Rannsachadh lùb (lussenonderzoek): dè cho earbsach 's a tha e mar fhianais trafaic?

Loop research (lussenonderzoek) is the forensic analysis of data from induction loops in the road surface, which can show to the millisecond when a vehicle passed a point and what the traffic light showed at that moment. Dutch courts regularly accept this data as evidence of speed or a red-light offence, but only if the expert report explains the method and the defence can test it; missing calibration or synchronisation records are the main point of attack.

The data reaches the file through an expert report. Under Article 51l of the Code of Criminal Procedure (Wetboek van Strafvordering, Sv) that report must be reasoned, and the suspect can ask for a further or counter-investigation (Articles 150a and 150b Sv). This article explains how the technique works, how courts assess it and how a defence is built.

How do induction loops work?

An induction loop is a coil of wire in the asphalt, connected to a detector in the traffic controller (verkeersregelinstallatie, VRI). When a metal object such as a car passes over it, the magnetic field changes and the detector registers a detection, with a time stamp stored in the controller’s log.

The loops are built for traffic management, such as switching lights to green or measuring congestion. In a criminal case the same log is reused as evidence, often together with the phase log (faselog) of the traffic lights.

What data does a loop record?

• Time stamps: the moment a detection begins and ends, often to the millisecond.
• Occupancy: how long a vehicle was over the loop.
• Calculated speed: with two loops at a known distance, the system calculates speed from the travel time between them.
• Vehicle class: some systems estimate whether a vehicle is a car, lorry or bus from the length of the signal.

A loop does not read a number plate. It registers that a metal object passed, which is why linking a detection to a specific vehicle is often the core issue in court.

How is loop data used as evidence?

Loop data is used to reconstruct intersection collisions, to establish whether a light was red at the moment of passing, to calculate speed where no radar or laser was used, and to check witness statements. It is often combined with other technical data, such as the event data recorder in a car’s airbag module.

In a judgment of 28 November 2022 (ECLI:NL:RBZWB:2022:7122), the District Court of Zeeland-West-Brabant used the phase log and loop detections of a signalled intersection in Tilburg to establish that a driver hit a pedestrian at well over the 80 km/h limit after running a red light. The defence argued the detections could belong to another vehicle. The court rejected that, partly because the car was not registered at the next loop after the collision.

What are the legal rules on technical evidence?

Under Article 338 Sv a court may only find a charge proven on the basis of legal means of evidence, and it must also be convinced. Article 339 Sv limits those means to five: the court’s own observation, statements of the suspect, statements of witnesses, statements of experts and written documents. Loop data usually enters the file as an expert report or as a written document containing the raw logs.

The court decides what weight to give the evidence and is not bound to accept a computer’s output. If the defence puts forward a reasoned position that the loop data is unreliable, Article 359 Sv requires the court to explain in its judgment why it nevertheless relies on it.

What must an expert report contain?

The public prosecutor or examining magistrate appoints the expert (Article 51i Sv), who must be expert and impartial. Under Article 51l Sv the expert must deliver a reasoned report that describes the assignment, the material examined, the methods used and the reasoning behind the conclusions.

A report that only says “the car was speeding”, without explaining how this follows from the loop data, does not meet that requirement. The defence can then argue that the finding cannot be verified.

How can you have loop research checked?

After the public prosecutor notifies you of the expert’s findings, you can ask for a further investigation or a counter-investigation (tegenonderzoek) under Articles 150a and 150b Sv. Act straight away: the notice opens a short period for the request, and a late request is hard to repair.

The request is made in writing to the public prosecutor. It should state which findings you dispute and why, and which expert you propose. If the prosecutor refuses, you can turn to the examining magistrate (rechter-commissaris).

What does the Supreme Court require?

In its judgment of 18 November 2025 (ECLI:NL:HR:2025:1711), the Supreme Court (Hoge Raad) held that Article 6 of the European Convention on Human Rights (ECHR) gives the suspect the right to test the reliability of an incriminating expert finding. The defence does not have to justify its interest in that, but must say which parts of the finding it disputes.

Hearing the expert is not required in every case. The court may choose another route, such as a supplementary report, further research by the same expert or an assessment by a second expert, and must check that the trial as a whole remains fair.

The District Court of Limburg applied this approach in an interlocutory decision of 19 November 2025 (ECLI:NL:RBLIM:2025:11395), a case about image comparison rather than loops. Instead of appointing a second expert, it allowed the defence to put written questions to the author of the report, leaving room for a counter-investigation if doubts remained.

When is a counter-investigation useful in a loop case?

• Several vehicles passed the loops within seconds of each other.
• The speed is calculated only from loop data, without radar or laser.
• There is no proof of clock synchronisation in the traffic controller.
• Calibration or maintenance records of the loops are missing.

Where is loop research vulnerable?

Loop data is automated, precise and hard to manipulate afterwards, which is why courts give it weight. Its weak points are technical and lie in what the report does not show.

• Technical faults: water, frost, damage from heavy vehicles or electrical interference can affect detections. Ask for maintenance and error logs for the date concerned.
• Identification: a loop detects “a metal object”, not your car. In multi-vehicle situations, ask how the expert linked a signal to your vehicle.
• Time synchronisation: a clock deviation of a second or two can make the difference between amber and red. Ask for proof of synchronisation.
• Interpretation: acceleration or deceleration between loops rests on assumptions. Ask for those assumptions to be made explicit.
• Documentation: without calibration and maintenance records the finding cannot be verified as Article 51l Sv requires.

Comparable technical evidence shows what makes such data hold up. In a judgment of 14 April 2025 (ECLI:NL:GHARL:2025:2243) the Court of Appeal of Arnhem-Leeuwarden based the speed in a fatal collision with a cyclist on the event data recorder in the airbag module. The data had been validated with test drives and corrected for a measured deviation of about 3.7%.

Can loop evidence be excluded?

Exclusion is possible but rare; more often the court gives the data less weight. The Supreme Court’s overview judgment of 1 December 2020 (ECLI:NL:HR:2020:1889) on procedural defects under Article 359a Sv sets three routes.

• Exclusion to prevent a violation of the right to a fair trial under Article 6 ECHR.
• Exclusion for another serious procedural violation, after weighing the seriousness, the blame and the cost to truth-finding.
• Not using evidence because irregularities have affected its reliability; this rule applies outside Article 359a Sv.

The Supreme Court applies this strictly. In its judgment of 27 June 2023 (ECLI:NL:HR:2023:975) it upheld a refusal to exclude evidence from a car search, partly because the defendant had not shown any actual disadvantage. For loop data, the third route is usually the realistic one: without calibration, synchronisation and raw data, reliability cannot be established. If the loop data is the only evidence, that can lead to an acquittal (vrijspraak).

How do you build a defence against loop evidence?

1. Have the file reviewed in the first days after you receive it, to see whether loop data is used and which documents are missing.
2. Ask the public prosecutor for the documents through your lawyer (Article 30 Sv): the full report with method, calibration and maintenance logs, error logs, proof of time synchronisation and the raw data.
3. Assess whether there is an alternative explanation, such as another vehicle or a lane change, and decide on a counter-investigation.
4. Request the counter-investigation in writing, with reasons and a proposed expert, without waiting.
5. If needed, ask the court to hear the expert and prepare concrete questions on error margin, identification and calibration.
6. At the hearing, argue that the evidence is unreliable or should be excluded under Article 359a Sv, and address the other evidence as well.

What can we do for you with loop research in a traffic case?

Ar luchd-lagha ann an imeachdan eucorach assist suspects in traffic cases where technical evidence is decisive, including cases after a tubaist trafaic throm.

• We review the case file and the expert report against the requirements of Article 51l Sv.
• We request the raw loop data, calibration, maintenance and synchronisation records from the public prosecutor.
• We draft and file the request for a counter-investigation and propose a qualified expert.
• We prepare questions for the expert and request a hearing of the expert where needed.
• We argue exclusion or reduced weight of the evidence at the hearing.

Geàrr-chunntas

• Loop research uses induction loop logs and traffic light phase logs to establish timing, speed and red-light offences.
• The evidence enters through an expert report, which must be reasoned and verifiable under Article 51l Sv.
• You can request a counter-investigation (Articles 150a and 150b Sv); act immediately after the notice.
• Under ECLI:NL:HR:2025:1711 you have the right to test an incriminating expert finding, though not always by hearing the expert.
• Missing calibration, synchronisation or raw data more often leads to reduced weight than to exclusion.

Ceistean Àbhaisteach