How the Car Notices Sleepiness

The obligation

Since 7 July 2024, every new car sold in the European Union must be equipped with a Driver Drowsiness and Attention Warning system - DDAW, in the language of the regulation. The requirement comes from the EU General Safety Regulation and applies to passenger cars, vans, trucks and buses alike.

A second system followed the same path: since 7 July 2026, all new vehicles must also carry an Advanced Driver Distraction Warning, which watches whether the driver's eyes stay on the road. Distraction and sleepiness are different states - but the direction is the same. The vehicle is now required, by law, to pay attention to the person driving it.

Switzerland is not bound by EU type-approval law, but in practice the vehicles on Swiss roads are the same vehicles: what becomes standard in the EU becomes standard here.

How it works

The regulation does not prescribe a technology. It prescribes a result: the system must recognise the signs of a sleepy driver and warn in time. Manufacturers choose the means. In practice, three families of methods exist:

• Steering analysis. A sleepy driver corrects differently - small drifts followed by abrupt corrections. The car reads its own steering data and looks for the pattern.
• Lane monitoring. A camera watches the road markings. Slow, unintended drift within the lane is one of the most consistent signatures of sleepiness at the wheel.
• Driver-facing cameras. The most direct method: eye closure, blink duration, head position. European rules require these systems to work without identifying the person and without keeping the images.

Whatever the method, the logic is the same: the system does not measure sleepiness itself. It measures behaviour, and infers.

What the law asks of the driver

Swiss road traffic law is older than any of these systems, and it already says something clear: a person who is overtired is not fit to drive (art. 31 LCStr). The law places the responsibility before the journey begins - not at the moment a warning light comes on.

This is where technology and law quietly diverge. The system intervenes during the drive, when the signs are already visible. The law expects the decision to have been made earlier, when the driver could still choose not to drive at all.

The limits

These systems are tested with human participants against a reference drowsiness scale, and they must meet statistical thresholds to be approved. That is a laboratory guarantee, not a promise about your drive tonight. Real roads, real nights and real drivers are more varied than any test protocol.

Three limits matter most:

• The warning can arrive late. A microsleep lasts a few seconds and can come before any pattern is clear enough to trigger an alert.
• The warning can be ignored. A chime does not make anyone less sleepy. Only sleep does.
• The system infers - it does not know. There is still no instrument in any car that measures sleepiness the way a breathalyser measures alcohol.

None of this makes the technology useless. It makes it what it is: an aid at the threshold, not a guardian across it.

See also