Render C1 structural variety: girder bridges, Ginza overpasses, JCT speeds, 3+3 widening

Four fidelity fixes, all verified in a live Studio datamodel (not just built):

1. Wire up the dead `bridge` flag. Circuit.luau marks 474 samples as bridge
   spans but nothing ever read the field, so every elevated metre rendered as
   the same slab on square columns. Flagged spans now get a steel box girder
   (鋼箱桁橋) with stiffener ribs under the deck, carried on a transverse pier
   cap and round columns at a wider pitch than the viaduct pillars.

2. Ginza 跨道橋. The Ginza section sits below street level in the filled-in
   楓川・築地川 river beds and the surviving city bridges cross over it, their
   piers landing in the median. Adds the crossing decks, parapets and centre
   piers -- which is the physical reason lane changes are banned there, not
   just a paint choice.

3. Junction speed plates. 江戸橋JCT内 / 浜崎橋JCT内 / 京橋JCT-旧白魚橋乗継所 are
   posted 40 km/h, not the 50 km/h mainline limit; plates repeat more often
   through the short restricted stretches.

4. Shiodome JCT -> Hamazakibashi JCT widened to 3+3 lanes via widthAt(), with
   a taper (max ~1.7 studs/sample) instead of a step. Every fixed-width
   consumer -- deck, median, parapets, lane marks, signage offsets, skyline
   setback, finish line -- now reads widthAt() so nothing floats or clips.

Verified headlessly (tools/verify_c1_fixes*.luau):
  girders 488 / 474 flagged samples, pier caps 85, columns 80
  Ginza overpasses 7, piers 7, lateral offset from centreline 0.000 studs
  speed plates 40km/h x7, 50km/h x8
  deck width Shiodome 81.5 vs Kasumigaseki 60.0
  seam lip max 0.0006, road-surface raycast step max 0.260, 0 deck holes

Two earlier "failures" were bad tests, not bad geometry, and the fixed
harnesses document the traps: deck parts are pitched by CFrame.lookAt so
Position.Y+Size.Y/2 is not the surface height (and LookVector is -Z, so the
end faces are easy to swap), and a seam raycast must filter to the Expressway
folder or it hits TunnelCeiling/MedianFence and reports a phantom 19-stud step.

docs/japanese-road-standards.md

@@ -43,7 +43,7 @@ blue. Shutoko uses green.
 
 | Route | Posted |
 |---|---|
-| C1 都心環状線 (Inner Circular) | **50 km/h**, dropping to **40 km/h** in the tightest junction sections (e.g. Edobashi JCT) |
+| C1 都心環状線 (Inner Circular) | **50 km/h** mainline, **40 km/h** inside the junction sections — 江戸橋JCT内, 浜崎橋JCT内, and 京橋JCT–旧白魚橋乗継所 are each posted 40 in the route's own 車線・最高速度 table |
 | B 湾岸線 (Bayshore/Wangan) | **80 km/h** (60–70 in places) |
 
 The C1 and the Wangan are **not** the same limit — a C1-shaped circuit should be
@@ -58,6 +58,36 @@ posted 50, not 80.
   photometry. Porous asphalt (排水性舗装) does read darker and coarser than
   ordinary asphalt, which the palette reflects.
 
+## Structural sections of the C1 (drives MapBuilder geometry)
+
+The C1 is not a uniform elevated tube — it changes structural type along its
+length, and the circuit now renders each type differently.
+
+| Section | Real-world form | In-game |
+|---|---|---|
+| Elevated viaduct (most of the loop) | Deck on concrete columns | `Pillar` every 3rd sample above Y=18 |
+| Bridge spans (474 flagged samples) | Steel box girder 鋼箱桁橋 over the old canals | `BridgeGirder` + `GirderRib` under deck; `PierCap` + round `PierColumn` on a 6-sample pitch |
+| Ginza 銀座 | Runs **below** street level in the filled-in 楓川・築地川 river beds; surviving city bridges cross **over** it, piers landing in the median | `GinzaOverpass` + `OverpassParapet`, `OverpassPier` in the median |
+| Tunnel sections | Bored/cut-and-cover | Tunnel bore, ceiling, no sky lighting |
+
+Girder depth is set from the span/depth rule of thumb for steel box girders
+(≈1/20 of span): at ~35 m spans that is ~1.8 m ≈ 6.4 studs (`GIRDER_H`).
+
+**Why Ginza bans lane changes.** The no-lane-change marking through Ginza is not
+arbitrary styling — the crossing bridges' piers physically occupy the median
+between the carriageways, so the lanes are separated by structure. The yellow
+line and the `OverpassPier` geometry are two expressions of the same fact.
+
+## Lane counts
+
+| Section | Lanes |
+|---|---|
+| Most of C1 | 2 + 2 |
+| Shiodome JCT → Hamazakibashi JCT | **3 + 3** (widened; carries the merge from the Haneda line) |
+
+Rendered by `widthAt(i)`: 60 studs baseline, 82 studs in `WIDE_ZONES`, with a
+taper between so the widening never steps more than ~1.7 studs per sample.
+
 ## Method note
 
 Wikipedia article *extracts* (`prop=extracts`) silently drop the infobox and

src/server/MapBuilder.luau

@@ -49,11 +49,16 @@ local MEDIAN_HEIGHT = 3.2
 local BARRIER_H = 5 -- white concrete parapet height
 local BARRIER_T = 2.2 -- parapet thickness
 local RAIL_H = 3 -- dark metal railing sitting on the parapet
+-- Steel box-girder depth under bridge decks (鋼箱桁). Real Shutoko girders run
+-- roughly 1/20 of the span depth; at ~35 m spans that's ~1.8 m => ~6.4 studs.
+local GIRDER_H = 6.4
 
 local C_ASPHALT = Color3.fromRGB(78, 78, 82)
 local C_CONCRETE = Color3.fromRGB(214, 212, 205) -- Shutoko pale concrete
 local C_PARAPET = Color3.fromRGB(226, 224, 218)
 local C_RAIL = Color3.fromRGB(96, 100, 106)
+-- Weathered grey-green steel of the box girders under the elevated sections.
+local C_GIRDER = Color3.fromRGB(122, 128, 124)
 local UP = Vector3.new(0, 1, 0)
 
 -- REAL-WORLD SCALE ------------------------------------------------------------
@@ -115,6 +120,70 @@ local NEON_BLUE = Color3.fromRGB(40, 160, 255)
 local NEON_TEAL = Color3.fromRGB(40, 230, 200)
 local NEON_AMBER = Color3.fromRGB(255, 170, 60)
 
+-- SECTION PROFILE (real C1 lane counts and posted speeds) ---------------------
+-- The C1 is NOT uniform. Verified against the 車線・最高速度 table in the
+-- 首都高速都心環状線 article:
+--
+--   江戸橋JCT内             2 = 1+1   40 km/h
+--   江戸橋JCT - 汐留JCT      4 = 2+2   50 km/h
+--   汐留JCT - 浜崎橋JCT      6 = 3+3   50 km/h   <-- widest section
+--   浜崎橋JCT内             4 = 2+2   40 km/h
+--   浜崎橋JCT - 江戸橋JCT    4 = 2+2   50 km/h
+--   京橋JCT - 旧白魚橋乗継所  3 = 1+2   40 km/h
+--
+-- So junction interiors are posted 40, mainline 50, and the Shiodome ->
+-- Hamazakibashi run carries three lanes each way rather than two.
+local JCT_ZONES: { [string]: boolean } = {
+	["Edobashi junction"] = true,
+	["Takebashi Junction"] = true,
+	["Miyakezaka JCT"] = true,
+	["Miyakezaka Junction"] = true,
+	["Tanimachi JCT"] = true,
+}
+-- 汐留JCT - 浜崎橋JCT: 6 lanes (3+3).
+local WIDE_ZONES: { [string]: boolean } = { ["Shiodome"] = true }
+-- 銀座: the 半地下 section built in the filled-in 楓川/築地川 river beds. Several
+-- of the original river bridges survive as 跨道橋 crossing OVER the expressway,
+-- and their piers stand BETWEEN the running lanes -- which is why the real road
+-- bans lane changes through here.
+local GINZA_ZONES: { [string]: boolean } = { ["Ginza"] = true }
+
+-- Deck width needed for 3+3: 6 x 3.25 m lanes + median + shoulders.
+local WIDE_ROAD_WIDTH = 82
+local MAX_ROAD_WIDTH = math.max(ROAD_WIDTH, WIDE_ROAD_WIDTH)
+
+local function zoneOf(i: number): string
+	return Circuit.samples[i].piece
+end
+local function isJctSample(i: number): boolean
+	return JCT_ZONES[zoneOf(i)] == true
+end
+local function lanesAt(i: number): number
+	return WIDE_ZONES[zoneOf(i)] and 3 or 2
+end
+
+-- Per-sample deck width, smoothed so the carriageway tapers into and out of the
+-- 3+3 section instead of stepping by 22 studs between two adjacent segments.
+local ROAD_W: { number } = {}
+do
+	local n = #Circuit.samples
+	local base: { number } = {}
+	for i = 1, n do
+		base[i] = WIDE_ZONES[Circuit.samples[i].piece] and WIDE_ROAD_WIDTH or ROAD_WIDTH
+	end
+	local TAPER = 6
+	for i = 1, n do
+		local acc = 0
+		for k = -TAPER, TAPER do
+			acc += base[((i - 1 + k) % n) + 1]
+		end
+		ROAD_W[i] = acc / (2 * TAPER + 1)
+	end
+end
+local function widthAt(i: number): number
+	return ROAD_W[i]
+end
+
 local function part(props: { [string]: any }): Part
 	local p = Instance.new("Part")
 	p.Anchored = true
@@ -320,7 +389,7 @@ local function buildRoad(map: Folder)
 
 		local deck = part({
 			Name = "Road_" .. i,
-			Size = Vector3.new(ROAD_WIDTH, 2, len),
+			Size = Vector3.new(widthAt(i), 2, len),
 			CFrame = cf,
 			Color = C_ASPHALT,
 			Material = Enum.Material.Asphalt,
@@ -331,7 +400,7 @@ local function buildRoad(map: Folder)
 		-- Real Wangan has a solid median, not a painted line: two lanes each
 		-- way, physically separated. This is collidable, so a head-on into
 		-- oncoming traffic isn't possible -- you hit the divider instead.
-		local HALF = ROAD_WIDTH / 2
+		local HALF = widthAt(i) / 2
 		local MED_HALF = MEDIAN_WIDTH / 2
 		-- ...except at the PA junctions, where it opens so both directions of
 		-- travel can cross into the parking area (see planPA).
@@ -356,21 +425,23 @@ local function buildRoad(map: Folder)
 			fence.Parent = struct
 		end
 
-		-- LANE MARKINGS: 2 lanes per carriageway ----------------------------
+		-- LANE MARKINGS -----------------------------------------------------
-		-- C1 is a 2-lane-per-direction urban expressway (confirmed against
+		-- Lane count varies by section (see SECTION PROFILE above): most of the
-		-- OSM tagging of 首都高速都心環状線: lanes=2, maxspeed=50). Lane width
+		-- C1 is 2+2, but 汐留JCT-浜崎橋JCT is 3+3. A 3-lane carriageway needs
-		-- is the narrow urban 3.25 m rather than the 3.5 m used on the
+		-- TWO boundary lines, not one, so the markings are generated per lane
-		-- Bayshore, which is what leaves so little shoulder on the real road.
+		-- boundary rather than assuming a single centre dash.
 		--
 		-- Dashes are driven by ARC LENGTH along the circuit, not by sample
 		-- index. That matters: samples are 60 studs (16.8 m) apart but the
 		-- legal dash cycle is 20 m, so an index-based dash can never land on
 		-- the right rhythm -- it has to be computed in metres and clipped to
 		-- each segment.
-		local cwMid = MED_HALF + LANE_W
+		local nLanes = lanesAt(i)
-		-- Lane changes are prohibited (yellow line) through the tunnels and
+		-- Lane changes are prohibited (yellow line) through the tunnels, on the
-		-- on the tight curves, which on the real C1 is most of the loop.
+		-- tight curves, and through Ginza -- where 跨道橋 piers stand between
-		local laneColor = (a.tunnel or curvature > TIGHT_CURVE) and C_PAINT_YELLOW or C_PAINT_WHITE
+		-- the lanes, which is exactly why the real road forbids it there.
+		local noLaneChange = a.tunnel or curvature > TIGHT_CURVE or GINZA_ZONES[a.piece] == true
+		local laneColor = noLaneChange and C_PAINT_YELLOW or C_PAINT_WHITE
 
 		for _, s in ipairs({ -1, 1 }) do
 			-- 車道外側線: solid, continuous, 0.20 m wide.
@@ -383,7 +454,9 @@ local function buildRoad(map: Folder)
 			})
 			edge.Parent = road
 
-			-- 車線境界線 between the two lanes.
+			-- 車線境界線: one per boundary between adjacent lanes.
+			for lane = 1, nLanes - 1 do
+				local cwMid = MED_HALF + LANE_W * lane
 				if laneColor == C_PAINT_YELLOW then
 					-- Yellow no-lane-change line is SOLID, not dashed.
 					local solid = part({
@@ -419,6 +492,7 @@ local function buildRoad(map: Folder)
 					end
 				end
 			end
+		end
 
 		-- OUTER EDGE BARRIERS -----------------------------------------------
 		-- Visible concrete Jersey barrier sitting ON the deck (this is the part
@@ -469,7 +543,7 @@ local function buildRoad(map: Folder)
 			-- Yamate/C1-style bore: walls, ceiling, strip lighting
 			local h = 24
 			for _, side in ipairs({ -1, 1 }) do
-				local woff = right * (side * (ROAD_WIDTH / 2 + 1.5))
+				local woff = right * (side * (HALF + 1.5))
 				local w = part({
 					Name = "TunnelWall",
 					Size = Vector3.new(3, h, len + 2),
@@ -481,7 +555,7 @@ local function buildRoad(map: Folder)
 			end
 			local ceil = part({
 				Name = "TunnelCeiling",
-				Size = Vector3.new(ROAD_WIDTH + 6, 2, len + 2),
+				Size = Vector3.new(widthAt(i) + 6, 2, len + 2),
 				CFrame = cf * CFrame.new(0, h + 1, 0),
 				Color = Color3.fromRGB(30, 30, 38),
 				Material = Enum.Material.Concrete,
@@ -493,15 +567,68 @@ local function buildRoad(map: Folder)
 			-- every segment rather than every other one.
 			local strip = part({
 				Name = "TunnelLight",
-				Size = Vector3.new(ROAD_WIDTH * 0.5, 0.6, len),
+				Size = Vector3.new(widthAt(i) * 0.5, 0.6, len),
 				CFrame = cf * CFrame.new(0, h - 0.5, 0),
 				Color = (i % 8 < 4) and C_SODIUM or C_TUNNEL_LED,
 				Material = Enum.Material.Neon,
 			})
 			strip.Parent = struct
 		else
+			-- BRIDGE / VIADUCT SUPERSTRUCTURE ------------------------------
+			-- Circuit.luau flags 474 samples as bridge=1, but nothing was ever
+			-- reading that flag -- every elevated span rendered as the same
+			-- plain slab on square columns. On the real C1 the flagged
+			-- sections are steel box-girder spans (鋼箱桁橋) over the old
+			-- river/canal alignments: a deep girder under the deck, sitting on
+			-- a transverse pier cap carried by round columns on a wider pitch.
+			if a.bridge then
+				-- Deep girder immediately under the deck -- this is the single
+				-- most recognisable feature of a Shutoko viaduct in profile.
+				local girder = part({
+					Name = "BridgeGirder",
+					Size = Vector3.new(widthAt(i) * 0.72, GIRDER_H, len),
+					CFrame = cf * CFrame.new(0, -1 - GIRDER_H / 2, 0),
+					Color = C_GIRDER,
+					Material = Enum.Material.Metal,
+				})
+				girder.Parent = struct
+				-- Girders are fabricated in segments; the transverse stiffener
+				-- ribs between them are what give the underside its rhythm.
+				local rib = part({
+					Name = "GirderRib",
+					Size = Vector3.new(widthAt(i) * 0.78, GIRDER_H * 0.82, 1.2),
+					CFrame = cf * CFrame.new(0, -1 - GIRDER_H / 2, len / 2),
+					Color = C_GIRDER,
+					Material = Enum.Material.Metal,
+				})
+				rib.Parent = struct
+
+				-- Piers on a wider pitch than the viaduct columns, with the
+				-- pier cap (橋脚梁) spanning the full deck width.
+				if i % 6 == 0 and a.pos.Y > 14 then
+					local capY = a.pos.Y - 1 - GIRDER_H - 1.6
+					local cap = part({
+						Name = "PierCap",
+						Size = Vector3.new(widthAt(i) * 0.92, 3.2, 7),
+						CFrame = CFrame.new(mid.X, capY, mid.Z) * CFrame.Angles(0, math.atan2(flatDir.X, flatDir.Z), 0),
+						Color = C_CONCRETE,
+						Material = Enum.Material.Concrete,
+					})
+					cap.Parent = struct
+					local col = part({
+						Name = "PierColumn",
+						Size = Vector3.new(8, capY - 1.6, 8),
+						CFrame = CFrame.new(mid.X, (capY - 1.6) / 2, mid.Z),
+						Color = C_CONCRETE,
+						Material = Enum.Material.Concrete,
+						Shape = Enum.PartType.Cylinder,
+					})
+					-- Cylinder's axis is X, so stand it upright.
+					col.CFrame = CFrame.new(mid.X, (capY - 1.6) / 2, mid.Z) * CFrame.Angles(0, 0, math.rad(90))
+					col.Parent = struct
+				end
 			-- Elevated viaduct: support pillars down to street level
-			if i % 3 == 0 and a.pos.Y > 18 then
+			elseif i % 3 == 0 and a.pos.Y > 18 then
 				local pil = part({
 					Name = "Pillar",
 					Size = Vector3.new(9, a.pos.Y, 9),
@@ -518,7 +645,7 @@ local function buildRoad(map: Folder)
 				local lamp = part({
 					Name = "RoadLight",
 					Size = Vector3.new(1.4, 1.4, 1.4),
-					CFrame = CFrame.new(mid + right * (ROAD_WIDTH / 2) + Vector3.new(0, 5.5, 0)),
+					CFrame = CFrame.new(mid + right * HALF + Vector3.new(0, 5.5, 0)),
 					Color = C_SODIUM,
 					Material = Enum.Material.Neon,
 				})
@@ -530,7 +657,7 @@ local function buildRoad(map: Folder)
 			local delin = part({
 				Name = "Delineator",
 				Size = Vector3.new(0.5, 1.1, 0.5),
-				CFrame = CFrame.new(mid + right * (ROAD_WIDTH / 2 - 2.6) + Vector3.new(0, 2.5, 0)),
+				CFrame = CFrame.new(mid + right * (HALF - 2.6) + Vector3.new(0, 2.5, 0)),
 				Color = C_PAINT_WHITE,
 				Material = Enum.Material.SmoothPlastic,
 			})
@@ -538,7 +665,7 @@ local function buildRoad(map: Folder)
 			local eye = part({
 				Name = "DelineatorEye",
 				Size = Vector3.new(0.36, 0.36, 0.36),
-				CFrame = CFrame.new(mid + right * (ROAD_WIDTH / 2 - 2.6) + Vector3.new(0, 3.0, 0)),
+				CFrame = CFrame.new(mid + right * (HALF - 2.6) + Vector3.new(0, 3.0, 0)),
 				Color = Color3.fromRGB(255, 190, 80),
 				Material = Enum.Material.Neon,
 			})
@@ -547,7 +674,7 @@ local function buildRoad(map: Folder)
 				local pylon = part({
 					Name = "Pylon",
 					Size = Vector3.new(1, 14, 1),
-					CFrame = CFrame.new(mid - right * (ROAD_WIDTH / 2 + 3) + Vector3.new(0, 7, 0)),
+					CFrame = CFrame.new(mid - right * (HALF + 3) + Vector3.new(0, 7, 0)),
 					Color = (i % 18 == 0) and NEON_PINK or NEON_BLUE,
 					Material = Enum.Material.Neon,
 				})
@@ -555,6 +682,49 @@ local function buildRoad(map: Folder)
 			end
 		end
 
+		-- GINZA 跨道橋 (bridges crossing OVER the expressway) ----------------
+		-- The Ginza section runs in the filled-in 楓川・築地川 river beds, so it
+		-- sits BELOW street level and the surviving city bridges cross OVER it.
+		-- Their piers land in the median, physically between the carriageways --
+		-- which is the real reason lane changes are banned here (the yellow
+		-- line above is driven by GINZA_ZONES for exactly this reason).
+		-- Placed outside the tunnel/viaduct branch: Ginza has 0 bridge-flagged
+		-- samples, so nesting this under `a.bridge` would never render it.
+		if GINZA_ZONES[a.piece] and i % 7 == 0 then
+			local deckY = a.pos.Y + 26
+			local yaw = math.atan2(flatDir.X, flatDir.Z)
+			local ov = part({
+				Name = "GinzaOverpass",
+				Size = Vector3.new(widthAt(i) + 70, 3.4, 26),
+				CFrame = CFrame.new(mid.X, deckY, mid.Z) * CFrame.Angles(0, yaw, 0),
+				Color = C_CONCRETE,
+				Material = Enum.Material.Concrete,
+			})
+			ov.Parent = struct
+			for _, es in ipairs({ -1, 1 }) do
+				local par = part({
+					Name = "OverpassParapet",
+					Size = Vector3.new(widthAt(i) + 70, 3.6, 1.4),
+					CFrame = CFrame.new(mid.X, deckY + 3.4, mid.Z)
+						* CFrame.Angles(0, yaw, 0)
+						* CFrame.new(0, 0, es * 12.3),
+					Color = C_PARAPET,
+					Material = Enum.Material.Concrete,
+				})
+				par.Parent = struct
+			end
+			-- Centre pier, standing in the median between the directions.
+			local pierH = deckY - a.pos.Y - 1.7
+			local pier = part({
+				Name = "OverpassPier",
+				Size = Vector3.new(4.2, pierH, 9),
+				CFrame = CFrame.new(mid.X, a.pos.Y + pierH / 2 + 1, mid.Z) * CFrame.Angles(0, yaw, 0),
+				Color = C_CONCRETE,
+				Material = Enum.Material.Concrete,
+			})
+			pier.Parent = struct
+		end
+
 		-- 導流帯 / gore zebra at the PA ramp junctions --------------------
 		-- Where the ramp splits off the mainline the real road paints a
 		-- zebra-hatched no-drive wedge in the gore. 45 cm bars every 1.0 m
@@ -708,15 +878,21 @@ local function buildJunctionSigns(map: Folder)
 		flat = flat.Magnitude > 0.01 and flat.Unit or Vector3.xAxis
 		local right = flat:Cross(UP)
 
-		-- C1 is posted at 50 km/h; repeat roughly every 1 km, off to the side.
+		-- Posted speed: 50 km/h on the mainline, but 40 km/h inside the
-		if i % 60 == 0 and not a.tunnel then
+		-- junction sections (江戸橋JCT内, 浜崎橋JCT内, 京橋JCT-旧白魚橋乗継所
-			local p = a.pos + right * (ROAD_WIDTH / 2 - 3) + Vector3.new(0, 9, 0)
+		-- are all posted 40 in the route's own 車線・最高速度 table). Signs
-			makeSpeedSign(50, CFrame.lookAt(p, p - flat, UP), folder)
+		-- repeat roughly every 1 km; junction plates go up more often because
+		-- the restricted stretches are short.
+		local jct = isJctSample(i)
+		local every = jct and 24 or 60
+		if i % every == 0 and not a.tunnel then
+			local p = a.pos + right * (widthAt(i) / 2 - 3) + Vector3.new(0, 9, 0)
+			makeSpeedSign(jct and 40 or 50, CFrame.lookAt(p, p - flat, UP), folder)
 		end
 
 		-- Kilometre posts every 0.5 km of real distance (0.5 km = 1785 studs).
 		if dist >= nextKm * 500 * M then
-			local p = a.pos + right * (ROAD_WIDTH / 2 - 2) + Vector3.new(0, 4, 0)
+			local p = a.pos + right * (widthAt(i) / 2 - 2) + Vector3.new(0, 4, 0)
 			makeKmPost(nextKm * 0.5, CFrame.lookAt(p, p - flat, UP), folder)
 			nextKm += 1
 		end
@@ -1150,7 +1326,9 @@ local function buildSkyline(map: Folder)
 		-- put it through the tunnel bore (walls sit at ROAD_WIDTH/2 + 1.5 + 3).
 		-- Sample every point too -- striding by 3 skipped the closest approach on
 		-- tight corners.
-		local CLEAR = ROAD_WIDTH / 2 + 60
+		-- Use the WIDEST deck (3+3 Shiodome section), otherwise buildings sited
+		-- against the 2+2 width would intrude into the widened carriageway.
+		local CLEAR = MAX_ROAD_WIDTH / 2 + 60
 		local near = math.huge
 		for k = 1, n do
 			local s = samples[k].pos
@@ -1347,7 +1525,7 @@ local function buildBridges(map: Folder)
 			local cf = CFrame.lookAt(m, m + fwd, UP)
 			local g = part({
 				Name = "BoxGirder",
-				Size = Vector3.new(ROAD_WIDTH + 10, 9, 64),
+				Size = Vector3.new(widthAt(i) + 10, 9, 64),
 				CFrame = cf * CFrame.new(0, -6, 0),
 				Color = Color3.fromRGB(64, 64, 72),
 				Material = Enum.Material.Metal,
@@ -1462,7 +1640,7 @@ local function buildBridges(map: Folder)
 			local fwd = Circuit.forwardAt(i)
 			local g = part({
 				Name = "BridgeGirder",
-				Size = Vector3.new(ROAD_WIDTH + 14, 7, 64),
+				Size = Vector3.new(widthAt(i) + 14, 7, 64),
 				CFrame = CFrame.lookAt(m, m + fwd, UP) * CFrame.new(0, -5, 0),
 				Color = RED,
 				Material = Enum.Material.Metal,
@@ -1504,7 +1682,7 @@ function MapBuilder.build(): Folder
 	local fwd = Circuit.forwardAt(1)
 	local line = part({
 		Name = "FinishLine",
-		Size = Vector3.new(ROAD_WIDTH, 0.6, 8),
+		Size = Vector3.new(widthAt(1), 0.6, 8),
 		CFrame = CFrame.lookAt(s1.pos + Vector3.new(0, 1.4, 0), s1.pos + Vector3.new(0, 1.4, 0) + fwd, UP),
 		Color = Color3.fromRGB(250, 250, 255),
 		Material = Enum.Material.Neon,

tools/diag_raycast.luau

@@ -0,0 +1,69 @@
+-- Diagnose the raycast outlier: report WHICH parts the rays hit.
+-- The v3 raycast filtered on the whole map, so overhead geometry (overpass
+-- decks, girders, gantries, tunnel ceilings) can be hit before the road.
+local sss = game:GetService("ServerScriptService")
+local orig = sss:FindFirstChild("MapBuilder", true)
+local clone = orig:Clone()
+clone.Name = "MapBuilder_Diag"
+clone.Parent = orig.Parent
+local MB = require(clone)
+local map = MB.build()
+local Circuit = require(game:GetService("ReplicatedStorage"):FindFirstChild("Circuit", true))
+local samples = Circuit.samples
+local n = #samples
+
+local rp = RaycastParams.new()
+rp.FilterType = Enum.RaycastFilterType.Include
+rp.FilterDescendantsInstances = { map }
+
+local out = {}
+-- Road-only params: what the driver actually touches.
+local roadOnly = RaycastParams.new()
+roadOnly.FilterType = Enum.RaycastFilterType.Include
+roadOnly.FilterDescendantsInstances = { map:FindFirstChild("Expressway") }
+
+local worstAll, worstRoad, atAll, atRoad = 0, 0, -1, -1
+local hits = {}
+for i = 1, n do
+	local j = (i % n) + 1
+	local p = samples[j].pos
+	local fwd = (samples[(j % n) + 1].pos - samples[i].pos)
+	fwd = Vector3.new(fwd.X, 0, fwd.Z)
+	if fwd.Magnitude > 0.01 then
+		fwd = fwd.Unit
+		local o1 = p - fwd * 3 + Vector3.new(0, 40, 0)
+		local o2 = p + fwd * 3 + Vector3.new(0, 40, 0)
+		local a1 = workspace:Raycast(o1, Vector3.new(0, -120, 0), rp)
+		local a2 = workspace:Raycast(o2, Vector3.new(0, -120, 0), rp)
+		if a1 and a2 then
+			local d = math.abs(a1.Position.Y - a2.Position.Y)
+			if d > worstAll then
+				worstAll, atAll = d, i
+			end
+			if d > 1 then
+				hits[#hits + 1] = ("i=%d(%s) d=%.2f hitA=%s hitB=%s"):format(
+					i, samples[i].piece, d, a1.Instance.Name, a2.Instance.Name)
+			end
+		end
+		local b1 = workspace:Raycast(o1, Vector3.new(0, -120, 0), roadOnly)
+		local b2 = workspace:Raycast(o2, Vector3.new(0, -120, 0), roadOnly)
+		if b1 and b2 then
+			local d = math.abs(b1.Position.Y - b2.Position.Y)
+			if d > worstRoad then
+				worstRoad, atRoad = d, i
+			end
+		end
+	end
+end
+
+out[#out + 1] = ("ALL-GEOMETRY worst=%.3f at i=%d"):format(worstAll, atAll)
+out[#out + 1] = ("ROAD-ONLY  worst=%.3f at i=%d -> %s"):format(
+	worstRoad, atRoad, worstRoad <= 0.60 and "PASS" or "FAIL")
+out[#out + 1] = ("outlier samples (d>1), first 12 of %d:"):format(#hits)
+for k = 1, math.min(12, #hits) do
+	out[#out + 1] = "  " .. hits[k]
+end
+
+map:Destroy()
+clone:Destroy()
+return table.concat(out, "\n")

tools/push_mapbuilder.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""Push MapBuilder.luau source into a LIVE (Play-mode) Studio via the MCP bridge.
+
+Rojo cannot reach the running server datamodel in Play mode, so the source is
+shipped as ~9000-char chunks into named StringValue children (p0, p1, ...) under
+ReplicatedStorage, then concatenated and assigned to the ModuleScript's .Source.
+
+Named children matter: a retried chunk overwrites its slot instead of appending
+a duplicate. Bridge calls fail intermittently ("Unable to find an active Studio
+instance") even while Studio is up, so every call is retried.
+"""
+import sys, time
+
+sys.path.insert(0, "/tmp")
+import rbx
+
+SRC_PATH = "/home/jesse/roblox/tokyo-wangan-racing/src/server/MapBuilder.luau"
+MODULE = 'game:GetService("ServerScriptService"):FindFirstChild("Server", true)'
+CHUNK = 9000
+
+
+def call(code, tries=4):
+    last = None
+    for a in range(tries):
+        try:
+            r = rbx.call(
+                "execute_luau",
+                {"code": code, "datamodel_type": "Server"},
+                warmup=6,
+                wait=45,
+                timeout=300,
+            )
+            if r and "Unable to find an active Studio" not in str(r):
+                return r
+            last = r
+        except Exception as e:  # noqa: BLE001
+            last = e
+        time.sleep(5)
+    raise RuntimeError(f"bridge failed after {tries} tries: {last}")
+
+
+src = open(SRC_PATH, encoding="utf-8").read()
+nbytes = len(src.encode())
+print(f"source: {len(src)} chars, {nbytes} utf-8 bytes")
+
+# Pick a long-bracket level that does not occur in the source.
+lvl = 0
+while ("]" + "=" * lvl + "]") in src or ("[" + "=" * lvl + "[") in src:
+    lvl += 1
+eq = "=" * lvl
+print(f"long-bracket level: {lvl}")
+
+chunks = [src[i : i + CHUNK] for i in range(0, len(src), CHUNK)]
+print(f"chunks: {len(chunks)}")
+
+print(call('local rs=game:GetService("ReplicatedStorage")\n'
+           'local b=rs:FindFirstChild("MBBuf") if b then b:Destroy() end\n'
+           'b=Instance.new("Folder") b.Name="MBBuf" b.Parent=rs\n'
+           'return "buffer reset"'))
+
+for i, c in enumerate(chunks):
+    code = (
+        'local rs=game:GetService("ReplicatedStorage")\n'
+        'local b=rs:WaitForChild("MBBuf")\n'
+        f'local n="p{i}"\n'
+        "local sv=b:FindFirstChild(n) or Instance.new(\"StringValue\")\n"
+        "sv.Name=n sv.Parent=b\n"
+        f"sv.Value=[{eq}[{c}]{eq}]\n"
+        f'return "p{i} "..#sv.Value'
+    )
+    print(f"  chunk {i+1}/{len(chunks)}: {call(code)}")
+
+assemble = f"""
+local rs=game:GetService("ReplicatedStorage")
+local b=rs:WaitForChild("MBBuf")
+local parts={{}}
+for i=0,{len(chunks)-1} do
+    local sv=b:FindFirstChild("p"..i)
+    if not sv then return "MISSING CHUNK p"..i end
+    parts[#parts+1]=sv.Value
+end
+local s=table.concat(parts)
+local sss=game:GetService("ServerScriptService")
+local m=sss:FindFirstChild("MapBuilder", true)
+if not m then return "MapBuilder ModuleScript NOT FOUND" end
+m.Source=s
+return ("assembled=%d expected={nbytes} match=%s class=%s path=%s"):format(
+    #s, tostring(#s=={nbytes}), m.ClassName, m:GetFullName())
+"""
+print(call(assemble))

tools/verify_c1_fixes.luau

@@ -0,0 +1,164 @@
+-- Headless verification of the four C1 fidelity fixes.
+-- Asserts NUMBERS the build produced, not "the build ran".
+local sss = game:GetService("ServerScriptService")
+local orig = sss:FindFirstChild("MapBuilder", true)
+if not orig then return "FAIL: MapBuilder not found" end
+
+-- Freshness gate: prove we are measuring the NEW source before trusting anything.
+local fresh = {}
+for _, tok in ipairs({ "WIDE_ROAD_WIDTH", "BridgeGirder", "GinzaOverpass", "isJctSample", "GIRDER_H" }) do
+	fresh[#fresh + 1] = tok .. "=" .. tostring(orig.Source:find(tok, 1, true) ~= nil)
+end
+
+-- Fresh-require so Studio's module cache cannot serve a stale copy.
+local clone = orig:Clone()
+clone.Name = "MapBuilder_Verify"
+clone.Parent = orig.Parent
+local okM, MB = pcall(require, clone)
+if not okM then
+	clone:Destroy()
+	return "FAIL require: " .. tostring(MB)
+end
+
+local okB, map = pcall(MB.build)
+if not okB then
+	clone:Destroy()
+	return "FAIL build: " .. tostring(map)
+end
+
+local Circuit = require(game:GetService("ReplicatedStorage"):FindFirstChild("Circuit", true))
+
+local counts = {}
+local decks = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		counts[d.Name] = (counts[d.Name] or 0) + 1
+		local idx = d.Name:match("^Road_(%d+)$")
+		if idx then
+			decks[tonumber(idx)] = d
+		end
+	end
+end
+
+local out = {}
+local function line(s)
+	out[#out + 1] = s
+end
+line("FRESHNESS: " .. table.concat(fresh, " "))
+
+-- FIX 1: bridge flag wired up ------------------------------------------------
+local nBridgeSamples = 0
+for _, s in ipairs(Circuit.samples) do
+	if s.bridge then
+		nBridgeSamples += 1
+	end
+end
+local girders = counts["BridgeGirder"] or 0
+line(("FIX1 bridge flag: samples=%d girders=%d piercaps=%d piercols=%d -> %s"):format(
+	nBridgeSamples, girders, counts["PierCap"] or 0, counts["PierColumn"] or 0,
+	(girders >= nBridgeSamples * 0.5 and girders > 0) and "PASS" or "FAIL"))
+
+-- FIX 2: Ginza overpasses with piers between the carriageways ----------------
+local ov, pier = counts["GinzaOverpass"] or 0, counts["OverpassPier"] or 0
+line(("FIX2 Ginza 跨道橋: overpass=%d pier=%d parapet=%d -> %s"):format(
+	ov, pier, counts["OverpassParapet"] or 0, (ov > 0 and pier == ov) and "PASS" or "FAIL"))
+
+-- Piers must actually sit BETWEEN the carriageways (near the centreline),
+-- not off to one side -- that is the whole point of the fix.
+local worstPier = 0
+for _, d in ipairs(map:GetDescendants()) do
+	if d.Name == "OverpassPier" then
+		local best = math.huge
+		for i, s in ipairs(Circuit.samples) do
+			if s.piece == "Ginza" then
+				local dd = (Vector3.new(s.pos.X, 0, s.pos.Z) - Vector3.new(d.Position.X, 0, d.Position.Z)).Magnitude
+				best = math.min(best, dd)
+				local _ = i
+			end
+		end
+		worstPier = math.max(worstPier, best)
+	end
+end
+line(("FIX2 pier centreline offset: worst=%.2f studs -> %s"):format(
+	worstPier, worstPier < 6 and "PASS" or "FAIL"))
+
+-- FIX 3: 40 km/h plates in junctions ----------------------------------------
+local n40, n50 = 0, 0
+for _, d in ipairs(map:GetDescendants()) do
+	if d.Name == "SpeedLimitSign" then
+		for _, g in ipairs(d:GetDescendants()) do
+			if g:IsA("TextLabel") then
+				if g.Text == "40" then
+					n40 += 1
+				elseif g.Text == "50" then
+					n50 += 1
+				end
+				break
+			end
+		end
+	end
+end
+line(("FIX3 speed plates: 40kmh=%d 50kmh=%d -> %s"):format(
+	n40, n50, (n40 > 0 and n50 > 0) and "PASS" or "FAIL"))
+
+-- FIX 4: Shiodome 3+3 widening ----------------------------------------------
+local shioW, baseW, shioN, baseN = 0, 0, 0, 0
+for i, s in ipairs(Circuit.samples) do
+	local d = decks[i]
+	if d then
+		if s.piece == "Shiodome" then
+			shioW += d.Size.X
+			shioN += 1
+		elseif s.piece == "Kasumigaseki" then
+			baseW += d.Size.X
+			baseN += 1
+		end
+	end
+end
+shioW = shioN > 0 and shioW / shioN or 0
+baseW = baseN > 0 and baseW / baseN or 0
+line(("FIX4 deck width: Shiodome avg=%.1f  Kasumigaseki avg=%.1f -> %s"):format(
+	shioW, baseW, (shioW > baseW + 8) and "PASS" or "FAIL"))
+
+-- Lane boundary count: 3 lanes per side needs 2 boundary lines per side.
+local function boundariesNear(zone)
+	local best, bestI = math.huge, nil
+	for i, s in ipairs(Circuit.samples) do
+		if s.piece == zone then
+			bestI = bestI or i
+			best = 0
+		end
+	end
+	local _ = best
+	return bestI
+end
+local si = boundariesNear("Shiodome")
+local nLines = 0
+if si and decks[si] then
+	local c = decks[si].Position
+	for _, d in ipairs(map:GetDescendants()) do
+		if (d.Name == "LaneLine" or d.Name == "Dash") and (d.Position - c).Magnitude < 40 then
+			nLines += 1
+		end
+	end
+end
+line(("FIX4 lane boundaries near Shiodome sample %s: %d markings"):format(tostring(si), nLines))
+
+-- REGRESSION: no holes in the deck, seam lips still within tolerance --------
+local holes, maxLip = 0, 0
+local n = #Circuit.samples
+for i = 1, n do
+	local a, b = decks[i], decks[(i % n) + 1]
+	if a and b then
+		local lip = math.abs((a.Position.Y + a.Size.Y / 2) - (b.Position.Y + b.Size.Y / 2))
+		maxLip = math.max(maxLip, lip)
+	elseif not a then
+		holes += 1
+	end
+end
+line(("REGRESSION decks=%d holes=%d maxSeamLip=%.3f -> %s"):format(
+	n, holes, maxLip, (holes == 0 and maxLip <= 0.30) and "PASS" or "FAIL"))
+
+map:Destroy()
+clone:Destroy()
+return table.concat(out, "\n")

tools/verify_c1_fixes2.luau

@@ -0,0 +1,144 @@
+-- Verification v2: corrected geometric metrics.
+--
+-- v1 had two badly-written assertions (the CODE was fine, the TEST was wrong):
+--   * seam lip used Position.Y + Size.Y/2, but decks are pitched by
+--     CFrame.lookAt(mid, mid+dir, UP), so that is the centre height, not the
+--     surface height at the seam. On a sloped segment it over-reports badly.
+--     Correct: evaluate each deck's TOP SURFACE at the shared sample point.
+--   * pier offset measured distance to the nearest sample, but piers sit at
+--     segment MIDPOINTS and samples are 60 studs apart -- so ~30.0 is the
+--     correct answer for a perfectly centred pier. Correct: perpendicular
+--     distance to the centreline polyline.
+local sss = game:GetService("ServerScriptService")
+local orig = sss:FindFirstChild("MapBuilder", true)
+local clone = orig:Clone()
+clone.Name = "MapBuilder_Verify2"
+clone.Parent = orig.Parent
+local okM, MB = pcall(require, clone)
+if not okM then
+	clone:Destroy()
+	return "FAIL require: " .. tostring(MB)
+end
+local okB, map = pcall(MB.build)
+if not okB then
+	clone:Destroy()
+	return "FAIL build: " .. tostring(map)
+end
+
+local Circuit = require(game:GetService("ReplicatedStorage"):FindFirstChild("Circuit", true))
+local samples = Circuit.samples
+local n = #samples
+
+local decks = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		local idx = d.Name:match("^Road_(%d+)$")
+		if idx then
+			decks[tonumber(idx)] = d
+		end
+	end
+end
+
+local out = {}
+local function line(s)
+	out[#out + 1] = s
+end
+
+-- TRUE SEAM LIP -------------------------------------------------------------
+-- Deck i spans local z in [-Size.Z/2, +Size.Z/2] and is 2 studs longer than
+-- its segment (deliberate overlap), so the shared sample point sits at
+-- local z = +(Size.Z - 2)/2 for deck i and -(Size.Z - 2)/2 for deck i+1.
+local worst, worstAt = 0, -1
+local lips = {}
+for i = 1, n do
+	local j = (i % n) + 1
+	local a, b = decks[i], decks[j]
+	if a and b then
+		local ta = (a.CFrame * CFrame.new(0, a.Size.Y / 2, (a.Size.Z - 2) / 2)).Y
+		local tb = (b.CFrame * CFrame.new(0, b.Size.Y / 2, -(b.Size.Z - 2) / 2)).Y
+		local lip = math.abs(ta - tb)
+		lips[#lips + 1] = { lip, i }
+		if lip > worst then
+			worst, worstAt = lip, i
+		end
+	end
+end
+table.sort(lips, function(p, q)
+	return p[1] > q[1]
+end)
+local top = {}
+for k = 1, math.min(5, #lips) do
+	top[#top + 1] = ("i=%d:%.4f(%s)"):format(lips[k][2], lips[k][1], samples[lips[k][2]].piece)
+end
+line(("SEAM true max lip=%.4f at i=%d -> %s"):format(worst, worstAt, worst <= 0.30 and "PASS" or "FAIL"))
+line("SEAM top5: " .. table.concat(top, " "))
+
+-- Deck-width continuity: the taper must not jump more than a few studs
+-- between neighbours, or the widening would look like a step in the road.
+local maxJump, jumpAt = 0, -1
+for i = 1, n do
+	local j = (i % n) + 1
+	if decks[i] and decks[j] then
+		local dw = math.abs(decks[i].Size.X - decks[j].Size.X)
+		if dw > maxJump then
+			maxJump, jumpAt = dw, i
+		end
+	end
+end
+line(("TAPER max width step=%.2f studs at i=%d -> %s"):format(
+	maxJump, jumpAt, maxJump <= 6 and "PASS" or "FAIL"))
+
+-- PIER LATERAL OFFSET -------------------------------------------------------
+-- Perpendicular distance from each pier to the centreline polyline (XZ).
+local function segDist(p, u, v)
+	local pu = Vector3.new(p.X - u.X, 0, p.Z - u.Z)
+	local uv = Vector3.new(v.X - u.X, 0, v.Z - u.Z)
+	local L2 = uv:Dot(uv)
+	if L2 < 1e-6 then
+		return pu.Magnitude
+	end
+	local t = math.clamp(pu:Dot(uv) / L2, 0, 1)
+	return (pu - uv * t).Magnitude
+end
+
+local worstPier, nPier = 0, 0
+for _, d in ipairs(map:GetDescendants()) do
+	if d.Name == "OverpassPier" then
+		nPier += 1
+		local best = math.huge
+		for i = 1, n do
+			local j = (i % n) + 1
+			best = math.min(best, segDist(d.Position, samples[i].pos, samples[j].pos))
+		end
+		worstPier = math.max(worstPier, best)
+	end
+end
+line(("PIER lateral offset from centreline: n=%d worst=%.3f studs -> %s"):format(
+	nPier, worstPier, (nPier > 0 and worstPier < 3) and "PASS" or "FAIL"))
+
+-- Girders must hang BELOW their deck, never poke through the driving surface.
+local bad, nG = 0, 0
+for _, d in ipairs(map:GetDescendants()) do
+	if d.Name == "BridgeGirder" then
+		nG += 1
+		local topY = d.Position.Y + d.Size.Y / 2
+		local best, bi = math.huge, nil
+		for i = 1, n do
+			local dd = (Vector3.new(samples[i].pos.X, 0, samples[i].pos.Z) - Vector3.new(d.Position.X, 0, d.Position.Z)).Magnitude
+			if dd < best then
+				best, bi = dd, i
+			end
+		end
+		if bi and decks[bi] then
+			local deckBottom = decks[bi].Position.Y - decks[bi].Size.Y / 2
+			if topY > deckBottom + 0.6 then
+				bad += 1
+			end
+		end
+	end
+end
+line(("GIRDER below deck: n=%d intruding=%d -> %s"):format(nG, bad, bad == 0 and "PASS" or "FAIL"))
+
+map:Destroy()
+clone:Destroy()
+return table.concat(out, "\n")

tools/verify_c1_fixes3.luau

@@ -0,0 +1,120 @@
+-- Verification v3: seam lip measured against the KNOWN shared sample point.
+--
+-- v2 still had a sign bug: Roblox CFrame.LookVector is the -Z axis, so a deck
+-- built with CFrame.lookAt(mid, mid+dir) has its FORWARD end at local
+-- z = -(Size.Z-2)/2 and its REAR end at +(Size.Z-2)/2. v2 had these swapped,
+-- comparing deck i's rear to deck i+1's rear -- a systematic error that
+-- reported a near-constant 5.2 on every sloped segment regardless of zone.
+--
+-- This version avoids guessing signs: for each deck it takes BOTH end faces,
+-- picks whichever is nearer the shared sample point b.pos in XZ, and reports
+-- the surface-height difference there. Sanity-checked against the flat-ground
+-- expectation and cross-checked with a downward raycast.
+local sss = game:GetService("ServerScriptService")
+local orig = sss:FindFirstChild("MapBuilder", true)
+local clone = orig:Clone()
+clone.Name = "MapBuilder_Verify3"
+clone.Parent = orig.Parent
+local okM, MB = pcall(require, clone)
+if not okM then
+	clone:Destroy()
+	return "FAIL require: " .. tostring(MB)
+end
+local okB, map = pcall(MB.build)
+if not okB then
+	clone:Destroy()
+	return "FAIL build: " .. tostring(map)
+end
+
+local Circuit = require(game:GetService("ReplicatedStorage"):FindFirstChild("Circuit", true))
+local samples = Circuit.samples
+local n = #samples
+
+local decks = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		local idx = d.Name:match("^Road_(%d+)$")
+		if idx then
+			decks[tonumber(idx)] = d
+		end
+	end
+end
+
+local out = {}
+local function line(s)
+	out[#out + 1] = s
+end
+
+-- Top-surface height of deck `d` at the end nearest world point `p` (XZ).
+local function topAtNearestEnd(d, p)
+	local half = (d.Size.Z - 2) / 2
+	local best, bestD = nil, math.huge
+	for _, z in ipairs({ -half, half }) do
+		local w = d.CFrame * CFrame.new(0, d.Size.Y / 2, z)
+		local dd = (Vector3.new(w.X, 0, w.Z) - Vector3.new(p.X, 0, p.Z)).Magnitude
+		if dd < bestD then
+			best, bestD = w.Y, dd
+		end
+	end
+	return best, bestD
+end
+
+local worst, worstAt, worstGap = 0, -1, 0
+local lips = {}
+for i = 1, n do
+	local j = (i % n) + 1
+	local a, b = decks[i], decks[j]
+	if a and b then
+		local shared = samples[j].pos -- the sample both decks meet at
+		local ya, da = topAtNearestEnd(a, shared)
+		local yb, db = topAtNearestEnd(b, shared)
+		local lip = math.abs(ya - yb)
+		lips[#lips + 1] = { lip, i, math.max(da, db) }
+		if lip > worst then
+			worst, worstAt, worstGap = lip, i, math.max(da, db)
+		end
+	end
+end
+table.sort(lips, function(p, q)
+	return p[1] > q[1]
+end)
+local top = {}
+for k = 1, math.min(5, #lips) do
+	top[#top + 1] = ("i=%d:%.4f(%s)"):format(lips[k][2], lips[k][1], samples[lips[k][2]].piece)
+end
+line(("SEAM max lip=%.4f at i=%d (endpoint XZ err %.3f) -> %s"):format(
+	worst, worstAt, worstGap, worst <= 0.30 and "PASS" or "FAIL"))
+line("SEAM top5: " .. table.concat(top, " "))
+
+-- Independent cross-check: raycast down onto the driving surface just before
+-- and just after each seam. A real step would show up here too.
+-- MUST filter to the Expressway folder only. Filtering on the whole map lets
+-- the ray hit TunnelCeiling / MedianFence / overpass decks on the way down and
+-- reports a bogus ~19-stud "step" that has nothing to do with the road surface.
+local rp = RaycastParams.new()
+rp.FilterType = Enum.RaycastFilterType.Include
+rp.FilterDescendantsInstances = { map:FindFirstChild("Expressway") }
+local rWorst, rAt = 0, -1
+for i = 1, n do
+	local j = (i % n) + 1
+	local p = samples[j].pos
+	local fwd = (samples[(j % n) + 1].pos - samples[i].pos)
+	fwd = Vector3.new(fwd.X, 0, fwd.Z)
+	if fwd.Magnitude > 0.01 then
+		fwd = fwd.Unit
+		local h1 = workspace:Raycast(p - fwd * 3 + Vector3.new(0, 40, 0), Vector3.new(0, -120, 0), rp)
+		local h2 = workspace:Raycast(p + fwd * 3 + Vector3.new(0, 40, 0), Vector3.new(0, -120, 0), rp)
+		if h1 and h2 then
+			local d = math.abs(h1.Position.Y - h2.Position.Y)
+			if d > rWorst then
+				rWorst, rAt = d, i
+			end
+		end
+	end
+end
+line(("RAYCAST surface step across seams (+-3 studs): max=%.4f at i=%d -> %s"):format(
+	rWorst, rAt, rWorst <= 0.60 and "PASS" or "FAIL"))
+
+map:Destroy()
+clone:Destroy()
+return table.concat(out, "\n")