Phase 1: road network and heat

Roads are a jittered 7x7 grid with ~a quarter of the edges thinned out, kept
connected. The loops are the point: "take a different road" is not a decision
without alternative routes.

Every metre driven adds heat to that segment; all roads shed it slowly. Crossing
a threshold escalates Clear -> Patrol -> Barricade -> Turret, each of which puts
real obstructions on the tarmac. Hysteresis stops a road on a boundary from
rebuilding its barricade every few seconds.

Level changes are deferred while the car is on the segment. Building a barricade
around the player would spawn a static collider inside the chassis, and the
player is meant to discover escalation by returning to a road, not by watching
it assemble behind them.

Checkpoint positions are seeded from the segment id, so a stretch of road always
fortifies in the same place — recognising it is part of learning the map.

Scenery now avoids the tarmac and the car spawns at a junction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -1,8 +1,9 @@
-# Drive Between the Lines — Phase 0 prototype
+# Drive Between the Lines — prototype
 
-Endless driving survival game. This is the **Phase 0** skeleton from the roadmap:
+Endless driving survival game.
-a drivable car on a flat plate with scattered obstacles, plus the persistent wear
+
-system that makes condition felt through the wheel.
+- **Phase 0** — a drivable car with persistent wear that is felt through the wheel.
+- **Phase 1** — a road network whose roads remember being driven, and escalate.
 
 ## Running
 
@@ -25,6 +26,38 @@ npm test        # sim + headless physics
 npm run build   # typecheck + production bundle
 ```
 
+## Road heat
+
+Every metre driven on a road adds heat to that segment; every road everywhere
+sheds it slowly. Cross a threshold and the road escalates:
+
+`Clear → Patrol → Barricade → Turret`
+
+- **Patrol** parks a vehicle on the verge. The road narrows.
+- **Barricade** puts concrete across it, leaving a gap you have to slow for.
+- **Turret** adds a tower overlooking the checkpoint.
+
+Roughly four traversals take a road from clear to turret; an untouched road
+cools off in about four minutes. Both numbers are in `sim/heat.ts` and both are
+guesses meant to be tuned.
+
+Three things are deliberate:
+
+- **No AI yet.** The props are stationary hazards. Escalation currently means the
+  road gets slower and more expensive to get wrong, which is enough to test the
+  phase's real question.
+- **Changes never happen on the road you are on.** They are deferred until you
+  leave. Otherwise a barricade would spawn a static collider inside your car —
+  and you are supposed to discover escalation by *coming back*, not by watching
+  it assemble behind you.
+- **A checkpoint's position is seeded from its segment**, so a given stretch of
+  road always fortifies in the same place. Recognising it is part of learning
+  the map.
+
+The road network is a jittered grid with roughly a quarter of its edges removed,
+keeping the whole thing connected. The loops are the point: "take a different
+road this time" is not a decision unless alternative routes exist.
+
 ## Layout
 
 The one rule worth keeping: **`src/sim/` imports neither three.js nor Rapier.**
@@ -35,14 +68,20 @@ ever opening a browser.
 
 ```
 src/
-  sim/        pure model — world generation, car condition → handling
+  sim/          pure model — roads, heat, world generation, condition → handling
-  physics/    Rapier world, raycast vehicle, input → wheel forces
+  physics/      Rapier world, raycast vehicle, input → wheel forces
-  render/     three.js scene, chase camera
+  render/       three.js scene, roads, chase camera
-  core/       fixed-timestep loop, seeded RNG, keyboard
+  core/         fixed-timestep loop, seeded RNG, keyboard
-  ui/         debug HUD
+  ui/           debug HUD
-  carSpec.ts  shared car dimensions, so body and mesh cannot drift apart
+  carSpec.ts    shared car dimensions, so body and mesh cannot drift apart
+  heatProps.ts  integration layer: heat levels → colliders + meshes
 ```
 
+`heatProps.ts` sits at the top level on purpose — it is the one module allowed to
+touch both Rapier and three.js, because it owns objects that must exist in both
+or neither. Its tests run headless: three.js scene graphs work fine in Node, so
+"the barricade's collider was removed along with its mesh" is a unit test.
+
 Data flows one way: `sim` → `physics` → `render`. Condition reaches the physics
 layer already digested into a `Handling` by `deriveHandling`, so there is exactly
 one place where "how broken the car is" turns into "how it drives".
@@ -56,8 +95,13 @@ one place where "how broken the car is" turns into "how it drives".
   rather than hours. Turn them down in `applyWear` once the curve reads right.
 - **No interpolation** between physics steps. Fine at 60 Hz; revisit if the step
   rate changes.
-- **Condition is not yet persisted** across reloads. IndexedDB comes with the
+- **Condition and heat are not yet persisted** across reloads. IndexedDB comes
-  campaign layer.
+  with the campaign layer.
+- **Heat has no diegetic signal at a distance.** You learn a road is hot by
+  arriving at the checkpoint. The brief wants it readable from patrol density and
+  wreckage before you commit — that needs the enemy presence Phase 5 brings.
+- **The heat HUD lines are debug scaffolding.** The brief is explicit that no
+  numeric heat meter ships; they exist to tune the curve and should come out.
 - **Bundle:** ~2.7 MB raw / ~945 KB gzipped, dominated by Rapier's WASM, which
   `rapier3d-compat` inlines as base64. Switching to the non-compat `@dimforge/rapier3d`
   package serves the WASM as a separate file (~570 KB gzipped, compiled in
@@ -66,5 +110,11 @@ one place where "how broken the car is" turns into "how it drives".
 
 ## Next
 
-Phase 0's gate: drive for 15–20 minutes, feel the condition changing how the car
+Phase 1's gate: **you catch yourself avoiding a road because of its history, not
-handles, and want to keep driving anyway. Everything else waits on that.
+its distance.** That cannot be checked by a test — it needs you driving the same
+routes for a while and noticing what you start doing.
+
+If it fails, the likely culprits, in order: escalation is too slow to matter
+within a session (`METRES_PER_HEAT`), decay is so fast that nothing accumulates
+(`DECAY_PER_SECOND`), or the props are not actually inconvenient enough to route
+around. Tune before building Phase 2 on top.

src/carSpec.ts

@@ -20,7 +20,8 @@ export const CAR = {
     offsetY: -0.15,
     suspensionRestLength: 0.32,
   },
-  spawn: { x: 0, y: 1.2, z: 0 },
+  /** Only the drop height; the horizontal spawn comes from the world model. */
+  spawn: { y: 1.2 },
 } as const;
 
 /** Wheel order used everywhere: front-left, front-right, rear-left, rear-right. */

src/heatProps.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, it } from 'vitest';
+import * as THREE from 'three';
+import { createHeatProps } from './heatProps';
+import { createPhysics } from './physics/physics';
+import { createHeat, stepHeat } from './sim/heat';
+import { generateWorld } from './sim/world';
+
+/**
+ * three.js scene graphs work without a browser (only WebGLRenderer needs one),
+ * so the prop lifecycle can be checked end to end — including that removing a
+ * barricade removes its collider, not just its mesh.
+ */
+describe('heat props lifecycle', () => {
+  it('builds and tears down colliders and meshes together', async () => {
+    const world = generateWorld(3, 0);
+    const physics = await createPhysics(world);
+    const scene = new THREE.Scene();
+    const props = createHeatProps(world.roads, physics, scene);
+    const heat = createHeat(world.roads);
+
+    const baseBodies = physics.rapier.bodies.len();
+    const baseMeshes = scene.children.length;
+
+    const drive = (metres: number) => {
+      for (let m = 0; m < metres; m++) {
+        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: 0, distance: 1 }), heat, 0);
+      }
+    };
+    const idle = (seconds: number) => {
+      for (let i = 0; i < seconds * 60; i++) {
+        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: null, distance: 0 }), heat, null);
+      }
+    };
+
+    drive(400);
+    expect(heat.level[0]).toBe('turret');
+    // Still on the road, so nothing has been built around the car yet.
+    expect(physics.rapier.bodies.len()).toBe(baseBodies);
+
+    // Leaving it lets the deferred work land.
+    idle(1);
+    const hotBodies = physics.rapier.bodies.len();
+    expect(hotBodies).toBeGreaterThan(baseBodies);
+    expect(scene.children.length - baseMeshes).toBe(hotBodies - baseBodies);
+
+    // Left alone, the road should give everything back.
+    idle(300);
+    expect(heat.level[0]).toBe('clear');
+    expect(physics.rapier.bodies.len()).toBe(baseBodies);
+    expect(scene.children.length).toBe(baseMeshes);
+  });
+
+  it('does not leak bodies when a road escalates through every level', async () => {
+    const world = generateWorld(3, 0);
+    const physics = await createPhysics(world);
+    const scene = new THREE.Scene();
+    const props = createHeatProps(world.roads, physics, scene);
+    const heat = createHeat(world.roads);
+    const baseBodies = physics.rapier.bodies.len();
+
+    for (let cycle = 0; cycle < 3; cycle++) {
+      for (let m = 0; m < 400; m++) {
+        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: 0, distance: 1 }), heat, 0);
+      }
+      for (let i = 0; i < 300 * 60; i++) {
+        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: null, distance: 0 }), heat, null);
+      }
+      expect(physics.rapier.bodies.len()).toBe(baseBodies);
+    }
+  });
+});

src/heatProps.ts

@@ -0,0 +1,87 @@
+/**
+ * Turns heat levels into things that physically exist on the road.
+ *
+ * This is an integration layer: it is allowed to touch both Rapier and three.js,
+ * which is why it sits outside src/sim/. The sim decides *what* should be there
+ * ({@link propsFor}); this only builds and tears it down.
+ */
+import * as THREE from 'three';
+import type RAPIER from '@dimforge/rapier3d-compat';
+import { propsFor, type HeatProp, type HeatState } from './sim/heat';
+import type { RoadNetwork } from './sim/roads';
+import type { PhysicsWorld } from './physics/physics';
+
+const MATERIALS: Record<HeatProp['kind'], THREE.MeshStandardMaterial> = {
+  patrol: new THREE.MeshStandardMaterial({ color: 0x4b5540, roughness: 0.7 }),
+  barricade: new THREE.MeshStandardMaterial({ color: 0x9aa0a4, roughness: 0.95 }),
+  tower: new THREE.MeshStandardMaterial({ color: 0x54493d, roughness: 0.9 }),
+};
+
+const BOX = new THREE.BoxGeometry(1, 1, 1);
+
+interface Placed {
+  bodies: RAPIER.RigidBody[];
+  meshes: THREE.Mesh[];
+}
+
+export function createHeatProps(
+  roads: RoadNetwork,
+  physics: PhysicsWorld,
+  scene: THREE.Scene,
+) {
+  const placed = new Map<number, Placed>();
+  /** Level changes waiting for the player to get off the road in question. */
+  const pending = new Set<number>();
+
+  const clear = (segmentId: number) => {
+    const existing = placed.get(segmentId);
+    if (!existing) return;
+    for (const body of existing.bodies) physics.removeBody(body);
+    for (const mesh of existing.meshes) scene.remove(mesh);
+    placed.delete(segmentId);
+  };
+
+  const build = (segmentId: number, heat: HeatState) => {
+    const segment = roads.segments[segmentId]!;
+    const props = propsFor(segment, heat.level[segmentId]!);
+    if (props.length === 0) return;
+
+    const entry: Placed = { bodies: [], meshes: [] };
+    for (const prop of props) {
+      entry.bodies.push(physics.addStaticBox(prop));
+
+      const mesh = new THREE.Mesh(BOX, MATERIALS[prop.kind]);
+      mesh.scale.set(prop.width, prop.height, prop.depth);
+      mesh.position.set(prop.x, prop.height / 2, prop.z);
+      mesh.rotation.y = prop.yaw;
+      mesh.castShadow = true;
+      mesh.receiveShadow = true;
+      scene.add(mesh);
+      entry.meshes.push(mesh);
+    }
+    placed.set(segmentId, entry);
+  };
+
+  return {
+    /**
+     * Rebuild the segments whose level changed — but never the one the car is
+     * currently on. A barricade appearing around the player would spawn a static
+     * collider inside the chassis, and watching a checkpoint assemble itself in
+     * the mirror would break the fiction anyway. Changes wait until they are
+     * out of sight, which is how the player is meant to find them: by returning.
+     *
+     * Call every step, not only when something changed, so deferred work drains.
+     */
+    sync(changedSegmentIds: readonly number[], heat: HeatState, occupiedSegmentId: number | null) {
+      for (const id of changedSegmentIds) pending.add(id);
+      if (pending.size === 0) return;
+
+      for (const id of [...pending]) {
+        if (id === occupiedSegmentId) continue;
+        pending.delete(id);
+        clear(id);
+        build(id, heat);
+      }
+    },
+  };
+}

src/main.ts

@@ -1,5 +1,8 @@
 import { generateWorld } from './sim/world';
 import { applyWear, deriveHandling, freshCondition } from './sim/car';
+import { createHeat, stepHeat } from './sim/heat';
+import { segmentAt } from './sim/roads';
+import { createHeatProps } from './heatProps';
 import { seedFromString } from './core/rng';
 import { startLoop } from './core/loop';
 import { createInput } from './core/input';
@@ -25,10 +28,13 @@ async function boot() {
   const input = createInput();
   const hud = createHud(seed);
   const driveState = createDriveState();
+  const heat = createHeat(model.roads);
+  const heatProps = createHeatProps(model.roads, physics, view.scene);
 
   let condition = freshCondition();
   let elapsed = 0;
   let respawnLatch = false;
+  let currentSegment: number | null = null;
 
   document.getElementById('boot')?.remove();
 
@@ -45,12 +51,18 @@ async function boot() {
 
       // Wear is applied from what actually happened this step, not from intent.
       const speed = physics.vehicle.currentVehicleSpeed();
+      const distance = Math.abs(speed) * dt;
       condition = applyWear(condition, {
         dt,
-        distance: Math.abs(speed) * dt,
+        distance,
         throttle: Math.abs(cmd.throttle),
         impactForce: physics.drainImpactForce(),
       });
+
+      // Heat: the road under the wheels remembers being used.
+      const at = physics.chassis.translation();
+      currentSegment = segmentAt(model.roads, at.x, at.z)?.id ?? null;
+      heatProps.sync(stepHeat(heat, { dt, segmentId: currentSegment, distance }), heat, currentSegment);
     },
 
     render(_alpha, frameDt) {
@@ -84,7 +96,12 @@ async function boot() {
       view.followSun();
       updateCamera(view, frameDt, physics.vehicle.currentVehicleSpeed());
       view.renderer.render(view.scene, view.camera);
-      hud.update(physics.vehicle.currentVehicleSpeed(), condition, elapsed);
+      hud.update(physics.vehicle.currentVehicleSpeed(), condition, elapsed, {
+        segmentId: currentSegment,
+        value: currentSegment === null ? 0 : heat.value[currentSegment]!,
+        level: currentSegment === null ? null : heat.level[currentSegment]!,
+        hottest: Math.max(...heat.value),
+      });
     },
   });
 }

src/physics/physics.test.ts

@@ -10,7 +10,9 @@ const IDLE: DriverInput = { throttle: 0, steer: 0, handbrake: false, respawn: fa
 
 /** Rapier runs headless, so vehicle tuning is checkable without a browser. */
 async function run(input: Partial<DriverInput>, seconds: number) {
-  const physics = await createPhysics(generateWorld(1, 0));
+  const world = generateWorld(1, 0);
+  const physics = await createPhysics(world);
+  const start = physics.chassis.translation();
   const state = createDriveState();
   const handling = deriveHandling(freshCondition());
   const cmd = { ...IDLE, ...input };
@@ -21,8 +23,10 @@ async function run(input: Partial<DriverInput>, seconds: number) {
     physics.step(STEP);
     maxYawRate = Math.max(maxYawRate, Math.abs(physics.chassis.angvel().y));
   }
+  const now = physics.chassis.translation();
   return {
-    pos: physics.chassis.translation(),
+    // Displacement from the spawn point, which is a road junction, not the origin.
+    pos: { x: now.x - start.x, y: now.y, z: now.z - start.z },
     speed: physics.vehicle.currentVehicleSpeed(),
     maxYawRate,
     grounded: [0, 1, 2, 3].every((i) => physics.vehicle.wheelIsInContact(i)),

src/physics/physics.ts

@@ -13,6 +13,18 @@ export interface PhysicsWorld {
   drainImpactForce(): number;
   step(dt: number): void;
   respawn(): void;
+  /** Static box, added and removed at runtime as road heat rises and falls. */
+  addStaticBox(box: StaticBox): RAPIER.RigidBody;
+  removeBody(body: RAPIER.RigidBody): void;
+}
+
+export interface StaticBox {
+  x: number;
+  z: number;
+  yaw: number;
+  width: number;
+  height: number;
+  depth: number;
 }
 
 /** Contacts weaker than this are just kerb-scrubbing, not damage. */
@@ -55,7 +67,7 @@ export async function createPhysics(model: WorldModel): Promise<PhysicsWorld> {
   // --- Car chassis ---
   const chassis = world.createRigidBody(
     RAPIER.RigidBodyDesc.dynamic()
-      .setTranslation(CAR.spawn.x, CAR.spawn.y, CAR.spawn.z)
+      .setTranslation(model.spawn.x, CAR.spawn.y, model.spawn.z)
       .setLinearDamping(0.1)
       .setAngularDamping(0.4)
       // The mass comes from here, not from collider density, so the centre of mass
@@ -127,8 +139,25 @@ export async function createPhysics(model: WorldModel): Promise<PhysicsWorld> {
       return v;
     },
 
+    addStaticBox(box) {
+      const body = world.createRigidBody(
+        RAPIER.RigidBodyDesc.fixed()
+          .setTranslation(box.x, box.height / 2, box.z)
+          .setRotation({ x: 0, y: Math.sin(box.yaw / 2), z: 0, w: Math.cos(box.yaw / 2) }),
+      );
+      world.createCollider(
+        RAPIER.ColliderDesc.cuboid(box.width / 2, box.height / 2, box.depth / 2).setFriction(0.8),
+        body,
+      );
+      return body;
+    },
+
+    removeBody(body) {
+      world.removeRigidBody(body);
+    },
+
     respawn() {
-      chassis.setTranslation({ x: CAR.spawn.x, y: CAR.spawn.y, z: CAR.spawn.z }, true);
+      chassis.setTranslation({ x: model.spawn.x, y: CAR.spawn.y, z: model.spawn.z }, true);
       chassis.setRotation({ x: 0, y: 0, z: 0, w: 1 }, true);
       chassis.setLinvel({ x: 0, y: 0, z: 0 }, true);
       chassis.setAngvel({ x: 0, y: 0, z: 0 }, true);

src/render/scene.ts

@@ -60,6 +60,27 @@ export function createScene(model: WorldModel): SceneView {
   grid.position.y = 0.02;
   scene.add(grid);
 
+  // --- Roads ---
+  const roadMat = new THREE.MeshStandardMaterial({ color: 0x23262a, roughness: 1 });
+  const quad = new THREE.PlaneGeometry(1, 1).rotateX(-Math.PI / 2);
+  for (const s of model.roads.segments) {
+    const strip = new THREE.Mesh(quad, roadMat);
+    strip.scale.set(s.width, 1, s.length);
+    strip.position.set((s.ax + s.bx) / 2, 0.03, (s.az + s.bz) / 2);
+    strip.rotation.y = Math.atan2(s.bx - s.ax, s.bz - s.az);
+    strip.receiveShadow = true;
+    scene.add(strip);
+  }
+  // Discs fill the wedge-shaped gaps where segments meet at an angle.
+  const junction = new THREE.CircleGeometry(1, 20).rotateX(-Math.PI / 2);
+  for (const n of model.roads.nodes) {
+    const disc = new THREE.Mesh(junction, roadMat);
+    disc.scale.setScalar(4.5);
+    disc.position.set(n.x, 0.031, n.z);
+    disc.receiveShadow = true;
+    scene.add(disc);
+  }
+
   // --- Obstacles ---
   const blockMat = new THREE.MeshStandardMaterial({ color: 0x767c82, roughness: 0.9 });
   const crateMat = new THREE.MeshStandardMaterial({ color: 0xa9773f, roughness: 0.8 });

src/sim/heat.test.ts

@@ -0,0 +1,147 @@
+import { describe, expect, it } from 'vitest';
+import { generateRoads, segmentAt, distanceToRoad, projectOntoSegment } from './roads';
+import { createHeat, levelFor, propsFor, stepHeat, type HeatState } from './heat';
+import { generateWorld } from './world';
+
+const roads = generateRoads(99, 220);
+
+describe('road network', () => {
+  it('is reproducible and connected', () => {
+    expect(generateRoads(99, 220)).toEqual(roads);
+    // Every node must be reachable, or part of the map is unusable.
+    const adjacency = new Map<number, number[]>();
+    for (const s of roads.segments) {
+      adjacency.set(s.a, [...(adjacency.get(s.a) ?? []), s.b]);
+      adjacency.set(s.b, [...(adjacency.get(s.b) ?? []), s.a]);
+    }
+    const seen = new Set([roads.nodes[0]!.id]);
+    const stack = [roads.nodes[0]!.id];
+    while (stack.length) {
+      for (const next of adjacency.get(stack.pop()!) ?? []) {
+        if (seen.has(next)) continue;
+        seen.add(next);
+        stack.push(next);
+      }
+    }
+    expect(seen.size).toBe(roads.nodes.length);
+  });
+
+  it('offers alternative routes, not just a tree', () => {
+    // More edges than nodes-1 means the graph contains loops — which is the
+    // whole premise of "take a different road this time".
+    expect(roads.segments.length).toBeGreaterThan(roads.nodes.length);
+  });
+
+  it('locates the segment under a point on it, and none off it', () => {
+    const s = roads.segments[3]!;
+    const mid = { x: (s.ax + s.bx) / 2, z: (s.az + s.bz) / 2 };
+    expect(segmentAt(roads, mid.x, mid.z)?.id).toBe(s.id);
+    expect(projectOntoSegment(s, mid.x, mid.z).t).toBeCloseTo(0.5, 5);
+    expect(distanceToRoad(roads, mid.x, mid.z)).toBeLessThan(0);
+  });
+});
+
+describe('world', () => {
+  it('spawns the car on the network and keeps scenery off the tarmac', () => {
+    const world = generateWorld(5);
+    expect(segmentAt(world.roads, world.spawn.x, world.spawn.z)).not.toBeNull();
+    for (const o of world.obstacles) {
+      const reach = Math.hypot(o.width, o.depth) / 2;
+      expect(distanceToRoad(world.roads, o.x, o.z)).toBeGreaterThan(reach);
+    }
+  });
+});
+
+/** Drive `metres` along one segment, then idle for `idleSeconds`. */
+function simulate(state: HeatState, segmentId: number, metres: number, idleSeconds = 0) {
+  const changed: number[] = [];
+  const stepMetres = 1;
+  for (let m = 0; m < metres; m += stepMetres) {
+    changed.push(...stepHeat(state, { dt: 1 / 60, segmentId, distance: stepMetres }));
+  }
+  for (let i = 0; i < idleSeconds * 60; i++) {
+    changed.push(...stepHeat(state, { dt: 1 / 60, segmentId: null, distance: 0 }));
+  }
+  return changed;
+}
+
+describe('heat', () => {
+  it('escalates through every level as a road is reused', () => {
+    const state = createHeat(roads);
+    const seen: string[] = [];
+    for (let trip = 0; trip < 6; trip++) {
+      simulate(state, 0, 80);
+      seen.push(state.level[0]!);
+    }
+    expect(seen).toContain('patrol');
+    expect(seen).toContain('barricade');
+    expect(seen).toContain('turret');
+  });
+
+  it('leaves unused roads alone', () => {
+    const state = createHeat(roads);
+    simulate(state, 0, 300);
+    expect(state.value[0]).toBeGreaterThan(0.5);
+    expect(state.value[1]).toBe(0);
+  });
+
+  it('cools a road that goes unused', () => {
+    const state = createHeat(roads);
+    simulate(state, 0, 300);
+    const hot = state.value[0]!;
+    simulate(state, 1, 0, 120);
+    expect(state.value[0]).toBeLessThan(hot - 0.4);
+  });
+
+  it('does not flap between levels while hovering on a threshold', () => {
+    const state = createHeat(roads);
+    // Park heat just above the patrol threshold, then hold it there.
+    simulate(state, 0, 90);
+    expect(state.level[0]).toBe('patrol');
+    let flips = 0;
+    for (let i = 0; i < 600; i++) {
+      // Roughly cancel the decay, so heat sits on the boundary.
+      const changed = stepHeat(state, { dt: 1 / 60, segmentId: 0, distance: 0.022 });
+      flips += changed.length;
+    }
+    expect(flips).toBe(0);
+  });
+
+  it('hysteresis means a level survives a small dip below its threshold', () => {
+    expect(levelFor(0.24, 'clear')).toBe('clear');
+    expect(levelFor(0.25, 'clear')).toBe('patrol');
+    expect(levelFor(0.2, 'patrol')).toBe('patrol');
+    expect(levelFor(0.16, 'patrol')).toBe('clear');
+  });
+});
+
+describe('heat props', () => {
+  const segment = roads.segments[0]!;
+
+  it('puts nothing on a clear road and escalates from there', () => {
+    expect(propsFor(segment, 'clear')).toEqual([]);
+    const counts = (['patrol', 'barricade', 'turret'] as const).map(
+      (l) => propsFor(segment, l).length,
+    );
+    expect(counts[0]).toBeLessThan(counts[1]!);
+    expect(counts[1]).toBeLessThan(counts[2]!);
+  });
+
+  it('is stable for a segment, so a checkpoint stays where you left it', () => {
+    expect(propsFor(segment, 'turret')).toEqual(propsFor(segment, 'turret'));
+  });
+
+  it('leaves a gap in the barricade rather than sealing the road', () => {
+    const blocks = propsFor(segment, 'barricade').filter((p) => p.kind === 'barricade');
+    const spanned = blocks.reduce((sum, b) => sum + b.width, 0);
+    expect(spanned).toBeLessThan(segment.width - 3);
+  });
+
+  it('keeps patrols and barricades within the road, and towers beside it', () => {
+    for (const prop of propsFor(segment, 'turret')) {
+      const { distance } = projectOntoSegment(segment, prop.x, prop.z);
+      if (prop.kind === 'tower') expect(distance).toBeGreaterThan(segment.width / 2);
+      else expect(distance).toBeLessThan(segment.width / 2);
+    }
+  });
+});

src/sim/heat.ts

@@ -0,0 +1,142 @@
+/**
+ * Road heat. Pure — no three.js, no Rapier.
+ *
+ * "Reuse has consequences": every metre driven on a road makes that road worse,
+ * and roads left alone quietly recover. The player is never shown a number — the
+ * escalation is meant to be read off what is physically sitting in the road.
+ */
+import { makeRng, randRange } from '../core/rng';
+import { pointOnSegment, type RoadNetwork, type RoadSegment } from './roads';
+
+export const HEAT_LEVELS = ['clear', 'patrol', 'barricade', 'turret'] as const;
+export type HeatLevel = (typeof HEAT_LEVELS)[number];
+
+/** Heat at which each level takes hold. */
+const THRESHOLDS: Record<Exclude<HeatLevel, 'clear'>, number> = {
+  patrol: 0.25,
+  barricade: 0.55,
+  turret: 0.85,
+};
+
+/**
+ * A level, once established, holds until heat drops this far below its threshold.
+ * Without it a road sitting on a boundary rebuilds and dismantles its barricade
+ * every few seconds.
+ */
+const HYSTERESIS = 0.08;
+
+/** Metres of driving that add a full point of heat. ~4 traversals to turret. */
+const METRES_PER_HEAT = 330;
+/** Heat shed per second everywhere. A hot road cools in roughly four minutes. */
+const DECAY_PER_SECOND = 0.004;
+
+export interface HeatState {
+  /** Indexed by segment id. */
+  value: number[];
+  level: HeatLevel[];
+}
+
+export function createHeat(roads: RoadNetwork): HeatState {
+  return {
+    value: new Array(roads.segments.length).fill(0),
+    level: new Array(roads.segments.length).fill('clear'),
+  };
+}
+
+export function levelFor(heat: number, current: HeatLevel): HeatLevel {
+  const rank = HEAT_LEVELS.indexOf(current);
+  let next: HeatLevel = 'clear';
+  for (const name of ['patrol', 'barricade', 'turret'] as const) {
+    // Rising uses the plain threshold; falling has to clear the hysteresis band.
+    const isCurrentOrLower = HEAT_LEVELS.indexOf(name) <= rank;
+    const bar = THRESHOLDS[name] - (isCurrentOrLower ? HYSTERESIS : 0);
+    if (heat >= bar) next = name;
+  }
+  return next;
+}
+
+export interface HeatStep {
+  dt: number;
+  /** Segment the car is on right now, or null if off-road. */
+  segmentId: number | null;
+  /** Metres travelled this step. */
+  distance: number;
+}
+
+/** Advances heat and returns the segments whose *level* changed. */
+export function stepHeat(state: HeatState, step: HeatStep): number[] {
+  const changed: number[] = [];
+  const decay = DECAY_PER_SECOND * step.dt;
+
+  for (let id = 0; id < state.value.length; id++) {
+    let heat = state.value[id]! - decay;
+    if (id === step.segmentId) heat += step.distance / METRES_PER_HEAT;
+    heat = Math.min(1, Math.max(0, heat));
+    state.value[id] = heat;
+
+    const next = levelFor(heat, state.level[id]!);
+    if (next !== state.level[id]) {
+      state.level[id] = next;
+      changed.push(id);
+    }
+  }
+  return changed;
+}
+
+export interface HeatProp {
+  x: number;
+  z: number;
+  yaw: number;
+  width: number;
+  height: number;
+  depth: number;
+  kind: 'patrol' | 'barricade' | 'tower';
+}
+
+/**
+ * What a given heat level physically puts on a road.
+ *
+ * There is no enemy AI yet, so these are stationary hazards: escalation shows up
+ * as the road getting harder to drive fast and more expensive to get wrong. That
+ * is enough to test the phase's real question — do you start avoiding a road
+ * because of its history?
+ */
+export function propsFor(segment: RoadSegment, level: HeatLevel): HeatProp[] {
+  if (level === 'clear') return [];
+
+  // Seeded off the segment so a road's checkpoint is always in the same place —
+  // recognising a specific stretch is part of learning the map.
+  const rng = makeRng((segment.id + 1) * 0x2545f491);
+  const props: HeatProp[] = [];
+  const half = segment.width / 2;
+
+  // Patrol: parked on the verge. Narrows the road, does not block it.
+  const patrolT = randRange(rng, 0.3, 0.7);
+  const patrolSide = rng() < 0.5 ? -1 : 1;
+  const patrol = pointOnSegment(segment, patrolT, patrolSide * (half - 1.4));
+  props.push({ ...patrol, width: 2, height: 1.6, depth: 4.4, kind: 'patrol' });
+
+  if (level === 'patrol') return props;
+
+  // Barricade: blocks across the road with a gap you have to slow down for.
+  const barricadeT = randRange(rng, 0.35, 0.65);
+  const gapSide = rng() < 0.5 ? -1 : 1;
+  const gapCentre = gapSide * randRange(rng, half * 0.35, half * 0.6);
+  for (const side of [-1, 1]) {
+    const inner = gapCentre + side * 2.1;
+    const outer = side * half;
+    const blockWidth = Math.abs(outer - inner);
+    if (blockWidth < 0.8) continue;
+    const at = pointOnSegment(segment, barricadeT, (inner + outer) / 2);
+    props.push({ ...at, width: blockWidth, height: 1.5, depth: 1.6, kind: 'barricade' });
+  }
+
+  if (level === 'barricade') return props;
+
+  // Turret: a tower overlooking the checkpoint. Inert for now — it is a landmark
+  // that says "this road has been noticed", and something to collide with.
+  const tower = pointOnSegment(segment, barricadeT + 0.06, (rng() < 0.5 ? -1 : 1) * (half + 2.5));
+  props.push({ ...tower, width: 3, height: 6, depth: 3, kind: 'tower' });
+
+  return props;
+}

src/sim/roads.ts

@@ -0,0 +1,178 @@
+/**
+ * The road network. Pure data — no three.js, no Rapier.
+ *
+ * A jittered grid with some edges removed. The grid matters: the central choice
+ * of the game is "reuse this road or take another one", which only exists if
+ * there are genuinely alternative routes between the same two places. Removed
+ * edges give the network character; the connectivity check stops removal from
+ * stranding a corner of the map.
+ */
+import { makeRng, randRange, type Rng } from '../core/rng';
+
+export interface RoadNode {
+  id: number;
+  x: number;
+  z: number;
+}
+
+export interface RoadSegment {
+  id: number;
+  a: number;
+  b: number;
+  ax: number;
+  az: number;
+  bx: number;
+  bz: number;
+  length: number;
+  width: number;
+}
+
+export interface RoadNetwork {
+  nodes: RoadNode[];
+  segments: RoadSegment[];
+}
+
+const GRID = 7;
+const ROAD_WIDTH = 9;
+/** Fraction of grid spacing a node may wander from its lattice point. */
+const JITTER = 0.3;
+/** Share of edges to try to remove, budget permitting. */
+const THINNING = 0.28;
+
+export function generateRoads(seed: number, extent: number): RoadNetwork {
+  const rng: Rng = makeRng(seed ^ 0x9e3779b9);
+  const spacing = (extent * 2) / (GRID - 1);
+
+  const nodes: RoadNode[] = [];
+  for (let row = 0; row < GRID; row++) {
+    for (let col = 0; col < GRID; col++) {
+      nodes.push({
+        id: row * GRID + col,
+        x: -extent + col * spacing + randRange(rng, -1, 1) * spacing * JITTER,
+        z: -extent + row * spacing + randRange(rng, -1, 1) * spacing * JITTER,
+      });
+    }
+  }
+
+  // Full lattice first, then thin it out.
+  const edges: Array<[number, number]> = [];
+  for (let row = 0; row < GRID; row++) {
+    for (let col = 0; col < GRID; col++) {
+      const i = row * GRID + col;
+      if (col + 1 < GRID) edges.push([i, i + 1]);
+      if (row + 1 < GRID) edges.push([i, i + GRID]);
+    }
+  }
+
+  const order = shuffle(edges.map((_, i) => i), rng);
+  const removed = new Set<number>();
+  const budget = Math.floor(edges.length * THINNING);
+  for (const idx of order) {
+    if (removed.size >= budget) break;
+    removed.add(idx);
+    // A road that cuts the map in two is worse than a boring one.
+    if (!isConnected(nodes.length, edges, removed)) removed.delete(idx);
+  }
+
+  const segments: RoadSegment[] = [];
+  edges.forEach((edge, idx) => {
+    if (removed.has(idx)) return;
+    const a = nodes[edge[0]]!;
+    const b = nodes[edge[1]]!;
+    segments.push({
+      id: segments.length,
+      a: a.id,
+      b: b.id,
+      ax: a.x,
+      az: a.z,
+      bx: b.x,
+      bz: b.z,
+      length: Math.hypot(b.x - a.x, b.z - a.z),
+      width: ROAD_WIDTH,
+    });
+  });
+
+  return { nodes, segments };
+}
+
+function shuffle<T>(items: T[], rng: Rng): T[] {
+  for (let i = items.length - 1; i > 0; i--) {
+    const j = Math.floor(rng() * (i + 1));
+    [items[i], items[j]] = [items[j]!, items[i]!];
+  }
+  return items;
+}
+
+function isConnected(
+  nodeCount: number,
+  edges: Array<[number, number]>,
+  removed: ReadonlySet<number>,
+): boolean {
+  const adjacency: number[][] = Array.from({ length: nodeCount }, () => []);
+  edges.forEach(([a, b], idx) => {
+    if (removed.has(idx)) return;
+    adjacency[a]!.push(b);
+    adjacency[b]!.push(a);
+  });
+
+  const seen = new Uint8Array(nodeCount);
+  const stack = [0];
+  seen[0] = 1;
+  let visited = 1;
+  while (stack.length) {
+    for (const next of adjacency[stack.pop()!]!) {
+      if (seen[next]) continue;
+      seen[next] = 1;
+      visited++;
+      stack.push(next);
+    }
+  }
+  return visited === nodeCount;
+}
+
+/** Squared distance from a point to a segment, plus how far along it fell (0..1). */
+export function projectOntoSegment(
+  s: RoadSegment,
+  x: number,
+  z: number,
+): { distance: number; t: number } {
+  const dx = s.bx - s.ax;
+  const dz = s.bz - s.az;
+  const lenSq = dx * dx + dz * dz;
+  const t = lenSq === 0 ? 0 : Math.max(0, Math.min(1, ((x - s.ax) * dx + (z - s.az) * dz) / lenSq));
+  return { distance: Math.hypot(x - (s.ax + dx * t), z - (s.az + dz * t)), t };
+}
+
+/** Nearest segment the point is actually *on*, or null if off-road. */
+export function segmentAt(roads: RoadNetwork, x: number, z: number): RoadSegment | null {
+  let best: RoadSegment | null = null;
+  let bestDistance = Infinity;
+  for (const s of roads.segments) {
+    const { distance } = projectOntoSegment(s, x, z);
+    if (distance < s.width / 2 && distance < bestDistance) {
+      bestDistance = distance;
+      best = s;
+    }
+  }
+  return best;
+}
+
+/** Distance to the nearest road surface, used to keep scenery off the tarmac. */
+export function distanceToRoad(roads: RoadNetwork, x: number, z: number): number {
+  let best = Infinity;
+  for (const s of roads.segments) {
+    best = Math.min(best, projectOntoSegment(s, x, z).distance - s.width / 2);
+  }
+  return best;
+}
+
+/** A point a fraction `t` along the segment, offset `side` metres to its left. */
+export function pointOnSegment(s: RoadSegment, t: number, side: number) {
+  const dx = (s.bx - s.ax) / s.length;
+  const dz = (s.bz - s.az) / s.length;
+  return {
+    x: s.ax + (s.bx - s.ax) * t + dz * side,
+    z: s.az + (s.bz - s.az) * t - dx * side,
+    yaw: Math.atan2(s.bx - s.ax, s.bz - s.az),
+  };
+}

src/sim/sim.test.ts

@@ -12,8 +12,9 @@ describe('world generation', () => {
   });
 
   it('keeps the spawn point clear', () => {
-    for (const o of generateWorld(7).obstacles) {
+    const world = generateWorld(7);
-      expect(Math.hypot(o.x, o.z)).toBeGreaterThanOrEqual(12);
+    for (const o of world.obstacles) {
+      expect(Math.hypot(o.x - world.spawn.x, o.z - world.spawn.z)).toBeGreaterThanOrEqual(12);
     }
   });
 });

src/sim/world.ts

@@ -6,6 +6,7 @@
  * of engine imports is what makes those systems unit-testable and fast-forwardable.
  */
 import { makeRng, randRange, type Rng } from '../core/rng';
+import { distanceToRoad, generateRoads, type RoadNetwork } from './roads';
 
 export interface Obstacle {
   x: number;
@@ -23,44 +24,62 @@ export interface WorldModel {
   seed: number;
   /** Half-width of the drivable plate, in metres. */
   extent: number;
+  roads: RoadNetwork;
   obstacles: Obstacle[];
+  /** Where the car starts: a junction, so the network is reachable immediately. */
+  spawn: { x: number; z: number };
 }
 
 const SPAWN_CLEARANCE = 12;
+/** Scenery this close to tarmac would read as a roadblock. Heat places those. */
+const ROAD_CLEARANCE = 2.5;
 
-export function generateWorld(seed: number, count = 140, extent = 220): WorldModel {
+export function generateWorld(seed: number, count = 200, extent = 220): WorldModel {
   const rng: Rng = makeRng(seed);
+  const roads = generateRoads(seed, extent);
   const obstacles: Obstacle[] = [];
 
-  while (obstacles.length < count) {
+  const spawnNode = roads.nodes.reduce((best, n) =>
+    Math.hypot(n.x, n.z) < Math.hypot(best.x, best.z) ? n : best,
+  );
+  const spawn = { x: spawnNode.x, z: spawnNode.z };
+
+  let attempts = 0;
+  while (obstacles.length < count && attempts < count * 40) {
+    attempts++;
     const x = randRange(rng, -extent, extent);
     const z = randRange(rng, -extent, extent);
     // Leave the player's spawn point clear so the car never starts inside a wall.
-    if (Math.hypot(x, z) < SPAWN_CLEARANCE) continue;
+    if (Math.hypot(x - spawn.x, z - spawn.z) < SPAWN_CLEARANCE) continue;
 
     const crate = rng() < 0.45;
-    obstacles.push(
+    const obstacle: Obstacle = crate
-      crate
+      ? {
-        ? {
+          x,
-            x,
+          z,
-            z,
+          width: randRange(rng, 0.8, 1.4),
-            width: randRange(rng, 0.8, 1.4),
+          height: randRange(rng, 0.8, 1.4),
-            height: randRange(rng, 0.8, 1.4),
+          depth: randRange(rng, 0.8, 1.4),
-            depth: randRange(rng, 0.8, 1.4),
+          yaw: rng() * Math.PI,
-            yaw: rng() * Math.PI,
+          kind: 'crate',
-            kind: 'crate',
+        }
-          }
+      : {
-        : {
+          x,
-            x,
+          z,
-            z,
+          width: randRange(rng, 1.5, 5),
-            width: randRange(rng, 1.5, 5),
+          height: randRange(rng, 1.2, 3.5),
-            height: randRange(rng, 1.2, 3.5),
+          depth: randRange(rng, 1.5, 5),
-            depth: randRange(rng, 1.5, 5),
+          yaw: rng() * Math.PI,
-            yaw: rng() * Math.PI,
+          kind: 'block',
-            kind: 'block',
+        };
-          },
+
-    );
+    // Roads have to stay drivable — obstruction is heat's job, not scenery's.
+    // Measured from the box's corner, since it may be rotated any which way.
+    const reach = Math.hypot(obstacle.width, obstacle.depth) / 2;
+    if (distanceToRoad(roads, x, z) < ROAD_CLEARANCE + reach) continue;
+
+    obstacles.push(obstacle);
   }
 
-  return { seed, extent, obstacles };
+  return { seed, extent, roads, obstacles, spawn };
 }

src/ui/hud.ts

@@ -1,4 +1,5 @@
 import type { CarCondition } from '../sim/car';
+import type { HeatLevel } from '../sim/heat';
 
 const BAR_WIDTH = 12;
 
@@ -7,12 +8,19 @@ function bar(value: number): string {
   return '█'.repeat(filled) + '·'.repeat(BAR_WIDTH - filled);
 }
 
+export interface HeatReadout {
+  segmentId: number | null;
+  value: number;
+  level: HeatLevel | null;
+  hottest: number;
+}
+
 export function createHud(seed: number) {
   const el = document.getElementById('hud')!;
   let last = 0;
 
   return {
-    update(speedMs: number, condition: CarCondition, now: number) {
+    update(speedMs: number, condition: CarCondition, now: number, heat: HeatReadout) {
       // The HUD is debug scaffolding, not the real interface — 10 Hz is plenty.
       if (now - last < 0.1) return;
       last = now;
@@ -23,6 +31,12 @@ export function createHud(seed: number) {
         `tires   ${bar(condition.tires)} ${(condition.tires * 100).toFixed(0)}%`,
         `chassis ${bar(condition.chassis)} ${(condition.chassis * 100).toFixed(0)}%`,
         '',
+        // Debug only. The real game signals heat through the road itself —
+        // see the design brief: no numeric meter ships.
+        `[debug] road    ${heat.segmentId === null ? 'off-road' : `#${heat.segmentId}`}`,
+        `[debug] heat    ${bar(heat.value)} ${heat.level ?? '—'}`,
+        `[debug] hottest ${bar(heat.hottest)}`,
+        '',
         `seed ${seed}`,
         'WASD drive · space handbrake · R respawn',
       ].join('\n');