This is the question we get asked most, and the answer varies by country. More importantly, it turns on a distinction most people do not make.
The distinction that decides everything
A radar detector is a device that picks up or jams the emissions of a speed measurement. That is prohibited across almost all of Europe, Belgium and the Netherlands included, and merely having one in the car is itself an offence.
A warning app does something else: it shares information — a location somebody reported, or a camera that is publicly known. Nothing is measured, intercepted or jammed. Legally that is a different object, and it is exactly why the rules differ from country to country.
Belgium
Warning about fixed cameras and zones is permitted. Radar detectors are banned, and carrying one in the vehicle risks seizure.
Netherlands
The same line: apps and navigation systems that warn about cameras are permitted; radar detectors are banned.
France
France is the outlier. Since 2012, apps may not show the exact location of a check. They work with wider danger zones instead of points, and during certain police operations the authorities can have the display of reports temporarily suspended. The app is legal; its precision is not, on French territory.
Germany
Since the 2020 revision of the StVO, a driver may not use a speed camera warning app while driving. The rule is aimed at the driver: a passenger may consult it. This is a real restriction, not a formality.
Luxembourg
Radar detectors are banned. There is no general prohibition on warning apps as there is in Germany, but the rules on handling a device at the wheel apply in full.
In short: sharing information is permitted almost everywhere, measuring or jamming almost nowhere — and France and Germany add their own rules on top.
What it means for how you drive
The practical conclusion is not that you need a different app per country. It is that the law of the country you are driving in applies, even on a Belgian plate with a Belgian subscription, and that it is the driver's job to respect it.
This article is an overview, not legal advice. Rules change; when in doubt, check the official source for the country concerned.
See how TrapSpotter warns you