Two data sources
StraitTraffic combines two independent sources. Official daily transit statistics come from IMF PortWatch, a project of the International Monetary Fund and the University of Oxford that counts ships passing 28 maritime chokepoints from satellite AIS. Live positions and our own transit counter come from terrestrial AIS receivers via AISStream. The two are always labelled separately on the site and are never mixed in one number.
Official statistics: IMF PortWatch
PortWatch publishes, for every chokepoint and every UTC day since 2019, the number of vessels that completed a transit, broken down into tankers, container ships, dry bulk, general cargo and ro-ro, together with an estimate of the capacity carried. The data is released weekly, usually on Tuesdays, so the latest available day is typically six to twelve days behind today; the exact date is shown next to every figure.
From these daily counts StraitTraffic derives the 7-day and 30-day averages, the “normal” level (the average of the previous calendar year unless a strait defines its own reference period), the share of normal, and the traffic status. Because PortWatch only sees vessels that broadcast AIS, ships that switch their transponders off are not counted, and true flows can be somewhat higher than reported.
Traffic status
The status compares the 7-day average of official PortWatch transits with the strait’s normal level. Within ±25% of normal the status is “Normal traffic”; 25–50% below is “Traffic below normal”; 50–75% below is “Traffic significantly below normal”; more than 75% below is “Significant disruption”; more than 25% above is “Traffic above normal”. A declared closure is shown separately, with the number of days since it was declared, and stays until a reopening is recorded on the timeline.
Live layer: terrestrial AIS
The live layer is built from AIS (Automatic Identification System) broadcasts. Ships above 300 gross tonnage on international voyages, and all passenger ships, are required to transmit their identity, position, speed and course. StraitTraffic receives these broadcasts from a network of shore-based receivers via AISStream and processes them on its own servers.
Terrestrial AIS coverage is limited by the range of shore receivers, typically 40–60 nautical miles from the coast. Vessels can also switch AIS off, transmit incorrect data, or be affected by GPS interference, which is a known issue in several of the regions we monitor.
What StraitTraffic calculates itself
- Vessels in zone now — vessels that have transmitted a position inside the monitored strait zone in the last 30 minutes.
- Transits — for each strait we define three areas: the strait zone and an approach area on each side. A transit is counted when a vessel is observed in one approach area, then inside the zone, then in the opposite approach area. A vessel that enters the zone and turns back is not counted. Repeated AIS messages never create additional transits: one physical passage is one transit.
- Direction — determined by which approach area the vessel came from and which one it went to.
- Ship type — taken from the AIS type code (tanker, cargo, passenger). Container ships and LNG/LPG carriers cannot be identified from the code alone, so they are recognised from the vessel’s name and dimensions; when in doubt the vessel stays in its base category. Vessels with unknown or unusual types are shown as “Other / Unknown” rather than being dropped.
- Daily history — our own completed transits are stored and summed per UTC day from the day monitoring began. Long-term history on the site comes from IMF PortWatch only.
Data quality safeguards
- Position reports that imply an impossible speed (a jump in coordinates) are discarded.
- Messages older than the last accepted one for the same vessel are ignored.
- A vessel that disappears inside the zone for longer than a plausible passage time is recorded as an unconfirmed passage and excluded from all counts.
- Comparisons with weekly or monthly averages are only shown once the history actually covers that period.
Where the live layer works
Coverage is checked automatically for every strait every few minutes, from the data itself. If no positions have arrived from the area in the last 24 hours, the page says there is no terrestrial AIS coverage and shows no live map. If vessels are seen but our own transit count stays below 30% of the official PortWatch level, the receivers only reach part of the strait: the map is shown, but our counters are hidden. Only where the receivers see both approaches does the page show our own transit count.
As of September 2026 our own counters work in the Suez Canal, the Strait of Dover and the Strait of Gibraltar. The Strait of Malacca, the Bosporus, the Panama Canal and the Taiwan Strait have partial coverage. There is no terrestrial coverage in the Strait of Hormuz, in Bab el-Mandeb or around the Cape of Good Hope; there the official PortWatch statistics are the only counts.
Data freshness
Official statistics always show the latest published day and when the next weekly release is expected. If PortWatch has not published for more than two weeks, the page marks the release as delayed and warns that the figures may not reflect the current situation.
Every page with a live map shows the state of the AIS feed. “Live” means a message was received within the last two minutes; “Delayed” means the last message is older than that but younger than 15 minutes; “Temporarily unavailable” means the feed has stopped and the positions shown must not be treated as current.
Limitations
StraitTraffic is not an official navigation or vessel-traffic service and must not be used for navigation, safety or legal purposes. Counts can be lower than reality where receiver coverage is poor, and vessel identities depend on what ships broadcast. Treat the figures as an indicator of activity and trend, not as an authoritative record.