diff --git a/bincio/segments/detect.py b/bincio/segments/detect.py
index af1626c..828d879 100644
--- a/bincio/segments/detect.py
+++ b/bincio/segments/detect.py
@@ -21,6 +21,15 @@ MATCH_RADIUS_M = 25          # max distance to segment start/end to open/close a
 CONFORMANCE_MAX_DEV_M = 50   # max allowed deviation for each interior segment point
 CONFORMANCE_MAX_FRAC = 0.30  # max fraction of interior points allowed to deviate
 
+# Minimum geometric speed (segment_distance / elapsed_s) per sport, in m/s.
+# Rejects false matches from long circuit rides where the track passes the
+# segment start early and the segment end hours later.
+_MIN_SPEED_MS: dict[str, float] = {
+    'cycling': 1.0,   # ~3.6 km/h — slowest realistic climb
+    'running': 0.5,   # ~1.8 km/h
+}
+_MIN_SPEED_DEFAULT = 0.2  # hiking / walking / unknown
+
 # ── fast distance approximation ───────────────────────────────────────────────
 
 _R = 6_371_000.0  # Earth radius in metres
@@ -250,6 +259,15 @@ def detect_one(track: ActivityTrack, seg: Segment) -> list[SegmentEffort]:
             # No end found — no more efforts possible starting at or after start_idx.
             break
 
+        # Reject implausibly slow matches (e.g. circuit rides matching start
+        # early and end hours later on a second pass through the area).
+        elapsed = track.times[end_idx] - track.times[start_idx]
+        if elapsed > 0:
+            min_speed = _MIN_SPEED_MS.get(track.sport, _MIN_SPEED_DEFAULT)
+            if seg.distance_m / elapsed < min_speed:
+                search_from = start_idx + 1
+                continue
+
         if _conformance_ok(track, seg, start_idx, end_idx):
             efforts.append(_extract_effort(track, seg, start_idx, end_idx))
             search_from = end_idx + 1