diff --git a/src/layers/traffic/flow.js b/src/layers/traffic/flow.js
index caf8aede2..ff5c7c189 100644
--- a/src/layers/traffic/flow.js
+++ b/src/layers/traffic/flow.js
@@ -1,6 +1,7 @@
 import { roadIdentity } from './retention.js';
 import * as Cesium from 'cesium';
 import {
+  floorCellReader,
   prepareRoadSurfaces,
   roadHeightStations,
   trafficSurfaceReady,
@@ -301,6 +302,7 @@ export function createFlow({ state: layerState, services, parts, source }) {
       }
     }
     if (depthHeight !== undefined) layerState._viewHeightEstimate = depthHeight;
+    const floorAt = floorCellReader(services.ground);
     for (const road of roads) {
       road.key ||= roadIdentity(road);
       const prepared = !refining && preparedRoads.get(road.key);
@@ -319,7 +321,7 @@ export function createFlow({ state: layerState, services, parts, source }) {
         );
         for (let i = 0; i < road.coords.length; i++) {
           const [lon, lat] = road.coords[i];
-          const floor = services.ground?.cachedGroundFloor?.(lat, lon);
+          const floor = floorAt(lat, lon);
           const height =
             Number.isFinite(floor) && Math.abs(floor) <= 9000
               ? Math.max(floor, layerState._viewHeightEstimate)
diff --git a/src/layers/traffic/index.js b/src/layers/traffic/index.js
index 4a5839f7d..eb0f8f544 100644
--- a/src/layers/traffic/index.js
+++ b/src/layers/traffic/index.js
@@ -39,10 +39,10 @@ export function createTrafficLayer({ services, source }) {
   parts.controls = createControls(context);
   parts.lifecycle = createLifecycle(context);
   state._parseRoads = TRAFFIC_TIMING_ENABLED
-    ? (data, trace) =>
+    ? (data, trace, parsedBySource) =>
         trace
-          ? parts.timing.parseRoadsTimed(data, trace)
-          : parts.model.parseRoads(data)
+          ? parts.timing.parseRoadsTimed(data, trace, parsedBySource)
+          : parts.model.parseRoads(data, trace, parsedBySource)
     : parts.model.parseRoads;
 
   state._loadRoadsForBounds = TRAFFIC_TIMING_ENABLED
diff --git a/src/layers/traffic/ingestion.js b/src/layers/traffic/ingestion.js
index a5e09f543..0065f5638 100644
--- a/src/layers/traffic/ingestion.js
+++ b/src/layers/traffic/ingestion.js
@@ -348,11 +348,16 @@ export function createIngestion({
     layerState._retainedPreview = false;
     layerState._detailRoadsReady = false;
     let paintRevision = 0;
+    // The snapshot that ends a pass replaces the streamed roads with the
+    // composed set, which is mostly the tile records already parsed and
+    // drawn. Remembering those per pass means only the records new to that
+    // snapshot (the clipped coarse roads) are parsed, not the whole set again.
+    let parsedBySource = new WeakMap();
     const onTile = (data, passSignal, preview) => {
       if (generation !== layerState._loadGeneration || passSignal.aborted)
         return;
       const start = performance.now();
-      const parsed = layerState._parseRoads(data, trace);
+      const parsed = layerState._parseRoads(data, trace, parsedBySource);
       phaseTiming('parse', start, { roads: parsed.length, incremental: true });
       if (data.replace) streamed = parsed;
       else streamed.push(...parsed);
@@ -531,6 +536,7 @@ export function createIngestion({
       if (altitude > FAST_FETCH_ALTITUDE) return;
 
       streamed = [];
+      parsedBySource = new WeakMap();
       majorPass = false;
       // Detailed pass: fetch the full road graph (tertiary, residential, etc.)
       console.log(`[Data:Traffic] Full fetch local roads [${cacheKey}]`);
diff --git a/src/layers/traffic/model.js b/src/layers/traffic/model.js
index ff1c89ddb..10b2a4bbb 100644
--- a/src/layers/traffic/model.js
+++ b/src/layers/traffic/model.js
@@ -1,5 +1,5 @@
 import * as Cesium from 'cesium';
-import { roadSurfaceChunks } from './surface.js';
+import { floorCellReader, roadSurfaceChunks } from './surface.js';
 import { t, translateKnown } from '../../i18n/index.js';
 import {
   flowDensityMult,
@@ -13,22 +13,35 @@ import {
 } from './policy.js';
 
 export function createModel({ state: layerState, services, parts, source }) {
-  /** Build scene waypoints from source records; thinning and terrain remain rendering policy. */
-  function parseRoads(roadData) {
+  /**
+   * Build scene waypoints from source records; thinning and terrain remain rendering policy.
+   *
+   * `parsedBySource` (source record → its parsed roads) is filled as records
+   * are parsed, and a record already in it is answered from it.
+   */
+  function parseRoads(roadData, _trace, parsedBySource) {
     if (!roadData || !roadData.roads) {
       return [];
     }
 
     const roads = [];
+    const floorAt = floorCellReader(services.ground);
     for (const road of roadData.roads) {
       if (!road.coordinates || road.coordinates.length < 2) continue;
+      // A replacing snapshot repeats records this load already parsed.
+      const known = parsedBySource?.get(road);
+      if (known) {
+        for (const parsed of known) roads.push(parsed);
+        continue;
+      }
+      const first = roads.length;
 
       for (const coords of roadSurfaceChunks(road.coordinates)) {
         const type = road.type;
         const oneway = road.oneway;
         // Pre-compute Cartesian3 waypoints (lon, lat, height) for fast lerp animation
         const waypoints = coords.map(([lng, lat]) => {
-          const floor = services.ground?.cachedGroundFloor?.(lat, lng);
+          const floor = floorAt(lat, lng);
           const h =
             (Number.isFinite(floor) && Math.abs(floor) <= 9000
               ? floor
@@ -60,6 +73,7 @@ export function createModel({ state: layerState, services, parts, source }) {
             flow: road.flow || null,
           });
       }
+      parsedBySource?.set(road, roads.slice(first));
     }
 
     return roads;
diff --git a/src/layers/traffic/roadLoadCost.test.mjs b/src/layers/traffic/roadLoadCost.test.mjs
new file mode 100644
index 000000000..c248624c4
--- /dev/null
+++ b/src/layers/traffic/roadLoadCost.test.mjs
@@ -0,0 +1,444 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createTrafficSource, outsideDetailTiles } from './source.js';
+import { createIngestion } from './ingestion.js';
+import { createModel } from './model.js';
+import { floorCellReader, roadSurfaceChunks } from './surface.js';
+import { DOT_HEIGHT_OFFSET } from './policy.js';
+import { clipTileLine } from '../../sources/openFreeMap.js';
+import { tileToBBox } from '../../data/tomtomTiles.js';
+
+// The clip as it was before the extent shortcut: every road against the four
+// regions around every tile. The shortcut must return exactly this.
+function clipEveryRoadAgainstEveryTile(roads, tiles) {
+  return roads.flatMap((road) => {
+    let lines = [road.coordinates];
+    for (const tile of tiles) {
+      const box = tileToBBox(tile.z, tile.x, tile.y);
+      const regions = [
+        { west: -180, east: box.west, south: -85, north: 85 },
+        { west: box.east, east: 180, south: -85, north: 85 },
+        { west: box.west, east: box.east, south: -85, north: box.south },
+        { west: box.west, east: box.east, south: box.north, north: 85 },
+      ];
+      lines = lines.flatMap((line) =>
+        regions.flatMap((region) => clipTileLine(line, region)),
+      );
+    }
+    return lines.map((coordinates) => ({ ...road, coordinates }));
+  });
+}
+
+function seeded(seed) {
+  let state = seed >>> 0;
+  return () => {
+    state = (Math.imul(state, 1664525) + 1013904223) >>> 0;
+    return state / 2 ** 32;
+  };
+}
+
+function tileBlock(x, y, size = 4) {
+  const tiles = [];
+  for (let dx = 0; dx < size; dx++)
+    for (let dy = 0; dy < size; dy++)
+      tiles.push({ z: 14, x: x + dx, y: y + dy });
+  return tiles;
+}
+
+/** Roads in and around a tile block: wandering, straight, tiny, and on tile edges. */
+function roadsAround(tiles, random, count = 400) {
+  const boxes = tiles.map((tile) => tileToBBox(tile.z, tile.x, tile.y));
+  const west = Math.min(...boxes.map((box) => box.west)),
+    east = Math.max(...boxes.map((box) => box.east)),
+    south = Math.min(...boxes.map((box) => box.south)),
+    north = Math.max(...boxes.map((box) => box.north));
+  const width = east - west,
+    height = north - south;
+  const roads = [];
+  for (let i = 0; i < count; i++) {
+    const step = i % 7 === 0 ? 0.00002 : i % 3 === 0 ? 0.01 : 0.002;
+    let lon = west - width / 2 + random() * width * 2,
+      lat = south - height / 2 + random() * height * 2;
+    const coordinates = [[lon, lat]];
+    const points = 1 + Math.floor(random() * 12);
+    for (let j = 0; j < points; j++) {
+      lon += (random() - 0.5) * step;
+      lat += (random() - 0.5) * step;
+      coordinates.push([lon, lat]);
+    }
+    roads.push({ type: 'primary', oneway: i % 2, id: i, coordinates });
+  }
+  const box = boxes[5];
+  const midLat = (box.south + box.north) / 2,
+    midLon = (box.west + box.east) / 2;
+  roads.push(
+    // Along a tile edge, ending on a corner, and touching an edge from outside.
+    {
+      id: 'edge',
+      coordinates: [
+        [box.west, box.south - 0.01],
+        [box.west, box.north + 0.01],
+      ],
+    },
+    {
+      id: 'corner',
+      coordinates: [
+        [box.west - 0.01, box.south - 0.01],
+        [box.west, box.south],
+        [box.east, box.north],
+      ],
+    },
+    {
+      id: 'touch',
+      coordinates: [
+        [box.west - 0.01, midLat],
+        [box.west, midLat],
+      ],
+    },
+    // Across the whole block, and a repeated vertex.
+    {
+      id: 'across',
+      coordinates: [
+        [west - 0.01, midLat],
+        [east + 0.01, midLat + 0.0003],
+      ],
+    },
+    {
+      id: 'repeat',
+      coordinates: [
+        [midLon, midLat],
+        [midLon, midLat],
+        [midLon + 0.001, midLat],
+      ],
+    },
+    // Records the clip drops or leaves alone.
+    { id: 'single', coordinates: [[west - 0.02, midLat]] },
+    { id: 'empty', coordinates: [] },
+    {
+      id: 'negative-zero',
+      coordinates: [
+        [-0, south - 0.02],
+        [0.001, south - 0.02],
+      ],
+    },
+    {
+      id: 'polar',
+      coordinates: [
+        [west - 0.02, 86],
+        [west - 0.02, 86.01],
+      ],
+    },
+  );
+  return roads;
+}
+
+for (const [place, x, y] of [
+  ['Manhattan', 4823, 6158],
+  ['London, across longitude 0', 8190, 5446],
+  ['longitude 0 and latitude 0', 8190, 8190],
+  ['the southern and eastern hemispheres', 15070, 9826],
+])
+  test(`coarse roads clipped around detail tiles match the every-tile clip: ${place}`, () => {
+    const tiles = tileBlock(x, y);
+    const roads = roadsAround(tiles, seeded(x * 31 + y));
+    for (const loaded of [tiles, tiles.slice(0, 5), tiles.slice(3, 4), []]) {
+      const expected = clipEveryRoadAgainstEveryTile(roads, loaded);
+      assert.ok(loaded.length === 0 || expected.length !== roads.length);
+      // deepStrictEqual separates -0 from 0 and compares every vertex exactly.
+      assert.deepStrictEqual(outsideDetailTiles(roads, loaded), expected);
+    }
+  });
+
+test('only a road that reaches a detail tile is clipped against it', () => {
+  const tiles = tileBlock(4823, 6158);
+  const block = tiles.map((tile) => tileToBBox(tile.z, tile.x, tile.y));
+  const west = Math.min(...block.map((box) => box.west)),
+    north = Math.max(...block.map((box) => box.north));
+  // 300 roads beside the block and between its tile edges, none touching one.
+  const clear = [];
+  for (let i = 0; i < 300; i++) {
+    const box = block[i % block.length];
+    const lon = i % 2 ? west - 0.02 - i * 1e-5 : box.west + 0.004;
+    const lat = i % 2 ? box.south + 0.004 : north + 0.01 + i * 1e-5;
+    clear.push({
+      coordinates: [
+        [lon, lat],
+        [lon + 0.002, lat + 0.0005],
+        [lon + 0.003, lat + 0.001],
+      ],
+    });
+  }
+  const first = block[0];
+  const crossing = {
+    coordinates: [
+      [first.west + 0.004, north + 0.01],
+      [first.west + 0.004, (first.south + first.north) / 2],
+    ],
+  };
+  let clips = 0;
+  const counted = (line, region) => {
+    clips++;
+    return clipTileLine(line, region);
+  };
+  const roads = outsideDetailTiles([...clear, crossing], tiles, counted);
+  assert.deepStrictEqual(
+    roads,
+    clipEveryRoadAgainstEveryTile([...clear, crossing], tiles),
+  );
+  assert.equal(roads.length, 301);
+  // Four regions for each tile the crossing road reaches, never the 19,264
+  // clips of every road against every tile.
+  assert.ok(clips >= 4, `expected the crossing road to be clipped: ${clips}`);
+  assert.ok(clips <= 4 * tiles.length, `clipped ${clips} times`);
+});
+
+/** A DEM-regime ground service: one floor per toFixed(3) cell, counting reads. */
+function cellGround({ mesh = false, warm = () => true } = {}) {
+  const ground = {
+    reads: 0,
+    meshFloorPreferred: () => mesh,
+    cachedGroundFloor(lat, lon) {
+      ground.reads++;
+      if (!Number.isFinite(lat) || !Number.isFinite(lon)) return null;
+      // Number() as the shared cache does it: "-0.000" and "0.000" are one cell.
+      const cell = `${Number(lat.toFixed(3))},${Number(lon.toFixed(3))}`;
+      if (!warm(cell)) return null;
+      let hash = 0;
+      for (const character of cell)
+        hash = (Math.imul(hash, 31) + character.charCodeAt(0)) | 0;
+      // The mesh regime answers per point, not per cell.
+      return (hash % 4000) / 10 + (mesh ? lat - Math.floor(lat * 1000) / 1000 : 0);
+    },
+  };
+  return ground;
+}
+
+test('floor reads are shared per cell and equal the per-coordinate reads', () => {
+  const random = seeded(7);
+  const points = [];
+  for (let i = 0; i < 4000; i++)
+    points.push([40.75 + random() * 0.01, -73.99 + random() * 0.01]);
+  // Cell edges, where toFixed(3) and Math.round can disagree, in each sign.
+  for (const lat of [40.7505, -40.7505, 0.0005, -0.0005, 1.0005, 8.5745])
+    for (const lon of [-73.9905, 73.9905, 0.0005, -0.0005, 1.0005, 2.3515])
+      points.push([lat, lon], [lat + 1e-9, lon - 1e-9], [lat - 1e-9, lon]);
+  points.push([Number.NaN, 0], [0, Infinity], [0, 0], [-0, -0]);
+
+  const direct = cellGround();
+  const expected = points.map(([lat, lon]) =>
+    direct.cachedGroundFloor(lat, lon),
+  );
+  const ground = cellGround();
+  const floorAt = floorCellReader(ground);
+  assert.deepStrictEqual(
+    points.map(([lat, lon]) => floorAt(lat, lon)),
+    expected,
+  );
+  // 4000 points in a 0.01 degree square share about 121 cells.
+  assert.ok(ground.reads < 600, `read the shared cache ${ground.reads} times`);
+
+  // A cold cell is remembered as cold for the pass, not asked again.
+  const cold = cellGround({ warm: () => false });
+  const coldAt = floorCellReader(cold);
+  assert.equal(coldAt(40.7512, -73.9812), null);
+  assert.equal(coldAt(40.7513, -73.9813), null);
+  assert.equal(cold.reads, 1);
+});
+
+test('mesh floors and unknown ground services are read per coordinate', () => {
+  const mesh = cellGround({ mesh: true });
+  const meshAt = floorCellReader(mesh);
+  const a = meshAt(40.7512, -73.9812),
+    b = meshAt(40.75121, -73.9812);
+  assert.equal(mesh.reads, 2);
+  assert.notEqual(a, b);
+
+  // No regime to ask: nothing says a floor is the same across a cell.
+  let reads = 0;
+  const bareAt = floorCellReader({ cachedGroundFloor: () => ++reads });
+  bareAt(40.7512, -73.9812);
+  bareAt(40.7512, -73.9812);
+  assert.equal(reads, 2);
+  assert.equal(floorCellReader(undefined)(1, 2), undefined);
+  assert.equal(floorCellReader({})(1, 2), undefined);
+});
+
+function road(lon, lat, points = 40) {
+  const coordinates = [];
+  for (let i = 0; i < points; i++)
+    coordinates.push([lon + i * 0.00004, lat + i * 0.00003]);
+  return { type: 'residential', oneway: 1, coordinates };
+}
+
+test('parsing reads each floor cell once and builds the same waypoints', () => {
+  const ground = cellGround({ warm: (cell) => !cell.endsWith('3') });
+  const state = { _viewHeightEstimate: 12 };
+  const model = createModel({
+    state,
+    services: { ground },
+    parts: {},
+    source: {},
+  });
+  const records = [road(-73.99, 40.75), road(-73.9899, 40.7501)];
+  const parsed = model.parseRoads({ roads: records });
+  const coordinates = records.reduce(
+    (count, record) => count + record.coordinates.length,
+    0,
+  );
+  assert.ok(
+    ground.reads < coordinates / 3,
+    `${ground.reads} floor reads for ${coordinates} coordinates`,
+  );
+
+  const direct = cellGround({ warm: (cell) => !cell.endsWith('3') });
+  const expected = records.flatMap((record) =>
+    roadSurfaceChunks(record.coordinates).map((coords) =>
+      coords.map(([lon, lat]) => {
+        const floor = direct.cachedGroundFloor(lat, lon);
+        return Cesium.Cartesian3.fromDegrees(
+          lon,
+          lat,
+          (floor ?? state._viewHeightEstimate) + DOT_HEIGHT_OFFSET,
+        );
+      }),
+    ),
+  );
+  assert.ok(direct.reads >= coordinates);
+  assert.deepStrictEqual(
+    parsed.map((entry) => entry.waypoints),
+    expected,
+  );
+});
+
+test('a record parsed earlier in the pass is not parsed again', () => {
+  const ground = cellGround();
+  const model = createModel({
+    state: {},
+    services: { ground },
+    parts: {},
+    source: {},
+  });
+  const first = { ...road(-73.99, 40.75), oneway: 0 },
+    second = road(-73.98, 40.76),
+    short = { type: 'primary', coordinates: [[-73.97, 40.77]] };
+  const parsedBySource = new WeakMap();
+  const tile = model.parseRoads({ roads: [first] }, null, parsedBySource);
+  assert.equal(tile.length, 2, 'a two-way record parses to both directions');
+  const reads = ground.reads;
+  const replaced = model.parseRoads(
+    { roads: [second, short, first], replace: true },
+    null,
+    parsedBySource,
+  );
+  assert.equal(replaced.length, 3);
+  // The same parsed roads, in the snapshot's order: dots stay attached.
+  assert.equal(replaced[1], tile[0]);
+  assert.equal(replaced[2], tile[1]);
+  const fresh = cellGround();
+  createModel({
+    state: {},
+    services: { ground: fresh },
+    parts: {},
+    source: {},
+  }).parseRoads({ roads: [second] });
+  assert.equal(ground.reads - reads, fresh.reads, 'only the new record read');
+
+  // Without the map every call parses, as before.
+  const again = model.parseRoads({ roads: [first] });
+  assert.notEqual(again[0], tile[0]);
+  assert.deepStrictEqual(again, tile);
+});
+
+test('the snapshot that ends a pass parses only the records it adds', async () => {
+  const detail = { z: 14, x: 3743, y: 6745 };
+  const core = tileToBBox(detail.z, detail.x, detail.y);
+  const local = {
+    type: 'residential',
+    oneway: 1,
+    coordinates: [
+      [core.west + 0.002, core.south + 0.002],
+      [core.west + 0.004, core.south + 0.004],
+    ],
+  };
+  const distant = {
+    type: 'primary',
+    oneway: 1,
+    coordinates: [
+      [core.east + 0.01, core.south],
+      [core.east + 0.02, core.south],
+    ],
+  };
+  const bounds = {
+    ...core,
+    east: core.east + 0.03,
+    coverage: {
+      detail: [detail],
+      coarse: [{ z: 12, x: 935, y: 1686 }],
+      key: 'test',
+    },
+  };
+  const source = createTrafficSource({
+    mapTiles: {
+      async fetchBounds(box, { zoom, onTile }) {
+        const tile = { roads: zoom === 14 ? [local] : [distant] };
+        onTile?.(tile);
+        return {
+          tiles: [tile],
+          loadedTiles: zoom === 14 ? [detail] : bounds.coverage.coarse,
+          partial: false,
+        };
+      },
+    },
+  });
+  const parsedRecords = [];
+  const painted = [];
+  const state = {
+    _enabled: true,
+    _loadGeneration: 0,
+    _tileCache: new Map(),
+    _roadMode: null,
+    _liveMode: false,
+    // The model's contract: fill the map, answer known records from it.
+    _parseRoads: (data, trace, parsedBySource) =>
+      data.roads.flatMap((record) => {
+        const known = parsedBySource?.get(record);
+        if (known) return known;
+        parsedRecords.push(record);
+        const parsed = [{ record }];
+        parsedBySource?.set(record, parsed);
+        return parsed;
+      }),
+  };
+  const ingestion = createIngestion({
+    state,
+    source,
+    services: {},
+    parts: {
+      viewport: {
+        clampBounds: (b) => b,
+        getBoundsCenter: () => ({ lat: 30.26, lon: -97.74 }),
+      },
+      flow: {
+        warmFlow: () => Promise.resolve([]),
+        applyFlowThenRender: async (roads, box, generation, altitude, label) => {
+          if (label === 'Loaded full') painted.push(roads);
+          return true;
+        },
+      },
+    },
+  });
+  await ingestion.loadRoadsForBounds(bounds, 350);
+  assert.equal(
+    parsedRecords.filter((record) => record === local).length,
+    1,
+    'the detail tile record is parsed when its tile streams, not again',
+  );
+  // The full graph is still the detail record plus the coarse road outside it.
+  assert.equal(painted.at(-1).length, 2);
+  assert.equal(painted.at(-1)[0].record, local);
+  assert.deepStrictEqual(
+    painted.at(-1)[1].record.coordinates,
+    distant.coordinates,
+  );
+});
diff --git a/src/layers/traffic/source.js b/src/layers/traffic/source.js
index 7327a3f98..135db0f38 100644
--- a/src/layers/traffic/source.js
+++ b/src/layers/traffic/source.js
@@ -33,23 +33,108 @@ export function trafficDetailBounds(box) {
   return detail;
 }
 
-/** Keep coarse roads only outside the detailed tile cores. */
-export function outsideDetailTiles(roads, tiles) {
+/** Longitude/latitude extent of one line; NaN coordinates leave it unusable. */
+function lineExtent(coordinates) {
+  let west = Infinity,
+    east = -Infinity,
+    south = Infinity,
+    north = -Infinity;
+  for (const [lon, lat] of coordinates) {
+    if (!(lon >= west)) west = lon;
+    if (!(lon <= east)) east = lon;
+    if (!(lat >= south)) south = lat;
+    if (!(lat <= north)) north = lat;
+  }
+  return { coordinates, west, east, south, north, settled: false };
+}
+
+/**
+ * What `clipTileLine` returns for a line lying wholly inside the clip region:
+ * the same line with each vertex recomputed from the one before it. Returns
+ * null for a line under the 12 m floor `clipTileLine` drops.
+ */
+function unclippedLine(coordinates) {
+  if (coordinates.length < 2) return null;
+  const out = [];
+  let metres = 0;
+  for (let i = 1; i < coordinates.length; i++) {
+    const a = coordinates[i - 1],
+      b = coordinates[i];
+    const dx = b[0] - a[0],
+      dy = b[1] - a[1];
+    if (i === 1) out.push([a[0] + 0 * dx, a[1] + 0 * dy]);
+    out.push([a[0] + dx, a[1] + dy]);
+    metres +=
+      Math.hypot(
+        (out[i][0] - out[i - 1][0]) * Math.cos((out[i][1] * Math.PI) / 180),
+        out[i][1] - out[i - 1][1],
+      ) * 111320;
+  }
+  return metres >= 12 ? out : null;
+}
+
+const sameLine = (a, b) =>
+  a.every(
+    (point, i) => Object.is(point[0], b[i][0]) && Object.is(point[1], b[i][1]),
+  );
+
+/**
+ * Keep coarse roads only outside the detailed tile cores.
+ *
+ * Each tile splits a line into the four regions around its core. Nearly every
+ * coarse road lies wholly inside one of those regions for nearly every tile,
+ * where the clip hands the line back unchanged, so that case is answered from
+ * the line's extent and only a line that reaches a tile's core or edge is
+ * clipped. Clipping every road against every tile froze the page for hundreds
+ * of milliseconds per road load at street level.
+ *
+ * `clip` is injectable for tests.
+ */
+export function outsideDetailTiles(roads, tiles, clip = clipTileLine) {
+  const boxes = tiles.map((tile) => tileToBBox(tile.z, tile.x, tile.y));
   return roads.flatMap((road) => {
-    let lines = [road.coordinates];
-    for (const tile of tiles) {
-      const box = tileToBBox(tile.z, tile.x, tile.y);
-      const regions = [
-        { west: -180, east: box.west, south: -85, north: 85 },
-        { west: box.east, east: 180, south: -85, north: 85 },
-        { west: box.west, east: box.east, south: -85, north: box.south },
-        { west: box.west, east: box.east, south: box.north, north: 85 },
-      ];
-      lines = lines.flatMap((line) =>
-        regions.flatMap((region) => clipTileLine(line, region)),
-      );
+    let lines = [lineExtent(road.coordinates)];
+    for (const box of boxes) {
+      const next = [];
+      for (const line of lines) {
+        // Strictly inside one region, and inside the bounds every region
+        // shares: the other three regions return nothing.
+        const clear =
+          line.west > -180 &&
+          line.east < 180 &&
+          line.south > -85 &&
+          line.north < 85 &&
+          (line.east < box.west ||
+            line.west > box.east ||
+            (line.west > box.west &&
+              line.east < box.east &&
+              (line.north < box.south || line.south > box.north)));
+        if (!clear) {
+          const regions = [
+            { west: -180, east: box.west, south: -85, north: 85 },
+            { west: box.east, east: 180, south: -85, north: 85 },
+            { west: box.west, east: box.east, south: -85, north: box.south },
+            { west: box.west, east: box.east, south: box.north, north: 85 },
+          ];
+          for (const region of regions)
+            for (const coordinates of clip(line.coordinates, region))
+              next.push(lineExtent(coordinates));
+        } else if (line.settled) {
+          next.push(line);
+        } else {
+          // The first unclipped pass can still move a vertex by a rounding
+          // step (a road crossing longitude or latitude 0); once a pass
+          // changes nothing, no later one will.
+          const coordinates = unclippedLine(line.coordinates);
+          if (!coordinates) continue;
+          const kept = lineExtent(coordinates);
+          kept.settled = sameLine(coordinates, line.coordinates);
+          next.push(kept);
+        }
+      }
+      lines = next;
     }
-    return lines.map((coordinates) => ({ ...road, coordinates }));
+    return lines.map(({ coordinates }) => ({ ...road, coordinates }));
   });
 }
 
diff --git a/src/layers/traffic/surface.js b/src/layers/traffic/surface.js
index 3344faa64..37a528901 100644
--- a/src/layers/traffic/surface.js
+++ b/src/layers/traffic/surface.js
@@ -69,6 +69,45 @@ export function trafficSurfaceReady(scene) {
   return currentRenderedFrame(scene)?.ready === true;
 }
 
+/**
+ * Ground-floor reads for one synchronous pass over road coordinates, asking
+ * the shared cache once per floor cell instead of once per coordinate.
+ *
+ * A road tile puts thousands of coordinates into a few hundred ~111 m cells,
+ * and each shared read formats and looks up two string keys, which added up
+ * to seconds per street-level load. Mesh floors answer per point (the nearest
+ * reading in the cell), so they are read through unchanged; only the DEM
+ * regime is one value per cell. Build a reader per pass: cells warm up
+ * between passes.
+ */
+export function floorCellReader(ground) {
+  const read = ground?.cachedGroundFloor;
+  if (!read) return () => undefined;
+  if (ground.meshFloorPreferred?.() !== false)
+    return (lat, lon) => read(lat, lon);
+  const cells = new Map();
+  return (lat, lon) => {
+    const y = lat * 1000,
+      x = lon * 1000;
+    const row = Math.round(y),
+      column = Math.round(x);
+    // The shared cache rounds with toFixed(3). At a cell edge that and
+    // Math.round can pick different cells, so an edge point asks directly.
+    if (!(
+      Math.abs(Math.abs(y - row) - 0.5) > 1e-6 &&
+      Math.abs(Math.abs(x - column) - 0.5) > 1e-6
+    ))
+      return read(lat, lon);
+    const key = row * 1e6 + column;
+    let floor = cells.get(key);
+    if (floor === undefined) {
+      floor = read(lat, lon) ?? null;
+      cells.set(key, floor);
+    }
+    return floor;
+  };
+}
+
 /** Preserve bends and insert height samples at most 150 m apart, splitting long roads. */
 export function roadSurfaceChunks(coordinates) {
   const chunks = [];
diff --git a/src/layers/traffic/timing.js b/src/layers/traffic/timing.js
index 0be95a330..bd1a3abb2 100644
--- a/src/layers/traffic/timing.js
+++ b/src/layers/traffic/timing.js
@@ -1,5 +1,5 @@
 import * as Cesium from 'cesium';
-import { roadSurfaceChunks } from './surface.js';
+import { floorCellReader, roadSurfaceChunks } from './surface.js';
 import { TRAFFIC_TIMING_ENABLED, DOT_HEIGHT_OFFSET } from './policy.js';
 
 export function createTiming({ state: layerState, services, parts, source }) {
@@ -190,7 +190,7 @@ export function createTiming({ state: layerState, services, parts, source }) {
    * waypoint-materialization time independently.
    */
 
-  function parseRoadsTimed(roadData, trace) {
+  function parseRoadsTimed(roadData, trace, parsedBySource) {
     /* TRACE_ONLY_BEGIN */
     const _trafficTimingState = trafficTimingPass(
       trace,
@@ -231,11 +231,19 @@ export function createTiming({ state: layerState, services, parts, source }) {
     }
 
     const roads = [];
+    const floorAt = floorCellReader(services.ground);
     /* TRACE_ONLY_BEGIN */
     let _trafficTimingWaypointMaterializationMs = 0;
     /* TRACE_ONLY_END */
     for (const road of roadData.roads) {
       if (!road.coordinates || road.coordinates.length < 2) continue;
+      // A replacing snapshot repeats records this load already parsed.
+      const known = parsedBySource?.get(road);
+      if (known) {
+        for (const parsed of known) roads.push(parsed);
+        continue;
+      }
+      const first = roads.length;
 
       for (const coords of roadSurfaceChunks(road.coordinates)) {
         const type = road.type;
@@ -245,7 +253,7 @@ export function createTiming({ state: layerState, services, parts, source }) {
         const _trafficTimingMaterializeStart = performance.now();
         /* TRACE_ONLY_END */
         const waypoints = coords.map(([lng, lat]) => {
-          const floor = services.ground?.cachedGroundFloor?.(lat, lng);
+          const floor = floorAt(lat, lng);
           const h =
             (Number.isFinite(floor) && Math.abs(floor) <= 9000
               ? floor
@@ -279,6 +287,7 @@ export function createTiming({ state: layerState, services, parts, source }) {
             flow: road.flow || null,
           });
       }
+      parsedBySource?.set(road, roads.slice(first));
     }
     /* TRACE_ONLY_BEGIN */
     const _trafficTimingMetrics = {