How does a geofence work?

A boundary is drawn on a map in software. When a tracked device's GPS position crosses it, the software records the event — arrival, departure — and can send an alert or start a timer.

How accurate is a geofence?

As accurate as the position feeding it. GPS.gov puts phone GPS at about 4.9 m under open sky, worse near buildings, bridges and trees, so very tight fences give false events.

Can geofencing prove detention?

It can time how long a truck was at a site. The US DOT Inspector General found GPS dwell data alone doesn't separate detention from normal loading time, so pair the fence with dock events.

Term

Geofence

A geofence is a virtual boundary drawn around a real place — a plant, a port, a city zone — so software can react when a tracked vehicle enters or leaves it. The US Federal Highway Administration describes geofencing as using GPS to create a virtual geographic boundary that triggers a response when a device crosses it.

Also called geofencing, geo-fence, virtual perimeter, geofence alert, geofence radiusUpdated 13 October 2026

What it is used for

Arrival and departure: a truck crossing the plant geofence logs gate-in and gate-out without anyone typing it.
Dwell and detention: time inside the fence is time at the site.
Arrival prediction: Audi's plant trial used zones of 1 km, 20 km and 50 km to track inbound trucks and estimate arrival precisely.
Rules by place: EU cities use geofences as a virtual perimeter inside which digital traffic rules apply — a Berlin trial let trucks in only in electric mode.
Port arrival: Rotterdam set a geofence 240 nautical miles around the Maascenter buoy to give ships its just-in-time arrival information.

Worked example: sizing a geofence

Warning needed: 30 minutes · truck at 80 km/h → 40 km radius · arrival fence at the plant: at least 25 m, about five times phone GPS accuracy of 4.9 m

GPS.gov says phones are typically accurate to within 4.9 m under open sky, worse near buildings, bridges and trees — so a fence tighter than a few tens of metres around a dock will fire falsely. The radii are illustrative.

Detention, measured

The US Department of Transportation's Inspector General defined detention in 2018 as time beyond the limit in shipping contracts, often 2 hours, and found a 15-minute increase in average dwell raises the expected crash rate by 6.2 percent. It also found carriers use GPS data to track dwell time but do not separate detention from legitimate loading and unloading time — which is what an arrival geofence plus a dock event can do.

The mistake people make

Drawing one fence for two jobs. A fence big enough to warn the dock an hour ahead is far too big to time the stay at the dock, and a fence around the dock is useless as a warning. Use nested fences — Audi used three — and give each event its own.

FAQ

Geofence: common questions

How does a geofence work?: A boundary is drawn on a map in software. When a tracked device's GPS position crosses it, the software records the event — arrival, departure — and can send an alert or start a timer.
How accurate is a geofence?: As accurate as the position feeding it. GPS.gov puts phone GPS at about 4.9 m under open sky, worse near buildings, bridges and trees, so very tight fences give false events.
Can geofencing prove detention?: It can time how long a truck was at a site. The US DOT Inspector General found GPS dwell data alone doesn't separate detention from normal loading time, so pair the fence with dock events.