So in the first post I said "I remember that back in Albany, my peak traffic counts were on Marin or San Pablo, about 10.3 cars per lane-minute."

But I remembered something key last night: left-turn lanes. Both streets had them. Traffic on Marin could keep flowing smoothly through an intersection because cars going left could get out of the way. But that takes space. Assuming that each direction can carry 10.3 cars a minute, that's nearly 21 cars a minute, but spread over three lanes -- two travel, one shared turn. And we're back down to 7 cars per lane minute.( Read more... )

Layman's conclusion: wide roads with little 'turbulence' can get up to 10 cars per lane-minute. More complicated streets are unlikely to get above 8, after accounting for turn lanes. This will have consequences for stuff like "is it physically possible for everyone to drive to work from here?"

I was re-reading old transit links (reminder: if someone says replacing buses with microtransit will improve ridership, you're being scammed), and specifically this post on ridership basics (or archive.org for a missing scatterplot.) It's worth reading, but a passage I don't remember from before notes that ridership can have super-linear response to density. Higher density means more potential riders, and makes driving less attractive, and may attract people who don't want to drive.

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There's this belief I've seen, exemplified by a recent Youtube comment:

everyone knows that cities can have grids.

but show a grid city and everyone will guess it's a North American city.

most cities in the world grew organically so grids aren't a big thing everywhere else. you might find a few grids here and there, but that's it. going all out on grids is a North America special.

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A video yesterday reminded me of the Saudi Line proposal to build a brand new very linear city (or linear arcology, one long building) in western Arabia. I looked again at the numbers, and wow it is nuts.

170 km long.

500 m high.

200 m wide.

(Area 34 km2.)

It's supposed to be higher than it's long! Crazy. You could probably bring the cost down just by flipping those two numbers (though maybe ventilating a 500 m wide building would be a bit more challenging, I dunno.)

For minimizing trip lengths you would want a circular city, or something close like a diamond or grid. But I can see some appeal of a linear city: simplifying your high speed transport by needing just one backbone route, and keeping it easy to go outside the city into a greenbelt/preserve. (Not sure how much point to that there is in western Arabia, but anyway.) So I wondered what a saner proposal might look like.

1) drop the arcology and just go with a conventional city with streets and buildings.

2) Have the width be at least a 5 minute walk from edge to spine, so 400 meters, making it 800 meters (10 minutes) edge to edge, which avoids the need to have any cross transit. This is 4x the width, so could reduce the length from 170 km to 42 km. (Though the original proposal used the height to be very high density, which I'm kind of waving away.)

You could double the width, for a 20 minute edge-edge walk; 1.6 km x 21 km.

But since you're trying to avoid cars, you should go in for bicycles and other micromobility, at let's say 3x expected walking speed. 2.4 km edge to spine, and 4.8 x 7 km in shape... which is actually almost a square, whoops. And you'd probably need cross-transit again for the minority who can't use any form of wheels, or the larger group who don't always want to. Still, it's a city where every point is a 10 minute bike/fast powerchair ride to the central spine, at 15 km/hr.

San Francisco is actually bigger than this, so I've just discovered that SF could be way nicer than it is (granted SF has hills.)

To keep a line shape better, go back to the 10 minute width of 800 meters, triple it for bikes, now you have a 2.4 x 14 km city, and can get some real rail use out of your backbone, while it's still a 15 minute walk from the center to the edge.

(I'd swear I've written this somewhere before, but I see no evidence of it anywhere.)

I've talked before about superblocks. There's the Barcelona model, of 3x3 blocks with one way loops so cars legally can't go through. (Disadvantage: may be complicated to scale.) I've privately imagined various other configurations, like putting diagonal diverters in so that cars have two-way access but physically can't go through the area, dividing the superblock into four half-diamond quadrants. (Possible disadvantage, access requires entering from the correct side.) ( Read more... )

In many dimensions of zoning code I want to go in a more laissez-faire direction than the US norm. More allowable height and units and rent-intention, less parking, more businesses, more choice of how much or how little land to use. But there are a couple of ways in which I'm tempted to tighten the screws.

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Over the years I've done various density and walkability calculations, estimating that the required density could go as low as 4000 people/km2 (8000 people in a 12 minute walk) or even 3000 people/km2 (9000 people in a 15 minute walk.) But in the literature, messy as it is, it seems more like 10 du (dwelling units)/acre is the expected minimum, which at 2.6 people/du is more like 6500 people/km2. So I want to poke at my assumptions.

Summary: yeah, my old assumptions were flawed, and I'm now looking at closer to 10,000 people/km2 for good walkable density. Data indicates you start getting more walking before that, like 5000/km2, but it levels off above 10,000, possibly because all the trips it is easy to make walkable, have been made walkble. And per older posts, you can reach these densities with single-family housing if you insist, though you'll need to accept small lots and yards.

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There are other benefits to higher density, of course: more taxpayers to pay for infrastructure, more riders to justify high transit frequency, letting more people live close to attractive points like subway stations, letting people have more and more interesting lives in walking or biking distance. But in terms of reducing car trips in favor of walking, the low-hanging fruit gets plucked by 10,000.

Revisiting yesterday's post in American units, since I mostly want to persuade Americans.

Thesis: low-density living, with single family houses and sizable yards, is compatible with low-car if not car-free living, if you go in heavily on bicycles.

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Various sources point to a minimum level of population density needed for walkability. A source I have lost said 10-20 dwelling units (du) per acre. This Australian model derived 25 du/hectare (2500 du/km2), which is the same as 10 du/acre, as a minimum, though 35 was notably better. At an assumption (as the paper used) of 2.6 people per du, 25 du/hectare is 6500 people/km2, 35 is 9100. My own personal experience, of places I have lived and looked up the densities of, is that nice walkability starts around 9000 people/m2, while 6000 tends to be doable but a bit anemic.

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And in reality, there is no reason for housing to be so uniform. Left alone, people would naturally build taller and live denser near high value locations like train stations, so walkability can be supported by a mix of SFH and multifamily/rental housing. But it's good to know that you can support it with pure SFH too... as long as you allow small lots.

Though it also means that bigger lots that don't support bigger households (via large household or various rental units) are kind of free-riding on higher density elsewhere, if the inhabitants enjoy walkability.

I thought of a new angle on just how badly most modern cities have let people down by not enabling safe biking.

Most cities have at least some bus service, whether as a semi-credible public transit system or as a sop to the poor and elderly. How fast are these buses? Pretty slow. NYC buses rarely break 9 MPH / 14 KPH though they're especially slow. City Observatory has easily-graphable data for multiple US cities, 2000-2013; the mean and median are 13 MPH / 21 KPH, big dense cities I'm familiar with are more like 11 MPH / 18 KPH, and very few cities break 15 MPH / 24 KPH. Absolute peak in the US was Salt Lake, almost hitting 19 MPH / 30 KPH for a few years. Even BRT systems around the world rarely break 30 KPH.

Bike speeds vary a lot, but en masse, one source says Dutch riders average 17 KPH. From my experience, it feels hard to go under 15 KPH and stay upright, even on a thick and heavy bike. [Edit: this says 12.4 KPH for the Dutch, and now that I have a bike I see I overestimated my default speeds. I don't know if either Dutch figure is "speed of motion" or door to door "speed of travel".] So being able to bike is like having personal bus-speed service, without the car necessities of a driver's license, insurance, and thousands of dollars per year spent on a car. (Or social cost of the 40,000 lives a year lost to cars, pollution, noise, etc.) Even if buses go faster than you do on a bike, not having to walk and wait means bikes win up to some distance.

Let's do an extremely bus-friendly case. Bike 15 KPH, bus 30 KPH, average of 10 minutes walk and wait to get on a bus. They cover equal distances at

1) 15*t = 30*(t-1/6), t = 1/3 hour, distance = 5 km. So for trips under 5 km or 3 miles, biking is faster.

If bus speed is 20 KPH and the time is 15 minutes to get on, we have

2) 15*t = 20*(t-1/4), t = 1 hour, distance = 15 km.

And this has been assuming that your destination is right at your bus stop; in reality there's potentially more delay there. (Also assuming bike parking right by your destination.)

Note what this means for a city planner wanting to reduce car use. You could invest a lot in public transit, including the high capital costs of metro or the high labor costs of frequent bus service... or you could shape your infrastructure so that lots of people view biking as safe and convenient, providing their own bikes (at a few hundred dollars/year) and labor, with your main job doing sweeping and snow removal.

But of course that low financial cost comes at the high political cost of taking street space away from cars. Easier to drop some buses in and call it a day... easier, but not very effective.

This post owes a lot to this kchoze post, on why buses have low mode share in Japan, and arguing buses have little role in a well-designed city (one with good walkability and metro, not to mention attractive biking.) I'd urge you to read the kchoze as well.