diff --git a/selfdrive/ui/onroad/model_renderer.py b/selfdrive/ui/onroad/model_renderer.py
index 441042bf6..189e9539a 100644
--- a/selfdrive/ui/onroad/model_renderer.py
+++ b/selfdrive/ui/onroad/model_renderer.py
@@ -711,21 +711,25 @@ class ModelRenderer(Widget):
     lane_width_left = float(plan.laneWidthLeft) if plan.laneWidthLeft > 0 else 0.0
     lane_width_right = float(plan.laneWidthRight) if plan.laneWidthRight > 0 else 0.0
 
-    if lane_width_left <= 0 or lane_width_right <= 0:
-      self._adjacent_path_vertices = [np.empty((0, 2), dtype=np.float32), np.empty((0, 2), dtype=np.float32)]
-      return
+    # Clear stale polygons, then build each available side independently.
+    self._adjacent_path_vertices = [
+      np.empty((0, 2), dtype=np.float32),
+      np.empty((0, 2), dtype=np.float32),
+    ]
 
     # Left adjacent: average of lane_lines[0] and lane_lines[1]
-    self._adjacent_path_vertices[0] = self._get_adjacent_path_polygon(
-      self._lane_lines[0].raw_points, self._lane_lines[1].raw_points,
-      lane_width_left / 2.0, max_idx, max_distance
-    )
+    if lane_width_left > 0:
+      self._adjacent_path_vertices[0] = self._get_adjacent_path_polygon(
+        self._lane_lines[0].raw_points, self._lane_lines[1].raw_points,
+        lane_width_left / 2.0, max_idx, max_distance
+      )
 
     # Right adjacent: average of lane_lines[2] and lane_lines[3]
-    self._adjacent_path_vertices[1] = self._get_adjacent_path_polygon(
-      self._lane_lines[2].raw_points, self._lane_lines[3].raw_points,
-      lane_width_right / 2.0, max_idx, max_distance
-    )
+    if lane_width_right > 0:
+      self._adjacent_path_vertices[1] = self._get_adjacent_path_polygon(
+        self._lane_lines[2].raw_points, self._lane_lines[3].raw_points,
+        lane_width_right / 2.0, max_idx, max_distance
+      )
 
   def _get_adjacent_path_polygon(self, line1: np.ndarray, line2: np.ndarray, y_off: float, max_idx: int, max_distance: float) -> np.ndarray:
     if line1.shape[0] == 0 or line2.shape[0] == 0:
diff --git a/selfdrive/ui/tests/test_adjacent_lane_renderer.py b/selfdrive/ui/tests/test_adjacent_lane_renderer.py
new file mode 100644
index 000000000..379cb3e37
--- /dev/null
+++ b/selfdrive/ui/tests/test_adjacent_lane_renderer.py
@@ -0,0 +1,89 @@
+import sys
+from types import ModuleType, SimpleNamespace
+
+import numpy as np
+import pytest
+
+
+def _stub_ui_dependencies() -> None:
+  """Keep this focused renderer test independent from a live raylib context."""
+  application = ModuleType("openpilot.system.ui.lib.application")
+  application.gui_app = SimpleNamespace(target_fps=20)
+  application.GL_VERSION = ""
+  application.FONT_SCALE = 1.0
+  application.font_fallback = lambda font: font
+  sys.modules[application.__name__] = application
+
+  shader_polygon = ModuleType("openpilot.system.ui.lib.shader_polygon")
+  shader_polygon.Gradient = lambda **kwargs: SimpleNamespace(**kwargs)
+  shader_polygon.draw_polygon = lambda *args, **kwargs: None
+  sys.modules[shader_polygon.__name__] = shader_polygon
+
+  text_measure = ModuleType("openpilot.system.ui.lib.text_measure")
+  text_measure.measure_text_cached = lambda *args, **kwargs: None
+  sys.modules[text_measure.__name__] = text_measure
+
+  widgets = ModuleType("openpilot.system.ui.widgets")
+  widgets.Widget = object
+  sys.modules[widgets.__name__] = widgets
+
+  ui_state = ModuleType("openpilot.selfdrive.ui.ui_state")
+  ui_state.ui_state = SimpleNamespace()
+  ui_state.UIStatus = SimpleNamespace()
+  sys.modules[ui_state.__name__] = ui_state
+
+
+_stub_ui_dependencies()
+import openpilot.selfdrive.ui.onroad.model_renderer as model_renderer
+
+
+class _FakeSubMaster:
+  def __init__(self, plan):
+    self.recv_frame = {"starpilotPlan": 1}
+    self._plan = plan
+
+  def __getitem__(self, key):
+    assert key == "starpilotPlan"
+    return self._plan
+
+
+@pytest.mark.parametrize(
+  ("lane_width_left", "lane_width_right", "expected_side"),
+  [
+    (3.5, 0.0, 0),
+    (0.0, 3.5, 1),
+  ],
+)
+def test_adjacent_path_keeps_a_single_valid_side(monkeypatch, lane_width_left, lane_width_right, expected_side):
+  fake_ui_state = SimpleNamespace(
+    sm=_FakeSubMaster(SimpleNamespace(laneWidthLeft=lane_width_left, laneWidthRight=lane_width_right)),
+    started_frame=0,
+  )
+  monkeypatch.setattr(model_renderer, "ui_state", fake_ui_state)
+
+  renderer = object.__new__(model_renderer.ModelRenderer)
+  left_outer = np.ones((2, 3), dtype=np.float32)
+  left_inner = np.ones((2, 3), dtype=np.float32)
+  right_inner = np.ones((2, 3), dtype=np.float32)
+  right_outer = np.ones((2, 3), dtype=np.float32)
+  renderer._lane_lines = [
+    SimpleNamespace(raw_points=left_outer),
+    SimpleNamespace(raw_points=left_inner),
+    SimpleNamespace(raw_points=right_inner),
+    SimpleNamespace(raw_points=right_outer),
+  ]
+  renderer._adjacent_path_vertices = [
+    np.ones((4, 2), dtype=np.float32),
+    np.ones((4, 2), dtype=np.float32),
+  ]
+
+  def fake_get_adjacent_path_polygon(line1, *_args):
+    side = 0 if line1 is left_outer else 1
+    return np.array([[side, 0], [side, 1], [side + 0.5, 1], [side + 0.5, 0]], dtype=np.float32)
+
+  monkeypatch.setattr(renderer, "_get_adjacent_path_polygon", fake_get_adjacent_path_polygon)
+
+  renderer._update_adjacent_paths(max_idx=1, max_distance=10.0)
+
+  assert renderer._adjacent_path_vertices[expected_side].size >= 4
+  assert renderer._adjacent_path_vertices[1 - expected_side].size == 0