Low-traffic neighbourhood tool v2

LTNs are a traffic management measure to stop excessive through-traffic cutting through residential streets. This is an interactive web app for rapidly exploring their design.

• Global version: https://a-b-street.github.io/ltn (import anywhere from OpenStreetMap)
• England: https://a-b-street.github.io/ltn/england.html
• Scotland: https://cnt.scot
• Read the user guide: https://a-b-street.github.io/ltn/user_guide.html

Developer docs

Installation

You'll need: npm, wasm-pack, and cargo.

cd web, and then:

• npm ci to install dependencies (ci to make sure the versions in package-lock.json are used)
• npm run wasm to rebuild the Rust backend
  • vite doesn't automatically rebuild when you edit things
• npm run dev to run locally
  • Changes to the Svelte/CSS usually auto-reload in your browser
• npm run fmt to auto-format code
• npm run check to see TypeScript errors

Faster local development

The tool operates on fixed study areas, generated from clips of OSM data. Custom areas imported by the user are always retrieved from the Overpass API, with the latest OSM data. For speed and for deterministic tests, there are also "built-in" study areas, consisting of pre-clipped osm.pbf files that do not automatically use the latest OSM data. These are manually managed and hosted by Dustin on assets.od2net.org and assets.cnt.scot.

If you're developing locally, you can cache assets locally by running bin/download-local-dev-data.sh.

Translations

If you're translating the app, you'll be editing .po files in web/src/locales/. You have to refresh the entire page to see changes. If you're using a tool to edit the .po file that's handy, please share it so we can list it here! And don't forget to add yourself to the credits in web/src/About.svelte.

When the app changes and new strings to translate appear, they should be detected and added to the .po automatically. Run npx wuchale init if something goes wrong.

Tests

(Don't spend too much time looking after these particular tests; they've been helpful to spot unexpected changes in calculating existing modal filters, but they're often just noisy when cell geometry slightly changes.)

The tests directory has some diff-based tests. cd backend; cargo test --release will run them. You'll need to follow the instructions above to ensure you have bristol and strasbourg areas set up. To accept the diffs, just commit the changed files.

There's a few ways to understand / verify the diffs.

First, you can manually try the tool before and after (running locally). You can load a .geojson file from tests/ as a project, then click the neighbourhood boundary (Bristol has two tests, Strasbourg just one). Then just visually compare things -- cells, shortcuts, and existing filters.

You can also try just diffing the output GeoJSON file as text. They're stored without pretty-printed newlines/indentation to save size in git, but you can do something like this to view, assuming you have jq and meld (or another diff tool):

function json_diff {
  cat $1 | jq > /tmp/after.json
  git show HEAD^:./$1 | jq > /tmp/before.json
  meld /tmp/before.json /tmp/after.json
}

cd tests/output
json_diff bristol_west.geojson

Finally, if you have the before and after output GeoJSON file (json_diff creates /tmp/before.geojson and /tmp/after.geojson), then you can try exploring in GeoDiffr. You'll want to "remove unchanged features", then try to understand the remaining changes. This tool is not very sophisticated; it's only helpful sometimes when filters change, but it's less useful if big cell polygons change.

Architecture

The Rust backend crate gets compiled to WASM, with generally type-unsafe APIs in lib.rs. On the frontend side, wasm.ts wraps these APIs in nicer TS APIs. The Svelte frontend calls APIs in the main thread (though moving to web workers later is a possibility). The "important" state is kept in the backend, while the frontend has more ephemeral UI state. The main state is serialized as GeoJSON and kept in the user's browser local storage. The frontend is effectively a single page app, with the Mode toggling between different pages.