Optimal public transport accessibility for a zero-car city.
A city designed from the ground up should be designed so that cars are unneeded:
- 90% of your reasons to leave the home are serviced within a short walk and no public transport.
- Travel from any point in the city to any other should require no more than a short walk to a public transport point and a short walk from one.
While designing this city, a guiding principle could be "No-one should live more than 5 minutes walk from a subway station!" - this results in a city design where circles 5 minutes walk (ie 500 meters) around a transport centre are tightly packed as much as possible. Eg we get a map of a single suburb like this:
Following the doctrine of Circle Packing we get a hexagonal grid
In that city design 90.7% of the city is 5 minutes walk from one of those hubs. The other 9.3% could be things that don't feature in peoples commutes - solar farms, wind turbines, water storage, etc. Or perhaps it could be parks.
Were this design to be based on squares instead of hexagons, only 78% ($\pi\over4$) of the city would be within 5 minutes of a subway station, and getting it up to 90% would create more hubs, adding costs.
Or - Domes are needed - the atmosphere is poisonous
Eg this Mars colony uses a hex grid layout (for the same circle packing reason):
Comments re bad traffic:
2 different people commented: "intersections will be harder cause cars will have to check more directions" - no they won't. There are zero cars in this city. You walk a few hundred meters to everything you typically need, and everything else is a subway trip. Retailers are at the hubs so can use freight networks directly. Foot paths out into the suburb can take the odd forklift delivering white goods or construction materials the few hundred meters from hub to home if needed.
More about the suburb design:
Each suburb is about 780,000 square meters
Your hub in the middle has everything most people need. Shops, post office, doctors, childcare, playgrounds, dentist, physio, pharmacist, restaurants, cafes, vet, a church or 2, and a small school (only a few year levels - not all, some students will need to travel for school). Few medical specialists (but not a full set - may need to travel for specialist medical) A police station (10 cops), fire brigade (3 vehicles), and 5-bed emergency department also feature in the hub. The whole hub complex is taking up about 180,000 square meters of space in the middle.
The inner ring around this is offices and dense living. 3 or 4 story buildings, 1-3 bedroom apartments. Maybe 200,000 square meters or so.
The outer ring is low density living. 3 bedroom houses on 400 square meter lots sorta thing. There's about 800 low density homes around the edge of the suburb.
Your looking about 10,000 people in total, 3000 commuting in peak hour. Well within the capabilities of modern subway systems. Freight is unloaded in side stations to avoid stalling the main lines.
Foot paths out to suburbs are wide enough for a vehicle to travel on, and are used by emergency services, as well as the occasional forklift delivering construction materials or really big eBay order.
Basic services are duplicated (eg 1 grocery store in each hub), but specialists require a commute, eg only one suburb in 4 has a Catholic church, one suburb in 5 has a pet store, only 1 in 50 has a brain surgery operating suite.