thins; the 400 m circle now serves 6,400 people |
| Move the clinic to a catchment shared by three cells | Cell’s share falls 2,400 → 800; closes the gap | The eleven-minute walk becomes a twenty-four-minute walk, or a trip by car |
The tradeoff of the week#
Livability against fiscal performance.
The clinic inside the circle is a service the cell cannot yet fund. The clinic outside the circle is a balance sheet that works and a walk that does not.
Assessment 1: the brief#
- Neighbourhood Unit Plan. One residential cell in Halstead Reach at Perry’s dimensions, designed to Moreno’s proximity standard, sited on the studio’s shared save.
- Weight: 25% of the studio.
- Submission: a set of in-simulation screenshots, plus a defence memo of no more than 800 words. Lodged today at 12:00 and defended at the jury at 14:00.
- Required in the screenshot set: the Land Value info view over the cell; the coverage radius of every service building you sited; the traffic volume layer showing the Kerrow corridor on the boundary.
Marking table:
| Criterion | Weight |
|---|---|
| Fiscal performance | 30 |
| Mobility performance | 30 |
| Livability | 40 |
What the jury will ask#
- The jury is chaired by Marisol Quaye. Sunniva Marek, principal at Kesselring & Marek, sits on it as guest juror.
- The first question is always the same: which two of the three metrics did you trade against each other, and where in the plan is that decision visible.
- Expect to be asked what the cell costs per month and what it returns. Have the number.
- Expect to be asked who the school catchment excludes. Perry’s diagram does not answer this and neither will yours unless you make it.
- Sunniva Marek marks from practice and will ask what happens to the plan in year ten, when the cell’s population has changed and the school has not.