Speed Checks from the Air: When Is Drone Footage Admissible as Evidence?

Drones can be used to document traffic offences and, under certain conditions, enable retrospective speed calculations. Whether such footage may be admitted as evidence depends on the technical reliability and traceability of the evaluation, as well as the lawfulness of the drone operation.

Not a Conventional Speed Measurement

In a conventional radar-based speed check, an approved measuring device records the vehicle’s speed. A drone does not do this: it merely records the vehicle’s journey on video. The speed is calculated afterwards using the footage. This involves determining how long the vehicle takes to travel between two clearly identifiable points along a previously measured stretch of road. Put simply, average speed is calculated by dividing the distance travelled by the time taken. The Higher Court of the Canton of Thurgau describes this method as a distance-time calculation based on video footage and fixed reference points measured on site (OGer TG RBOG 2024 No. 39, section 3.3.2).

For the calculation to be reliable, it must be possible to verify the length of the measured section, how the travel time was determined from the video footage, and whether the same vehicle is continuously tracked throughout the recording. The calculation must also be capable of independent verification, and any measurement uncertainties must be taken into account. In the Thurgau case, the court considered the specific evaluation sufficiently transparent and comprehensible, relying, among other things, on the safety margins applied (OGer TG RBOG 2024 No. 39, section 3.4).

Technical Traceability

A reliable evaluation requires, in particular, that the frame rate, reference points and distance measurements can be verified. It must also be established that the same vehicle was tracked throughout the evaluation.The distance-time calculation produces an average speed over the selected section of road. Safety margins can be applied to account for potential measurement inaccuracies.

The Swiss Federal Supreme Court accepted such a calculation in proceedings concerning the provisional withdrawal of a driving licence. It did so because the lower court had provided a comprehensible justification for its assessment and had taken potential inaccuracies into account. However, the decision does not establish that all drone-based speed measurements are automatically admissible. Furthermore, it concerned provisional driving licence withdrawal proceedings rather than a final criminal-law determination (Federal Supreme Court, 1C_683/2023, sections 3.2 and 3.4).

Lawfulness of Drone Operations

The purpose and scope of the operation are decisive. Controls carried out without any specific prior suspicion are not automatically unlawful. However, systematic or continuous surveillance must be assessed separately. In the Thurgau case, the police only began recording after observing suspicious driving behaviour. There was no permanent recording of road traffic. The Higher Court therefore considered the operation compatible with the applicable cantonal data protection legislation (OGer TG RBOG 2024 No. 39, section 4.3). Furthermore, following the driver’s U-turn manoeuvre, the police activity shifted from general traffic surveillance to a specific investigation (OGer TG RBOG 2024 No. 39, section 5.5.6).

Limits on the Admissibility of Evidence

Not every violation of the rules governing the collection of evidence automatically renders that evidence inadmissible. The first consideration is which legal provision has been violated. Under Article 141(2) of the Swiss Code of Criminal Procedure (CrimPC), unlawfully obtained evidence may only exceptionally be admitted if it is indispensable for investigating serious criminal offences. Whether an offence qualifies as serious must be determined on the basis of the circumstances of the individual case and not solely by reference to the applicable statutory penalty (BGE 147 IV 16, section 6).

Conclusion

Drone footage can be used to document traffic offences and may enable retrospective speed calculations. However, its admissibility as evidence requires a technically transparent and verifiable evaluation, reliable identification and tracking of the vehicle, and a lawful and proportionate drone operation. These decisions do not establish a general rule that all aerial speed checks using drones are permissible.