diff --git a/selfdrive/locationd/test/test_lagd.py b/selfdrive/locationd/test/test_lagd.py
new file mode 100644
index 0000000000..6de0130cde
--- /dev/null
+++ b/selfdrive/locationd/test/test_lagd.py
@@ -0,0 +1,102 @@
+import random
+import numpy as np
+import time
+from unittest import mock
+import pytest
+
+from cereal import messaging
+from openpilot.selfdrive.locationd.lagd import LateralLagEstimator, retrieve_initial_lag, masked_normalized_cross_correlation
+from openpilot.selfdrive.test.process_replay.migration import migrate, migrate_carParams
+from openpilot.selfdrive.locationd.test.test_locationd_scenarios import TEST_ROUTE
+from openpilot.common.params import Params
+from openpilot.tools.lib.logreader import LogReader
+from openpilot.system.hardware import PC
+
+MAX_ERR_FRAMES = 1
+
+
+class TestLagd:
+  def test_read_saved_params(self):
+    params = Params()
+
+    lr = migrate(LogReader(TEST_ROUTE), [migrate_carParams])
+    CP = next(m for m in lr if m.which() == "carParams").carParams
+
+    msg = messaging.new_message('liveDelay')
+    msg.liveDelay.lateralDelayEstimate = random.random()
+    msg.liveDelay.validBlocks = random.randint(1, 10)
+    params.put("LiveLag", msg.to_bytes())
+    params.put("CarParamsPrevRoute", CP.as_builder().to_bytes())
+
+    saved_lag_params = retrieve_initial_lag(params, CP)
+    assert saved_lag_params is not None
+
+    lag, valid_blocks = saved_lag_params
+    assert lag == msg.liveDelay.lateralDelayEstimate
+    assert valid_blocks == msg.liveDelay.validBlocks
+
+  def test_ncc(self):
+    lag_frames = random.randint(1, 20)
+
+    desired_sig = np.sin(np.arange(0.0, 10.0, 0.1))
+    actual_sig = np.sin(np.arange(0.0, 10.0, 0.1) - lag_frames * 0.1)
+    mask = np.ones(len(desired_sig), dtype=bool)
+
+    corr = masked_normalized_cross_correlation(desired_sig, actual_sig, mask, 200)[len(desired_sig) - 1:len(desired_sig) + 20]
+    assert np.argmax(corr) == lag_frames
+
+    # add some noise
+    desired_sig += np.random.normal(0, 0.05, len(desired_sig))
+    actual_sig += np.random.normal(0, 0.05, len(actual_sig))
+    corr = masked_normalized_cross_correlation(desired_sig, actual_sig, mask, 200)[len(desired_sig) - 1:len(desired_sig) + 20]
+    assert np.argmax(corr)  in range(lag_frames - MAX_ERR_FRAMES, lag_frames + MAX_ERR_FRAMES + 1)
+
+  def test_estimator(self):
+    class ZeroMock(mock.Mock):
+      def __getattr__(self, *args):
+        return 0
+
+    dt = 0.05
+    lag_frames = random.randint(1, 20)
+
+    mocked_CP = mock.Mock(steerActuatorDelay=1.0)
+    estimator = LateralLagEstimator(mocked_CP, 0.05,
+                                    block_count=10, min_valid_block_count=0,
+                                    block_size=1, okay_window_sec=100 * dt,
+                                    min_recovery_buffer_sec=0, min_yr=0)
+    for i in range(100):
+      t = i * dt
+      vego = 20.0
+      desired_cuvature = np.cos(t) * 100 / (vego ** 2)
+      actual_yr = np.cos(t - lag_frames * dt) * 100 / vego
+      msgs = [
+        (t, "carControl", mock.Mock(latActive=True)),
+        (t, "carState", mock.Mock(vEgo=vego, steeringPressed=False)),
+        (t, "controlsState", mock.Mock(desiredCurvature=desired_cuvature, lateralControlState=mock.Mock(which=mock.Mock(return_value='debugControlState'), debugControlState=ZeroMock()))),
+        (t, "livePose", mock.Mock(orientationNED=ZeroMock(), velocityDevice=ZeroMock(), accelerationDevice=ZeroMock(), angularVelocityDevice=ZeroMock(z=actual_yr))),
+      ]
+      for t, w, m in msgs:
+        estimator.handle_log(t, w, m)
+      estimator.update_points()
+    estimator.update_estimate()
+
+    # expect one block filled, with lateralDelayEstimate equal to lateralDelay equal to lag_frames
+    output = estimator.get_msg(True)
+    assert np.allclose(output.liveDelay.lateralDelay, lag_frames * dt, atol=0.01)
+    assert output.liveDelay.status == 'estimated'
+    assert output.liveDelay.validBlocks == 1
+
+  @pytest.mark.skipif(PC, reason="only on device")
+  def test_estimator_performance(self):
+    mocked_CP = mock.Mock(steerActuatorDelay=0.1)
+    estimator = LateralLagEstimator(mocked_CP, 0.05)
+
+    ds = []
+    for i in range(1000):
+      st = time.perf_counter()
+      estimator.update_points()
+      estimator.update_estimate()
+      d = time.perf_counter() - st
+      ds.append(d)
+
+    assert np.mean(ds) < 0.05