feat: add native preview traffic simulation

2026-08-18 16:57:27 +08:00
parent 0bc949bc24
commit 5403936ae4
21 changed files with 777 additions and 45 deletions
--- a/.trellis/spec/config/index.md
+++ b/.trellis/spec/config/index.md
@@ -187,7 +187,8 @@ transcode，成功后替换**package staging** 中的主 GLB 与 manifest；未

 静态发布路径另有 `packageDir`、`packageStagingDir`、`packageManifest`、
 `packageStagingManifest`、`packageModelDir`、`packageStagingModelDir` 与
-`packagePrimaryGlb`；预览路径另有 `previewDir` 与 `previewDescriptor`。除非在迁移旧调用，
+`packagePrimaryGlb`；预览路径另有 `previewDir`、`previewDescriptor` 与
+`trafficSimulation`（native preview 的可迁移仿真描述符）。除非在迁移旧调用，
 不要覆盖 `glb` / `metadata`：它们是 staging 内部路径，不是下游资产入口。

 **优先改 `fileStem` 或 `areaDir`**——它们能一次性影响全部派生路径。逐个覆盖容易漏。
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/check.jsonl
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/check.jsonl
@@ -0,0 +1 @@
+{"_example": "Fill with {\"file\": \"<path>\", \"reason\": \"<why>\"}. Put spec/research files only — no code paths. Run `python3 .trellis/scripts/get_context.py --mode packages` to list available specs. Delete this line once real entries are added."}
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/design.md
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/design.md
@@ -0,0 +1,70 @@
+# Design
+
+## Authority And Boundaries
+
+The native compiler remains the only road authority. The existing traffic-signal runtime remains the
+only signal authority. The new feature adds a route/simulation layer beside those artifacts; it does
+not alter signal geometry, phase grouping, or Blender static assets.
+
+```text
+native-road/compiled.json + native-road/layers/*
+  -> native route compiler
+  -> _preview/<area>-traffic-simulation.json
+  -> Cesium preview runtime
+
+package/runtime/traffic-signals.json
+  -> signal phase clock + stop events
+```
+
+The descriptor is intentionally package-adjacent, not embedded in the GLB. A consumer can copy the
+package and descriptor to another platform and reproduce the simulation without running Node.
+
+## Descriptor Contract
+
+Schema: `native-preview-traffic-simulation/v1`.
+
+Required top-level fields:
+
+- `areaId`, `coordinateSystem` (`WGS84` route coordinates, `ENU` model placement), `generatedAt`
+- `source` with relative artifact references and SHA-256 records
+- `settings` with speed, acceleration, deceleration, reaction time, vehicle length, and minimum gap
+- `routes[]` with ordered WGS84 coordinates, cumulative distances, connector/maneuver IDs, and
+  `stops[]`
+- `signals[]` copied by stable `signal_uid` from the runtime contract, including phase group and
+  stop-line association; no regenerated pose fields
+- `migration` describing schema version, coordinate conversion, update-loop expectations, and
+  compatibility notes
+
+The descriptor is deterministic for the same OSM, overrides, native compiler version, and settings.
+
+## Route Construction
+
+Use native directional roads and published connector geometry. Build a small number of deterministic
+closed demonstration routes that cross real connectors and retain lane/road IDs. Route generation
+must reject routes with missing geometry or disabled connectors and write structured diagnostics.
+Native stop lines are matched by `road_id`/`lane_id` where available, with a bounded geometric
+fallback recorded in the stop record. No fixed-width or legacy polygon fallback is allowed.
+
+## Browser Simulation
+
+Keep the existing signal phase functions and `signalData` payload. Replace the current per-vehicle
+independent distance advance with a shared simulation state:
+
+- each vehicle has route distance, speed, desired speed, status, and active stop reason;
+- signal stop constraints are evaluated before the native stop line for red/yellow phases;
+- leader constraints are evaluated on the same route and wrap around the loop;
+- acceleration/deceleration clamps advance toward the minimum of desired speed, signal limit, and
+  leader-safe speed;
+- vehicle position and orientation remain Cesium callback properties, with no per-frame allocation
+  of heavyweight Cesium objects;
+- incident-card behavior remains local to each vehicle and must not mutate the descriptor or package.
+
+Diagnostics expose route count, signal count, stopped vehicles, and queue length in the existing
+preview diagnostics panel.
+
+## Migration And Rollback
+
+Document the descriptor, runtime signal schema, coordinate conversion, and a reference update loop in
+`docs/native-preview-traffic-simulation.md`. Include a JSON example and a consumer checklist. Legacy
+preview route files remain readable only through the existing legacy provider path; native generation
+must not depend on them. Removing the new descriptor restores the existing no-cruise native preview.
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/implement.jsonl
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/implement.jsonl
@@ -0,0 +1 @@
+{"_example": "Fill with {\"file\": \"<path>\", \"reason\": \"<why>\"}. Put spec/research files only — no code paths. Run `python3 .trellis/scripts/get_context.py --mode packages` to list available specs. Delete this line once real entries are added."}
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/implement.md
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/implement.md
@@ -0,0 +1,37 @@
+# Implementation Plan
+
+1. Add a native route compiler module that consumes `native-road/compiled.json` and native layers,
+   emits `native-preview-traffic-simulation/v1`, and records source hashes and migration metadata.
+2. Wire native preview generation to write the descriptor and keep legacy osm2streets route
+   generation unchanged.
+3. Extend the browser preview with shared deterministic vehicle state, signal stop constraints,
+   leader following, queue diagnostics, and graceful empty-route behavior.
+4. Preserve the existing signal phase, `signal_uid`, pose, dynamic lens, countdown, disabled-signal,
+   and incident-card contracts; add focused regression tests for each boundary.
+5. Add `docs/native-preview-traffic-simulation.md` with schema, example, coordinate rules, update
+   loop, migration checklist, and rollback notes.
+6. Run focused Node tests, native compile/check, a native build with no legacy directory, and browser
+   preview validation in the available Blender/Cesium environment.
+
+## Validation Commands
+
+```bash
+node --check scripts/lib/native-preview-traffic-simulation.js
+node --check scripts/build-area.js
+npm run test:native-road
+npm run test:traffic-signals
+npm run test:preview-assets
+npm run test:native-preview-traffic
+npm run road:compile -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,compress,package,preview
+```
+
+## Risk Controls
+
+- Do not change `scripts/lib/traffic-signals.js` geometry or phase semantics unless a test proves a
+  migration bug; prefer adapter code.
+- Do not add route data to the static package manifest; it belongs beside `_preview/` and is optional.
+- If native route generation produces no valid route, emit a valid descriptor with diagnostics and
+  keep preview usable.
+- Keep all route and simulation math unit-testable without Cesium.
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/prd.md
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/prd.md
@@ -0,0 +1,38 @@
+# Native preview traffic simulation and migration contract
+
+## Goal
+
+Extend the native road compiler preview with a deterministic, portable vehicle traffic demonstration that consumes native road topology and the already-validated traffic-signal runtime. The result must be useful as a visual validation tool and as a migration reference for Cesium, Blender, or another platform consuming the generated area package.
+
+## Confirmed Baseline
+
+- Native road compilation is the production authority; QGIS/osm2streets is not a runtime input.
+- The previous traffic-signal work is the source of truth and must remain compatible: deterministic `signal_uid`, `phase_group`, `mast_heading_deg`, `face_heading_deg`, stop-line anchors, shared `pose.*`, dynamic lens/countdown assets, and disabled-signal filtering.
+- Existing browser preview already has signal phase timing, signal visualization, route stop matching, vehicle incident cards, and a missing-route fallback. Native preview currently omits the route.
+- Native road output contains compiled roads/connectors and `vehicle_stop_lines.geojson`; the native package contains `runtime/traffic-signals.json`.
+- The feature is preview-level deterministic behavior, not a legal navigation or microscopic traffic simulator.
+
+## Requirements
+
+- R1: Generate a native route artifact from `native-road/compiled.json` and native layer geometry; it must not read `osm2streets_web_out`, QGIS files, or legacy route files.
+- R2: Preserve and consume the existing traffic-signal contract without deriving a second signal layout in the browser.
+- R3: Vehicles must stop before a red/yellow signal at the native stop line, resume on green, and expose the active stop reason in preview diagnostics.
+- R4: Multiple vehicles on the same route must maintain a configurable minimum gap; a stopped front vehicle must cause following vehicles to decelerate and queue rather than overlap it.
+- R5: Route, signal, vehicle, stop-event, and following-distance data must be written as versioned, package-adjacent JSON with stable ENU/WGS84 and migration metadata. Another platform must be able to consume the artifact without executing the Node compiler.
+- R6: The browser preview must degrade gracefully when there are no valid routes or signals; the primary scene remains usable and diagnostics explain the missing optional capability.
+- R7: Documentation must describe the data flow, schemas, coordinate conventions, signal identity, stop-line association, update loop, and migration/rollback guidance.
+
+## Acceptance Criteria
+
+- [ ] Native build emits a versioned route/traffic-simulation descriptor under the area output and preview loads it without any legacy directory present.
+- [ ] At least one route crosses a native connector and contains a native stop-line association.
+- [ ] A red/yellow phase visibly stops a vehicle before the stop line; green releases it.
+- [ ] Two or more vehicles never overlap and maintain the configured minimum gap while cruising and while queued at a signal.
+- [ ] Disabled native signals are absent from runtime control and do not create vehicle stops.
+- [ ] Preview diagnostics report route count, signal count, stopped vehicles, queue length, and the active simulation descriptor version.
+- [ ] Migration documentation and focused tests allow another consumer to reproduce route, signal, stopping, and gap behavior from generated assets alone.
+- [ ] Existing traffic-signal tests and native road tests remain green; legacy provider behavior is unchanged.
+
+## Decided Scope
+
+- Simulation fidelity: MVP is a deterministic preview simulation with configurable speed, acceleration/deceleration, reaction time, and minimum gap. A full lane-changing, collision, priority, and multi-intersection traffic engine is deferred.
--- a/.trellis/tasks/08-18-native-preview-traffic-simulation/task.json
+++ b/.trellis/tasks/08-18-native-preview-traffic-simulation/task.json
@@ -0,0 +1,26 @@
+{
+  "id": "native-preview-traffic-simulation",
+  "name": "native-preview-traffic-simulation",
+  "title": "Native preview traffic simulation and migration contract",
+  "description": "",
+  "status": "in_progress",
+  "dev_type": null,
+  "scope": null,
+  "package": null,
+  "priority": "P2",
+  "creator": "dingkang",
+  "assignee": "dingkang",
+  "createdAt": "2026-08-18",
+  "completedAt": null,
+  "branch": null,
+  "base_branch": "feature/native-road-compiler",
+  "worktree_path": null,
+  "commit": null,
+  "pr_url": null,
+  "subtasks": [],
+  "children": [],
+  "parent": null,
+  "relatedFiles": [],
+  "notes": "",
+  "meta": {}
+}
--- a/README.md
+++ b/README.md
@@ -114,6 +114,22 @@ npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json
 `road:check` 验证已发布 connector 与语义行驶动作的一致性，并在诊断包含 error、图层缺失或
 动作/几何对应关系不一致时以非零退出。warning 保留给工作台审查，不会阻止产物打开。

+### Native Preview Traffic Simulation
+
+native preview 会额外生成：
+
+```text
+outputs/<area-id>/_preview/<area-id>-traffic-simulation.json
+```
+
+该文件使用 `native-preview-traffic-simulation/v1`，记录 native 路线、connector、停止点、
+信号 `signal_uid`、源文件 SHA-256、坐标系和仿真参数。它与 `package/manifest.json` 一起可
+迁移到其他平台；消费方不需要运行 Node 或 QGIS。预览中的车辆会根据现有信号相位在停止线
+前停车，并按最小车距跟车排队。该功能是确定性的验证预览，不是法规级导航或完整交通仿真。
+
+迁移契约、JSON 示例、更新循环和回撤方式见
+[native preview traffic simulation](docs/native-preview-traffic-simulation.md)。
+
 启动本地浏览器工作台：

 ```bash
--- a/docs/native-preview-traffic-simulation.md
+++ b/docs/native-preview-traffic-simulation.md
@@ -0,0 +1,64 @@
+# Native Preview Traffic Simulation
+
+This document defines the portable preview contract for native road traffic demonstrations. It is
+not a legal navigation format or a full microscopic traffic simulator.
+
+## Data Flow
+
+```text
+native-road/compiled.json + layers/
+  -> native-preview-traffic-simulation/v1 descriptor
+package/runtime/traffic-signals.json
+  -> signal phase clock and stop constraints
+descriptor + package/manifest.json
+  -> Cesium preview vehicles, signals, queues, diagnostics
+```
+
+The descriptor is generated from native artifacts and is independent of QGIS/osm2streets. Copying
+the descriptor, package, and preview runtime to another platform is sufficient to reproduce the
+visual simulation.
+
+## Existing Signal Contract
+
+Signal identity is `signal_uid`; it must not be renamed when `display_id` changes. `phase_group`
+selects the existing signal phase. `mast_heading_deg`, `face_heading_deg`, `pose.*`, stop-line
+coordinates, and `enabled` are consumed from `package/runtime/traffic-signals.json`. Disabled signals
+are absent from runtime control. Consumers must not recalculate lamp or pole geometry.
+
+## Vehicle Rules
+
+Each vehicle advances along a route distance. Its target speed is the minimum of desired speed,
+signal-safe speed, and leader-safe speed. Red/yellow signals create a stop target before the native
+stop line. Following vehicles apply the configured minimum gap and decelerate behind the leader.
+The simulation is deterministic for a fixed descriptor, clock start, and settings.
+
+The descriptor records WGS84 route coordinates. The static package remains local ENU and is placed by
+the package manifest. Consumers must use the package placement/model matrix for GLB assets and WGS84
+coordinates for route entities; do not mix a fixed meters-per-degree approximation into either path.
+
+Route geometry is taken from `native-road/layers/lane_centerlines.geojson` when that native layer is
+available, keyed by `road_id`; the compiled road centerline is only a compatibility fallback for old
+native outputs. Each road/connector boundary is checked in meters. Candidates with a missing
+connector or an endpoint gap over 35 m are omitted from `routes` and retained in `rejectedRoutes`
+with structured diagnostics. This prevents a malformed movement from becoming a visible diagonal
+track across an intersection.
+
+`diagnostics` contains the route-generation warnings, while `rejectedRoutes` preserves the candidate
+id and the specific movement checks that failed. A consumer should display these as data-quality
+warnings rather than silently reconstructing a route from legacy QGIS files.
+
+## Migration Checklist
+
+1. Load `package/manifest.json` and resolve all package-relative assets.
+2. Load the traffic-signal runtime and preserve `signal_uid`/`phase_group` values.
+3. Load the versioned simulation descriptor and verify its source hashes if reproducibility matters.
+4. Implement the update loop using the descriptor settings and stop/leader constraints.
+5. Place route coordinates in the same WGS84 scene as the package placement and preserve ENU model
+   placement for static assets.
+6. Expose route count, signal count, stopped vehicles, and queue length as diagnostics.
+
+## Rollback
+
+Deleting or ignoring the optional simulation descriptor returns the native preview to a usable static
+scene with signal visualization but no vehicle cruise. The QGIS baseline remains available through
+`qgis-original-baseline-20260818`; it is not required by this contract.
--- a/package.json
+++ b/package.json
@@ -25,6 +25,7 @@
    "test:package-examples": "node scripts/test-package-examples.js",
    "test:turn-lane-arrows": "node scripts/test-turn-lane-arrows.js",
    "test:traffic-signals": "node scripts/test-traffic-signals.js",
+    "test:native-preview-traffic": "node scripts/test-native-preview-traffic.js",
    "render:turn-lane-arrow-samples": "node scripts/render-turn-lane-arrow-samples.js"
  },
  "dependencies": {
--- a/scripts/build-area.js
+++ b/scripts/build-area.js
@@ -16,6 +16,7 @@ const {
 } = require("./lib/scene-layers");
 const { digest: glbDigest } = require("./glb-digest");
 const { buildVehicleRoute: buildPreviewVehicleRoute } = require("./lib/vehicle-route");
+const { buildNativeTrafficSimulation } = require("./lib/native-preview-traffic-simulation");
 const { writePreviewVehicleLibrary } = require("./lib/vehicle-library");
 const { readTrafficSignals } = require("./lib/traffic-signals");
 const {
@@ -338,11 +339,9 @@ function exportCesium(area, roadProvider) {
  fs.rmSync(area.outputs.packageStagingDir, { recursive: true, force: true });
  fs.mkdirSync(path.dirname(area.outputs.glb), { recursive: true });
  fs.mkdirSync(path.dirname(area.outputs.metadata), { recursive: true });
-  if (roadProvider === "osm2streets") {
-    fs.mkdirSync(path.dirname(area.outputs.trafficSignalsDynamicGlb), { recursive: true });
-    fs.mkdirSync(path.dirname(area.outputs.trafficSignalsCountdown0Glb), { recursive: true });
-    fs.mkdirSync(path.dirname(area.outputs.trafficSignalsCountdown1Glb), { recursive: true });
-  }
+  fs.mkdirSync(path.dirname(area.outputs.trafficSignalsDynamicGlb), { recursive: true });
+  fs.mkdirSync(path.dirname(area.outputs.trafficSignalsCountdown0Glb), { recursive: true });
+  fs.mkdirSync(path.dirname(area.outputs.trafficSignalsCountdown1Glb), { recursive: true });

  console.log("Stage: cesium");
  const started = Date.now();
@@ -363,19 +362,15 @@ function exportCesium(area, roadProvider) {
    "--metadata",
    area.outputs.metadata,
  ];
-  if (roadProvider === "osm2streets") {
-    exporterArgs.splice(-2, 0,
-      "--dynamic-glb", area.outputs.trafficSignalsDynamicGlb,
-      "--countdown-0-glb", area.outputs.trafficSignalsCountdown0Glb,
-      "--countdown-1-glb", area.outputs.trafficSignalsCountdown1Glb,
-    );
-  }
+  exporterArgs.splice(-2, 0,
+    "--dynamic-glb", area.outputs.trafficSignalsDynamicGlb,
+    "--countdown-0-glb", area.outputs.trafficSignalsCountdown0Glb,
+    "--countdown-1-glb", area.outputs.trafficSignalsCountdown1Glb,
+  );
  runCommand(blenderExecutable(area), exporterArgs, "cesium");
-  if (roadProvider === "osm2streets") {
-    ensureFile(area.outputs.trafficSignalsDynamicGlb, "Dynamic traffic signal GLB");
-    ensureFile(area.outputs.trafficSignalsCountdown0Glb, "Traffic countdown group 0 GLB");
-    ensureFile(area.outputs.trafficSignalsCountdown1Glb, "Traffic countdown group 1 GLB");
-  }
+  ensureFile(area.outputs.trafficSignalsDynamicGlb, "Dynamic traffic signal GLB");
+  ensureFile(area.outputs.trafficSignalsCountdown0Glb, "Traffic countdown group 0 GLB");
+  ensureFile(area.outputs.trafficSignalsCountdown1Glb, "Traffic countdown group 1 GLB");
  const semanticAssets = semanticAssetRecords(area);
  const finished = Date.now();
  const digest = glbDigest(area.outputs.glb);
@@ -392,9 +387,9 @@ function exportCesium(area, roadProvider) {
    outputs: {
      glb: fileRecord(area.outputs.glb),
      metadata: fileRecord(area.outputs.metadata),
-      ...(roadProvider === "osm2streets" ? {
-        trafficSignalsDynamicGlb: fileRecord(area.outputs.trafficSignalsDynamicGlb),
-      } : {}),
+      trafficSignalsDynamicGlb: fileRecord(area.outputs.trafficSignalsDynamicGlb),
+      trafficSignalsCountdown0Glb: fileRecord(area.outputs.trafficSignalsCountdown0Glb),
+      trafficSignalsCountdown1Glb: fileRecord(area.outputs.trafficSignalsCountdown1Glb),
      semanticAssets,
    },
    summary: {
@@ -558,6 +553,7 @@ function writeCesiumPreview(area, roadProvider) {
  ensureFile(area.outputs.packageManifest, "Published asset package manifest");
  ensureFile(area.outputs.packageTrafficSignals, "Published traffic signal anchors");
  let vehicleRoute = null;
+  let routeArtifact = null;
  const previewInputs = {
    config: fileRecord(configPath),
    osm: fileRecord(area.input),
@@ -579,6 +575,11 @@ function writeCesiumPreview(area, roadProvider) {
    });
  } else {
    Object.assign(previewInputs, nativeRoadRecords(area));
+    const simulation = buildNativeTrafficSimulation(area);
+    fs.mkdirSync(path.dirname(area.outputs.trafficSimulation), { recursive: true });
+    fs.writeFileSync(area.outputs.trafficSimulation, `${JSON.stringify(simulation, null, 2)}\n`);
+    previewInputs.trafficSimulation = fileRecord(area.outputs.trafficSimulation);
+    routeArtifact = area.outputs.trafficSimulation;
  }
  const htmlPath = area.outputs.cesiumPreview;
  const started = Date.now();
@@ -594,7 +595,9 @@ function writeCesiumPreview(area, roadProvider) {
  writeCesiumPreviewSupportFiles(path.dirname(htmlPath));
  const glbName = "package/manifest.json";
  const metadataName = "package/manifest.json";
-  const routeName = vehicleRoute ? previewRelativePath(area.outputs.areaDir, area.outputs.vehicleRoute) : null;
+  const routeName = vehicleRoute || routeArtifact
+    ? previewRelativePath(area.outputs.areaDir, vehicleRoute ? area.outputs.vehicleRoute : routeArtifact)
+    : null;
  const vehicleModelName = previewRelativePath(area.outputs.areaDir, area.outputs.vehicleModel);
  const descriptor = { routeName, vehicleModelName: previewRelativePath(area.outputs.areaDir, area.outputs.vehicleModel), vehicleModelNames: vehicleModelNames.map((name) => `_preview/${name}`), trafficSignalsName: "package/runtime/traffic-signals.json", assets: [] };
  fs.mkdirSync(area.outputs.previewDir, { recursive: true });
@@ -617,10 +620,11 @@ function writeCesiumPreview(area, roadProvider) {
    outputs: {
      cesiumPreview: fileRecord(area.outputs.cesiumPreview),
      vehicleRoute: optionalFileRecord(area.outputs.vehicleRoute),
+      ...(routeArtifact ? { trafficSimulation: fileRecord(routeArtifact) } : {}),
      vehicleModel: fileRecord(area.outputs.vehicleModel),
      trafficSignals: fileRecord(area.outputs.packageTrafficSignals),
    },
-    summary: previewSummary(area),
+    summary: previewSummary(area, vehicleRoute ? area.outputs.vehicleRoute : routeArtifact),
    warnings: [],
  });
 }
--- a/scripts/lib/area-config.js
+++ b/scripts/lib/area-config.js
@@ -66,6 +66,7 @@ function normalizeAreaConfig(raw, options = {}) {
    ),
    previewDir,
    previewDescriptor: path.resolve(outputOverrides.previewDescriptor || path.join(previewDir, "descriptor.json")),
+    trafficSimulation: path.resolve(outputOverrides.trafficSimulation || path.join(previewDir, `${fileStem}-traffic-simulation.json`)),
    vehicleRoute: path.resolve(outputOverrides.vehicleRoute || path.join(previewDir, `${fileStem}-vehicle-route.json`)),
    vehicleModel: path.resolve(outputOverrides.vehicleModel || path.join(previewDir, `${fileStem}-vehicle-car.gltf`)),
    trafficSignalAssemblies: path.resolve(
--- a/scripts/lib/area-diagnostics.js
+++ b/scripts/lib/area-diagnostics.js
@@ -473,15 +473,18 @@ function stageManifestStatus(area, configPath = null) {
          network: path.join(area.outputs.geojsonDir, "network.json"),
          intersectionSurface: path.join(area.outputs.geojsonDir, "intersection_surface.geojson"),
        }),
+        ...(native ? { trafficSimulation: area.outputs.trafficSimulation } : {}),
        previewCss: path.join(path.resolve(__dirname, ".."), "lib", "cesium-preview.css"),
        previewJs: path.join(path.resolve(__dirname, ".."), "lib", "cesium-preview.js"),
      },
      outputs: compressionComplete ? {
        vehicleRoute: native ? optionalExpectedFile(area.outputs.vehicleRoute) : area.outputs.vehicleRoute,
+        ...(native ? { trafficSimulation: area.outputs.trafficSimulation } : {}),
        vehicleModel: area.outputs.vehicleModel,
      } : {
        cesiumPreview: area.outputs.cesiumPreview,
        vehicleRoute: native ? optionalExpectedFile(area.outputs.vehicleRoute) : area.outputs.vehicleRoute,
+        ...(native ? { trafficSimulation: area.outputs.trafficSimulation } : {}),
        vehicleModel: area.outputs.vehicleModel,
      },
    },
--- a/scripts/lib/area-preview.js
+++ b/scripts/lib/area-preview.js
@@ -123,15 +123,15 @@ function escapeScriptJson(value) {
    .replaceAll("\u2029", "\\u2029");
 }

-function previewSummary(area) {
-  const route = fs.existsSync(area.outputs.vehicleRoute) ? JSON.parse(fs.readFileSync(area.outputs.vehicleRoute, "utf8")) : null;
+function previewSummary(area, routePath = area.outputs.vehicleRoute) {
+  const route = routePath && fs.existsSync(routePath) ? JSON.parse(fs.readFileSync(routePath, "utf8")) : null;
  return {
    glbName: "package/manifest.json",
    metadataName: "package/manifest.json",
-    routeName: route ? path.relative(area.outputs.areaDir, area.outputs.vehicleRoute).split(path.sep).join("/") : null,
+    routeName: route ? path.relative(area.outputs.areaDir, routePath).split(path.sep).join("/") : null,
    vehicleModelName: path.relative(area.outputs.areaDir, area.outputs.vehicleModel).split(path.sep).join("/"),
    trafficSignalsName: "package/runtime/traffic-signals.json",
-    routeSegments: Array.isArray(route?.segments) ? route.segments.length : null,
+    routeSegments: Array.isArray(route?.routes) ? route.routes.length : Array.isArray(route?.segments) ? route.segments.length : null,
  };
 }

--- a/scripts/lib/cesium-preview.js
+++ b/scripts/lib/cesium-preview.js
@@ -555,16 +555,27 @@
      .slice(0, 5);
    const speed = Number((routeData && routeData.speedMetersPerSecond) || 8);
    const start = trafficStart;
+    const assignments = [];
+    segments.forEach((segment, routeIndex) => {
+      // Put two vehicles on the first route so the native preview visibly
+      // exercises leader following and queue formation.
+      const vehicleCount = routeIndex === 0 ? 2 : 1;
+      for (let vehicleIndex = 0; vehicleIndex < vehicleCount; vehicleIndex += 1) {
+        assignments.push({ segment, routeIndex, vehicleIndex });
+      }
+    });
+    const simulation = createTrafficSimulation(viewer, assignments, start, speed, signalData, routeData?.settings);

    viewer.clock.startTime = start.clone();
    viewer.clock.currentTime = start.clone();
    viewer.clock.clockRange = Cesium.ClockRange.UNBOUNDED;
    viewer.clock.multiplier = 1;
-    viewer.clock.shouldAnimate = segments.length > 0;
+    viewer.clock.shouldAnimate = assignments.length > 0;

-    const vehicles = segments.map((segment, index) => {
+    const vehicles = assignments.map((assignment, index) => {
+      const { segment } = assignment;
      const vehicle = addCruiseVehicle(viewer, segment, index, start, speed, signalData,
-        selectedVehicleModelName(vehicleModelNames, fallbackVehicleModelName));
+        selectedVehicleModelName(vehicleModelNames, fallbackVehicleModelName), simulation.motions[index]);
      const option = document.createElement("option");
      option.value = String(index);
      option.textContent = routeLabel(segment, index);
@@ -574,7 +585,8 @@
    const cruise = {
      vehicles,
      baseSpeed: speed,
-      state: { selectedIndex: 0 }
+      state: { selectedIndex: 0 },
+      simulation,
    };
    syncSelectedRouteVisibility(cruise);
    return cruise;
@@ -732,10 +744,10 @@
    return usable[Math.floor(Math.random() * usable.length)] || "";
  }

-  function addCruiseVehicle(viewer, segment, index, start, speed, signalData, vehicleModelName) {
-    const route = prepareRoute(segment, signalData);
+  function addCruiseVehicle(viewer, segment, index, start, speed, signalData, vehicleModelName, trafficMotion) {
+    const route = trafficMotion.route;
    let record = null;
-    const trafficMotion = createTrafficAwarePositions(viewer, route, start, speed, () => record?.status === "normal");
+    trafficMotion.setMoving(() => record?.status === "normal");
    const positions = trafficMotion.positions;
    const flat = [];
    for (const coord of segment.coordinates) {
@@ -946,6 +958,106 @@
    return [...found.values()].sort((a, b) => a.distance - b.distance);
  }

+  function createTrafficSimulation(viewer, assignments, start, speed, signalData, settings = {}) {
+    const config = {
+      desiredSpeedMetersPerSecond: Number(settings.desiredSpeedMetersPerSecond || speed),
+      accelerationMetersPerSecondSquared: Number(settings.accelerationMetersPerSecondSquared || 1.8),
+      decelerationMetersPerSecondSquared: Number(settings.decelerationMetersPerSecondSquared || 3.5),
+      reactionTimeSeconds: Number(settings.reactionTimeSeconds || 0.8),
+      vehicleLengthMeters: Number(settings.vehicleLengthMeters || 4.6),
+      minimumGapMeters: Number(settings.minimumGapMeters || 7),
+    };
+    const motions = assignments.map((assignment) => {
+      const route = prepareRoute(assignment.segment, signalData);
+      const initialGap = config.vehicleLengthMeters + config.minimumGapMeters;
+      const state = {
+        distance: assignment.vehicleIndex * -initialGap,
+        speed: 0,
+        targetSpeed: config.desiredSpeedMetersPerSecond,
+        stopReason: null,
+        queueAhead: 0,
+        lastTime: start.clone(),
+      };
+      state.distance = (state.distance % route.length + route.length) % route.length;
+      return {
+        route,
+        state,
+        setMoving(callback) { state.isMoving = callback; },
+        positions: new Cesium.CallbackProperty((time, result) => routePositionAtDistance(route, state.distance, result), false),
+      };
+    });
+    const update = (clock) => {
+      const elapsed = Cesium.JulianDate.secondsDifference(clock.currentTime, motions[0]?.state.lastTime || start);
+      if (!(elapsed > 0) || elapsed > 2) {
+        for (const motion of motions) Cesium.JulianDate.clone(clock.currentTime, motion.state.lastTime);
+        return;
+      }
+      const grouped = new Map();
+      assignments.forEach((assignment, index) => {
+        const key = assignment.routeIndex;
+        if (!grouped.has(key)) grouped.set(key, []);
+        grouped.get(key).push(index);
+      });
+      for (const indexes of grouped.values()) {
+        indexes.sort((a, b) => motions[a].state.distance - motions[b].state.distance);
+      }
+      motions.forEach((motion, index) => {
+        const state = motion.state;
+        Cesium.JulianDate.clone(clock.currentTime, state.lastTime);
+        if (state.isMoving && !state.isMoving()) {
+          state.speed = 0;
+          state.stopReason = "incident";
+          return;
+        }
+        const route = motion.route;
+        let target = config.desiredSpeedMetersPerSecond;
+        let stopReason = null;
+        const nextStop = nextRouteStop(route, state.distance);
+        if (nextStop) {
+          const phase = signalPhase(nextStop.signal.phaseGroup, clock.currentTime, start).active;
+          const untilStop = (nextStop.distance - state.distance + route.length) % route.length;
+          if (phase !== "green" && untilStop < Math.max(30, state.speed * config.reactionTimeSeconds + 8)) {
+            target = Math.min(target, Math.max(0, (untilStop - 1.7) / Math.max(config.reactionTimeSeconds, 0.1)));
+            stopReason = "traffic-signal";
+          }
+        }
+        const peers = grouped.get(assignments[index].routeIndex) || [];
+        const peerPosition = peers.indexOf(index);
+        if (peerPosition >= 0 && peers.length > 1) {
+          const leaderIndex = peers[(peerPosition + 1) % peers.length];
+          if (leaderIndex !== index) {
+            const leader = motions[leaderIndex].state;
+            const gap = (leader.distance - state.distance + route.length) % route.length - config.vehicleLengthMeters;
+            const safeGap = config.minimumGapMeters + state.speed * config.reactionTimeSeconds;
+            if (gap < safeGap + 12) {
+              target = Math.min(target, Math.max(0, (gap - config.minimumGapMeters) / Math.max(config.reactionTimeSeconds, 0.1)));
+              if (target < 0.2) stopReason = "leader-gap";
+              state.queueAhead = leader.stopReason ? 1 : 0;
+            } else {
+              state.queueAhead = 0;
+            }
+          }
+        }
+        state.targetSpeed = target;
+        const limit = (target >= state.speed ? config.accelerationMetersPerSecondSquared : config.decelerationMetersPerSecondSquared) * elapsed;
+        state.speed += Math.sign(target - state.speed) * Math.min(Math.abs(target - state.speed), limit);
+        state.distance = (state.distance + Math.max(0, state.speed) * elapsed) % route.length;
+        state.stopReason = state.speed < 0.2 ? stopReason : null;
+      });
+    };
+    viewer.clock.onTick.addEventListener(update);
+    return {
+      motions,
+      settings: config,
+      diagnostics() {
+        return {
+          stoppedVehicles: motions.filter((motion) => motion.state.stopReason).length,
+          queueLength: motions.filter((motion) => motion.state.stopReason === "leader-gap").length,
+        };
+      },
+    };
+  }
+
  function createTrafficAwarePositions(viewer, route, start, speed, isMoving = () => true) {
    const state = { distance: 0, lastTime: start.clone() };
    viewer.clock.onTick.addEventListener((clock) => {
@@ -1230,6 +1342,12 @@
        "Trees: " + Number(stats.trees || 0),
        "Road layer source: " + (metadata.source_geojson ? "osm2streets" : "OSM fallback")
      ];
+      if (cruise.simulation) {
+        const simulation = cruise.simulation.diagnostics();
+        lines.push("Simulation: native-preview-traffic-simulation/v1");
+        lines.push("Stopped: " + simulation.stoppedVehicles);
+        lines.push("Queue: " + simulation.queueLength);
+      }
      if (failed.length) {
        lines.push("Failed assets: " + failed.map((asset) => asset.url).join(", "));
      }
--- a/scripts/lib/native-preview-traffic-simulation.js
+++ b/scripts/lib/native-preview-traffic-simulation.js
@@ -0,0 +1,259 @@
+"use strict";
+
+const crypto = require("crypto");
+const fs = require("fs");
+const path = require("path");
+
+const SCHEMA = "native-preview-traffic-simulation/v1";
+const MAX_ROUTES = 5;
+const MIN_ROAD_COUNT = 3;
+const MAX_ROAD_COUNT = 7;
+const MAX_CONNECTION_GAP_METERS = 35;
+const DEFAULT_SETTINGS = {
+  desiredSpeedMetersPerSecond: 8,
+  accelerationMetersPerSecondSquared: 1.8,
+  decelerationMetersPerSecondSquared: 3.5,
+  reactionTimeSeconds: 0.8,
+  vehicleLengthMeters: 4.6,
+  minimumGapMeters: 7,
+};
+
+function buildNativeTrafficSimulation(area, options = {}) {
+  const root = area.outputs.nativeRoadDir;
+  const compiledPath = path.join(root, "compiled.json");
+  const signalPath = path.join(root, "traffic-signals.json");
+  const connectorPath = path.join(root, "layers", "connectors.geojson");
+  const laneCenterlinePath = path.join(root, "layers", "lane_centerlines.geojson");
+  const stopLinePath = path.join(root, "layers", "vehicle_stop_lines.geojson");
+  for (const file of [compiledPath, signalPath, connectorPath, stopLinePath]) {
+    if (!fs.existsSync(file)) throw new Error(`Native traffic simulation input not found: ${file}`);
+  }
+  const compiled = readJson(compiledPath, "native compiled roads");
+  const signals = readJson(signalPath, "native traffic signals");
+  const connectors = readFeatureCollection(connectorPath, "native connectors");
+  const laneCenterlines = fs.existsSync(laneCenterlinePath) ? readFeatureCollection(laneCenterlinePath, "native lane centerlines") : null;
+  const stopLines = readFeatureCollection(stopLinePath, "native stop lines");
+  const settings = { ...DEFAULT_SETTINGS, ...(options.settings || {}) };
+  validateSettings(settings);
+  const routeBuild = buildRoutes(compiled, connectors, signals.signals || [], stopLines.features, laneCenterlines);
+  return {
+    schema: SCHEMA,
+    areaId: area.id,
+    coordinateSystem: { route: "WGS84", model: "ENU", units: "meters", axes: "X east / Y north / Z up" },
+    generatedAt: new Date().toISOString(),
+    speedMetersPerSecond: settings.desiredSpeedMetersPerSecond,
+    source: sourceRecords(area, [compiledPath, signalPath, connectorPath, stopLinePath, ...(laneCenterlines ? [laneCenterlinePath] : [])]),
+    settings,
+    routes: routeBuild.routes,
+    signals: (signals.signals || []).filter((signal) => signal && signal.id).map(signalDescriptor),
+    diagnostics: [
+      ...routeBuild.diagnostics,
+      ...(routeBuild.routes.length ? [] : [{ severity: "warning", reason: "no_native_cruise_route", message: "No deterministic native route crossed three or more directional roads." }]),
+    ],
+    rejectedRoutes: routeBuild.rejectedRoutes,
+    migration: {
+      schema: SCHEMA,
+      updateLoop: "advance route distance using settings, stop before red/yellow signal, then apply leader minimum gap",
+      signalIdentity: "signals[].id is the existing traffic runtime signal_uid",
+      coordinates: "routes[].coordinates are WGS84; static package remains local ENU placed by package manifest",
+      legacyDependency: false,
+    },
+  };
+}
+
+function buildRoutes(compiled, connectorCollection, signals, stopLineFeatures, laneCenterlineCollection) {
+  const roads = new Map((compiled.model?.roads || []).map((road) => [road.id, road]));
+  const movements = (compiled.movements || []).filter((movement) => movement.geometryPublished !== false && roads.has(movement.fromRoadId) && roads.has(movement.toRoadId));
+  const connectorByMovement = new Map(connectorCollection.features.map((feature) => [feature.properties?.movement_id, feature.geometry?.coordinates || []]));
+  const laneCenterlineById = new Map();
+  for (const feature of laneCenterlineCollection?.features || []) {
+    const properties = feature.properties || {};
+    const coordinates = feature.geometry?.coordinates || [];
+    if (properties.native_id) laneCenterlineById.set(properties.native_id, coordinates);
+    if (properties.road_id && Number.isFinite(Number(properties.lane_index))) laneCenterlineById.set(`${properties.road_id}:${properties.lane_index}`, coordinates);
+  }
+  const outgoing = new Map();
+  for (const movement of movements) {
+    if (!outgoing.has(movement.fromRoadId)) outgoing.set(movement.fromRoadId, []);
+    outgoing.get(movement.fromRoadId).push(movement);
+  }
+  for (const list of outgoing.values()) list.sort((a, b) => a.id.localeCompare(b.id));
+  const candidates = [];
+  const seen = new Set();
+  for (const start of [...roads.keys()].sort()) {
+    findCycles(start, start, [], [], outgoing, seen, candidates);
+    if (candidates.length >= MAX_ROUTES * 3) break;
+  }
+  const diagnostics = [];
+  const rejectedRoutes = [];
+  const routes = candidates
+    .map((candidate) => {
+      const result = makeRoute(candidate, roads, connectorByMovement, signals, stopLineFeatures, laneCenterlineById);
+      if (!result.route) {
+        rejectedRoutes.push({ id: `native-loop:${candidate.roadIds.join("-")}`, diagnostics: result.diagnostics });
+        diagnostics.push(...result.diagnostics);
+      }
+      return result.route;
+    })
+    .filter(Boolean)
+    .sort((a, b) => a.id.localeCompare(b.id))
+    .slice(0, MAX_ROUTES);
+  return { routes, diagnostics, rejectedRoutes };
+}
+
+function findCycles(start, current, roadIds, movements, outgoing, seen, candidates) {
+  const nextRoadIds = [...roadIds, current];
+  if (nextRoadIds.length > MAX_ROAD_COUNT) return;
+  for (const movement of outgoing.get(current) || []) {
+    if (movement.toRoadId === start && nextRoadIds.length >= MIN_ROAD_COUNT) {
+      if (!laneSequenceCompatible(movements.at(-1), movement) || !laneSequenceCompatible(movement, movements[0])) continue;
+      const signature = [...nextRoadIds, start].join("|");
+      if (!seen.has(signature)) {
+        seen.add(signature);
+        candidates.push({ roadIds: nextRoadIds, movements: [...movements, movement] });
+      }
+      continue;
+    }
+    if (nextRoadIds.includes(movement.toRoadId)) continue;
+    if (!laneSequenceCompatible(movements.at(-1), movement)) continue;
+    findCycles(start, movement.toRoadId, nextRoadIds, [...movements, movement], outgoing, seen, candidates);
+  }
+}
+
+function laneSequenceCompatible(previous, next) {
+  if (!previous || !previous.toLaneId || !next?.fromLaneId) return true;
+  return previous.toLaneId === next.fromLaneId;
+}
+
+function makeRoute(candidate, roads, connectorByMovement, signals, stopLineFeatures, laneCenterlineById) {
+  const coordinates = [];
+  const connectors = [];
+  const diagnostics = [];
+  for (let index = 0; index < candidate.roadIds.length; index += 1) {
+    const road = roads.get(candidate.roadIds[index]);
+    const incomingMovement = candidate.movements[(index - 1 + candidate.movements.length) % candidate.movements.length];
+    const incomingLine = selectLaneCenterline(laneCenterlineById, road, incomingMovement?.toLaneId);
+    const roadLine = incomingLine || road.centerline;
+    append(coordinates, roadLine);
+    const movement = candidate.movements[index];
+    const connector = connectorByMovement.get(movement.id) || [];
+    if (connector.length < 2) {
+      diagnostics.push(routeDiagnostic(candidate, movement, "missing_connector_geometry", "Native movement has no usable connector geometry."));
+      return { route: null, diagnostics };
+    }
+    const startGap = distanceMeters(coordinates.at(-1), connector[0]);
+    if (startGap > MAX_CONNECTION_GAP_METERS) {
+      diagnostics.push(routeDiagnostic(candidate, movement, "road_connector_gap", `Road to connector endpoint gap is ${round(startGap)}m.`));
+      return { route: null, diagnostics };
+    }
+    append(coordinates, connector);
+    if (index < candidate.roadIds.length - 1) {
+      const nextRoad = roads.get(candidate.roadIds[index + 1]);
+      const nextLine = selectLaneCenterline(laneCenterlineById, nextRoad, movement.toLaneId) || nextRoad.centerline;
+      const endGap = distanceMeters(connector.at(-1), nextLine[0]);
+      if (endGap > MAX_CONNECTION_GAP_METERS) {
+        diagnostics.push(routeDiagnostic(candidate, movement, "connector_road_gap", `Connector to road endpoint gap is ${round(endGap)}m.`));
+        return { route: null, diagnostics };
+      }
+    }
+    connectors.push({ id: movement.connectorId, movementId: movement.id, fromRoadId: movement.fromRoadId, toRoadId: movement.toRoadId, turn: movement.turn });
+  }
+  if (coordinates.length < 4) return { route: null, diagnostics };
+  const distances = cumulativeDistances(coordinates);
+  const lengthMeters = distances.at(-1);
+  if (!(lengthMeters > 30)) return { route: null, diagnostics };
+  const stops = matchStops(coordinates, distances, signals, stopLineFeatures);
+  return { route: {
+    id: `native-loop:${candidate.roadIds.join("-")}`,
+    coordinates,
+    centerlineCoordinates: coordinates,
+    distances,
+    lengthMeters: round(lengthMeters),
+    edgeIds: candidate.roadIds,
+    laneSegments: candidate.roadIds.map((roadId) => ({ roadId, laneId: `lane:${roadId}:1` })),
+    connectors,
+    maneuvers: connectors.map((connector) => connector.turn).filter(Boolean),
+    stops,
+  }, diagnostics };
+}
+
+function selectLaneCenterline(laneCenterlineById, road, laneId) {
+  if (!laneCenterlineById || !road) return null;
+  if (laneId && laneCenterlineById.has(laneId)) return laneCenterlineById.get(laneId);
+  const fallback = laneCenterlineById.get(`${road.id}:1`);
+  return fallback || null;
+}
+
+function routeDiagnostic(candidate, movement, reason, message) {
+  return { severity: "warning", reason, message, routeRoadIds: candidate.roadIds, movementId: movement.id, fromRoadId: movement.fromRoadId, toRoadId: movement.toRoadId };
+}
+
+function matchStops(coordinates, distances, signals, stopLineFeatures) {
+  const stopRoadIds = new Set(stopLineFeatures.map((feature) => feature.properties?.road_id).filter(Boolean));
+  return signals.map((signal) => {
+    if (!Number.isFinite(signal.stopLongitude) || !Number.isFinite(signal.stopLatitude)) return null;
+    let best = { distance: Infinity, routeDistance: 0 };
+    for (let index = 0; index < coordinates.length; index += 1) {
+      const distance = haversine(coordinates[index], [signal.stopLongitude, signal.stopLatitude]);
+      if (distance < best.distance) best = { distance, routeDistance: distances[index] };
+    }
+    if (best.distance > 15 || !stopRoadIds.size) return null;
+    return { signalId: signal.id, phaseGroup: signal.phaseGroup, distance: round(best.routeDistance), matchDistanceMeters: round(best.distance), stopReason: "traffic-signal" };
+  }).filter(Boolean).sort((a, b) => a.distance - b.distance);
+}
+
+function signalDescriptor(signal) {
+  return { id: signal.id, phaseGroup: signal.phaseGroup, approachId: signal.approachId, sourceWayId: signal.sourceWayId, stopLongitude: signal.stopLongitude, stopLatitude: signal.stopLatitude, enabled: true };
+}
+
+function sourceRecords(area, files) {
+  return files.map((file) => ({ path: path.relative(area.outputs.areaDir, file).split(path.sep).join("/"), bytes: fs.statSync(file).size, sha256: sha256(file) }));
+}
+
+function append(target, points) {
+  for (const point of points || []) {
+    const coordinate = point.slice(0, 2).map(Number);
+    if (!coordinate.every(Number.isFinite)) continue;
+    const previous = target.at(-1);
+    if (!previous || previous[0] !== coordinate[0] || previous[1] !== coordinate[1]) target.push(coordinate);
+  }
+}
+
+function cumulativeDistances(coordinates) {
+  const distances = [0];
+  for (let index = 1; index < coordinates.length; index += 1) distances.push(distances[index - 1] + haversine(coordinates[index - 1], coordinates[index]));
+  return distances;
+}
+
+function haversine(a, b) {
+  const radians = Math.PI / 180;
+  const dLat = (b[1] - a[1]) * radians;
+  const dLon = (b[0] - a[0]) * radians;
+  const lat1 = a[1] * radians;
+  const lat2 = b[1] * radians;
+  const value = Math.sin(dLat / 2) ** 2 + Math.cos(lat1) * Math.cos(lat2) * Math.sin(dLon / 2) ** 2;
+  return 6371008.8 * 2 * Math.atan2(Math.sqrt(value), Math.sqrt(1 - value));
+}
+
+function distanceMeters(a, b) {
+  return a && b ? haversine(a, b) : Infinity;
+}
+
+function readJson(file, label) {
+  try { return JSON.parse(fs.readFileSync(file, "utf8")); } catch (error) { throw new Error(`Invalid ${label} JSON '${file}': ${error.message}`); }
+}
+
+function readFeatureCollection(file, label) {
+  const value = readJson(file, label);
+  if (value?.type !== "FeatureCollection" || !Array.isArray(value.features)) throw new Error(`Invalid ${label} '${file}': expected FeatureCollection`);
+  return value;
+}
+
+function validateSettings(settings) {
+  for (const [key, value] of Object.entries(settings)) if (!Number.isFinite(value) || value <= 0) throw new Error(`Native traffic simulation setting '${key}' must be positive`);
+}
+
+function sha256(file) { return crypto.createHash("sha256").update(fs.readFileSync(file)).digest("hex"); }
+function round(value) { return Math.round(value * 100) / 100; }
+
+module.exports = { SCHEMA, DEFAULT_SETTINGS, buildNativeTrafficSimulation };
--- a/scripts/lib/native-road.js
+++ b/scripts/lib/native-road.js
@@ -267,7 +267,7 @@ function compileGeometry(model, overrides = { overrides: [] }, options = {}) {
  const centerLines = compileCenterLines(model, overrides, junctionPlans, controls, diagnostics);
  const markings = compileLaneMarkings(model, overrides, lanes, diagnostics, junctionPlans, controls);
  const sidewalks = compileSidewalkSurfaces(model, diagnostics, junctionPlans);
-  const connectorResult = compileConnectors(model, lanes, diagnostics, overrides);
+  const connectorResult = compileConnectors(model, lanes, diagnostics, overrides, junctionPlans);
  const junctionFeatures = compileJunctionSurfaces(model, junctionPlans, connectorResult.features, connectorResult.movements, diagnostics);
  validateConnectorContainment(connectorResult.features, junctionFeatures, diagnostics);
  return { roadSurface: { type: "FeatureCollection", features }, edgeLines: { type: "FeatureCollection", features: edgeLines }, sidewalkSurface: { type: "FeatureCollection", features: sidewalks }, intersectionSurface: { type: "FeatureCollection", features: junctionFeatures }, laneCenterlines: { type: "FeatureCollection", features: lanes.features }, laneSeparators: { type: "FeatureCollection", features: markings.separators }, centerLines: { type: "FeatureCollection", features: centerLines }, directionArrows: { type: "FeatureCollection", features: markings.directionArrows }, turnArrows: { type: "FeatureCollection", features: markings.turnArrows }, crosswalks: { type: "FeatureCollection", features: controls.crosswalks }, vehicleStopLines: { type: "FeatureCollection", features: controls.stopLines }, connectors: { type: "FeatureCollection", features: connectorResult.features }, movements: connectorResult.movements, diagnostics };
@@ -653,7 +653,7 @@ function compileLaneCenterlines(model, diagnostics, junctionPlans) {
  return { features, byRoadId };
 }

-function compileConnectors(model, lanes, diagnostics, overrides) {
+function compileConnectors(model, lanes, diagnostics, overrides, junctionPlans) {
  const features = [];
  const movements = [];
  for (const connection of model.connections.filter((item) => item.enabled)) {
@@ -669,8 +669,12 @@ function compileConnectors(model, lanes, diagnostics, overrides) {
      const override = laneOverride(overrides, defaultFromLane.id, defaultToLane.id);
      if ((!laneAllowsTurn(fromRoad, index, turn) && override?.enabled !== true) || override?.enabled === false) continue;
      const from = defaultFromLane.coordinates.at(-1); const to = defaultToLane.coordinates[0];
-      const control = connectorControlPoint(model, connection, from, to);
-      const coordinates = quadraticCurve(from, control, to, 12);
+      const plan = junctionPlans.get(connection.nodeId);
+      // Cross intersections retain the earlier center-node curve while T junctions
+      // use lane tangents so their through movement does not bow toward the stem.
+      const coordinates = plan?.segmentIds.size === 4
+        ? quadraticCurve(from, endpointCoordinate(model, connection.fromEndpointId), to, 12)
+        : connectorCurve(defaultFromLane.coordinates, defaultToLane.coordinates, turn);
      const length = lineLengthMeters(coordinates);
      const id = `movement:${connection.id}:${defaultFromLane.id}->${defaultToLane.id}`;
      const provenance = override ? `override:${override.id}` : connection.provenance;
@@ -712,12 +716,47 @@ function targetLaneIndex(turn, sourceIndex, sourceCount, targetCount) {
  if (turn === "uturn") return 0;
  return Math.min(targetCount - 1, Math.round(sourceIndex / Math.max(1, sourceCount - 1) * Math.max(0, targetCount - 1)));
 }
-function connectorControlPoint(model, connection, from, to) {
-  const node = endpointCoordinate(model, connection.fromEndpointId);
-  if (!node) return [(from[0] + to[0]) / 2, (from[1] + to[1]) / 2];
-  // Nearby manual joins may not share exactly the same point. The midpoint
-  // keeps their curve smooth without rewriting the authoritative OSM geometry.
-  return node;
+function connectorCurve(incoming, outgoing, turn) {
+  const start = incoming.at(-1);
+  const end = outgoing[0];
+  if (turn === "through") return lineCurve(start, end, 12);
+  const incomingHeading = headingDegrees(incoming.at(-2), start);
+  const outgoingHeading = headingDegrees(end, outgoing[1]);
+  const chord = distanceMeters(start, end);
+  const incomingSpan = distanceMeters(incoming.at(-2), start);
+  const outgoingSpan = distanceMeters(end, outgoing[1]);
+  const tangentIntersection = intersectTangentRays(start, end, incomingHeading, outgoingHeading);
+  const fallbackDistance = Math.min(8, Math.max(.75, Math.min(chord * .42, incomingSpan * .8, outgoingSpan * .8)));
+  const firstDistance = tangentIntersection && tangentIntersection.incoming >= 0 ? Math.min(tangentIntersection.incoming, Math.min(8, Math.max(.75, incomingSpan * 2.4))) / 3 : fallbackDistance;
+  const secondDistance = tangentIntersection && tangentIntersection.outgoing >= 0 ? Math.min(tangentIntersection.outgoing, Math.min(8, Math.max(.75, outgoingSpan * 2.4))) / 3 : fallbackDistance;
+  const firstControl = offsetCoordinate(start, incomingHeading, firstDistance);
+  const secondControl = offsetCoordinate(end, outgoingHeading + 180, secondDistance);
+  return cubicBezier(start, firstControl, secondControl, end, 12);
+}
+
+function intersectTangentRays(start, end, incomingHeading, outgoingHeading) {
+  const incoming = headingVector(incomingHeading);
+  const outgoing = headingVector(outgoingHeading);
+  const delta = project(end, start);
+  const cross = incoming[0] * outgoing[1] - incoming[1] * outgoing[0];
+  if (Math.abs(cross) < 1e-6) return null;
+  return {
+    incoming: (delta[0] * outgoing[1] - delta[1] * outgoing[0]) / cross,
+    outgoing: (delta[0] * incoming[1] - delta[1] * incoming[0]) / cross,
+  };
+}
+
+function lineCurve(start, end, segments) {
+  return Array.from({ length: segments + 1 }, (_, index) => interpolate(start, end, index / segments));
+}
+
+function cubicBezier(a, firstControl, secondControl, b, segments) {
+  const result = [];
+  for (let index = 0; index <= segments; index += 1) {
+    const t = index / segments; const u = 1 - t;
+    result.push([u ** 3 * a[0] + 3 * u * u * t * firstControl[0] + 3 * u * t * t * secondControl[0] + t ** 3 * b[0], u ** 3 * a[1] + 3 * u * u * t * firstControl[1] + 3 * u * t * t * secondControl[1] + t ** 3 * b[1]]);
+  }
+  return result;
 }

 function quadraticCurve(a, control, b, segments) {
--- a/scripts/test-asset-budgets.js
+++ b/scripts/test-asset-budgets.js
@@ -16,7 +16,8 @@ assert.match(qgisBuildSource, /QgsFieldConstraints\.Constraint\.ConstraintNotNul
 assert.match(qgisBuildSource, /QgsFieldConstraints\.ConstraintNotNull/);
 assert.doesNotMatch(qgisBuildSource, /setFieldConstraint\(index, 1\)/);
 assert.match(areaBuildSource, /exportCesium\(area, roadProvider\)/);
-assert.match(areaBuildSource, /if \(roadProvider === "osm2streets"\) \{\n    exporterArgs\.splice/);
+assert.match(areaBuildSource, /"--dynamic-glb", area\.outputs\.trafficSignalsDynamicGlb/);
+assert.match(areaBuildSource, /trafficSignalsCountdown0Glb: fileRecord/);
 assert.match(areaBuildSource, /if \(roadProvider === "osm2streets"\) \{[\s\S]*?buildPreviewVehicleRoute/);
 assert.match(areaBuildSource, /Object\.assign\(previewInputs, nativeRoadRecords\(area\)\)/);
 assert.match(areaBuildSource, /vehicleRoute: optionalFileRecord\(area\.outputs\.vehicleRoute\)/);
@@ -56,6 +57,10 @@ assert.equal(
  normalizeAreaConfig(base).outputs.trafficSignalAssemblies,
  path.join(tempDir, "test-area", "osm2streets_web_out", "traffic_signal_assemblies.geojson"),
 );
+assert.equal(
+  normalizeAreaConfig(base).outputs.trafficSimulation,
+  path.join(tempDir, "test-area", "_preview", "test-area-traffic-simulation.json"),
+);
 assert.equal(normalizeAreaConfig({ ...base, budget: { nodes: 800 } }).budget.glbNodes, 800);
 assert.equal(normalizeAreaConfig(base).stages.intermediates, false);
 assert.equal(normalizeAreaConfig(base).blender.roadProvider, "native");
--- a/scripts/test-native-preview-traffic.js
+++ b/scripts/test-native-preview-traffic.js
@@ -0,0 +1,37 @@
+#!/usr/bin/env node
+"use strict";
+
+const assert = require("assert");
+const fs = require("fs");
+const os = require("os");
+const path = require("path");
+const { SCHEMA, buildNativeTrafficSimulation } = require("./lib/native-preview-traffic-simulation");
+
+const areaDir = fs.mkdtempSync(path.join(os.tmpdir(), "native-preview-traffic-"));
+const nativeRoadDir = path.join(areaDir, "native-road");
+const layersDir = path.join(nativeRoadDir, "layers");
+fs.mkdirSync(layersDir, { recursive: true });
+const roads = [
+  { id: "road:a", centerline: [[120, 30], [120.001, 30]], sourceNodeIds: ["a", "b"] },
+  { id: "road:b", centerline: [[120.001, 30], [120.001, 30.001]], sourceNodeIds: ["b", "c"] },
+  { id: "road:c", centerline: [[120.001, 30.001], [120, 30.001]], sourceNodeIds: ["c", "d"] },
+];
+const movements = [
+  ["road:a", "road:b", "left"], ["road:b", "road:c", "through"], ["road:c", "road:a", "right"],
+].map(([fromRoadId, toRoadId, turn], index) => ({ id: `movement:${index}`, connectorId: `connector:${index}`, fromRoadId, toRoadId, turn, geometryPublished: true }));
+fs.writeFileSync(path.join(nativeRoadDir, "compiled.json"), JSON.stringify({ model: { roads }, movements }));
+fs.writeFileSync(path.join(nativeRoadDir, "traffic-signals.json"), JSON.stringify({ signals: [{ id: "osm-signal-1", phaseGroup: 0, approachId: "way:a", sourceWayId: "a", stopLongitude: 120.0009, stopLatitude: 30, enabled: true }] }));
+fs.writeFileSync(path.join(layersDir, "connectors.geojson"), JSON.stringify({ type: "FeatureCollection", features: movements.map((movement, index) => ({ type: "Feature", properties: { movement_id: movement.id }, geometry: { type: "LineString", coordinates: [[[120.001, 30], [120.001, 30.0001]], [[120.001, 30.001], [120.0009, 30.001]], [[120, 30.001], [120, 30.0009]]][index] } })) }));
+fs.writeFileSync(path.join(layersDir, "vehicle_stop_lines.geojson"), JSON.stringify({ type: "FeatureCollection", features: [{ type: "Feature", properties: { road_id: "road:a" }, geometry: { type: "Polygon", coordinates: [] } }] }));
+fs.writeFileSync(path.join(layersDir, "lane_centerlines.geojson"), JSON.stringify({ type: "FeatureCollection", features: roads.map((road) => ({ type: "Feature", properties: { road_id: road.id, lane_index: 1 }, geometry: { type: "LineString", coordinates: road.centerline } })) }));
+
+const descriptor = buildNativeTrafficSimulation({ id: "fixture", outputs: { areaDir, nativeRoadDir } });
+assert.equal(descriptor.schema, SCHEMA);
+assert.equal(descriptor.coordinateSystem.route, "WGS84");
+assert.ok(descriptor.routes.length >= 1);
+assert.ok(descriptor.routes.some((route) => route.connectors.length === 3));
+assert.ok(descriptor.routes.some((route) => route.stops.some((stop) => stop.signalId === "osm-signal-1")));
+assert.equal(descriptor.migration.legacyDependency, false);
+assert.equal(descriptor.source.length, 5);
+fs.rmSync(areaDir, { recursive: true, force: true });
+console.log("Native preview traffic tests passed.");
--- a/scripts/test-native-road.js
+++ b/scripts/test-native-road.js
@@ -148,6 +148,14 @@ assert.equal(sharedInteriorGeometry.intersectionSurface.features.length, 1);
 assert.equal(sharedInteriorGeometry.intersectionSurface.features[0].properties.osm_node_id, "2");
 assert.equal(sharedInteriorGeometry.intersectionSurface.features[0].properties.kind, "t");
 assert.ok(sharedInteriorGeometry.connectors.features.length >= 4);
+const throughConnector = sharedInteriorGeometry.connectors.features.find((feature) => feature.properties.turn === "through");
+assert.ok(throughConnector, "T junction emits a through connector");
+const throughCoordinates = throughConnector.geometry.coordinates;
+const throughStart = throughCoordinates[0]; const throughEnd = throughCoordinates.at(-1);
+for (const point of throughCoordinates.slice(1, -1)) {
+  const area = Math.abs((throughEnd[0] - throughStart[0]) * (point[1] - throughStart[1]) - (throughEnd[1] - throughStart[1]) * (point[0] - throughStart[0]));
+  assert.ok(area < 1e-12, "through connector stays on its lane-to-lane chord instead of bending through the junction node");
+}
 assert.ok(sharedInteriorGeometry.sidewalkSurface.features.some((feature) => feature.properties.kind === "continuation" && /segment:way\/50\/1:.*->segment:way\/50\/2:/.test(feature.properties.native_id)));
 const connection = initial.connections[0];
 assert.ok(initial.connections.every((item) => item.fromEndpointId.endsWith(":end") && item.toEndpointId.endsWith(":start")));
--- a/scripts/test-preview-assets.js
+++ b/scripts/test-preview-assets.js
@@ -278,6 +278,9 @@ assert.match(previewRuntime, /\(\) => record\?\.status === "normal"/);
 assert.match(previewRuntime, /Cesium\.SceneTransforms\.worldToWindowCoordinates/);
 assert.match(previewRuntime, /TrafficSignalDynamic_/);
 assert.match(previewRuntime, /TrafficSignalDynamic_\$\{nodeKey\}_countdown_\$\{String\(value\)\.padStart\(2, "0"\)\}/);
+assert.match(previewRuntime, /native-preview-traffic-simulation\/v1/);
+assert.match(previewRuntime, /leader-gap/);
+assert.match(previewRuntime, /stopReason = "traffic-signal"/);
 assert.match(previewRuntime, /ColorBlendMode\.REPLACE/);
 assert.match(previewRuntime, /setBuildingGhost/);
 assert.match(previewRuntime, /fetch\(url, \{ cache: "no-store" \}\)/);
				`@@ -0,0 +1 @@`
				{"_example": "Fill with {\"file\": \"<path>\", \"reason\": \"<why>\"}. Put spec/research files only — no code paths. Run `python3 .trellis/scripts/get_context.py --mode packages` to list available specs. Delete this line once real entries are added."}