Common Speed Detection Errors: What Can Go Wrong With Radar and Lidar


Radar and lidar are sophisticated technologies, but they're not perfect. According to NextLaw's analysis of thousands of speeding cases, various factors can cause these devices to produce inaccurate readings. Understanding potential error sources helps drivers evaluate whether challenging their ticket makes sense.


Cosine Effect (Angle Error)


Jon Cohen, who has analyzed speed measurement evidence in thousands of cases, identifies the cosine effect as one of the most misunderstood potential errors.


When a radar or lidar beam is aimed at an angle to a vehicle's path rather than directly head-on, it measures only the component of speed coming directly toward (or away from) the device. The reading will be lower than the actual speed—not higher.


This means the cosine effect generally works in the driver's favor, not against them. However, it becomes relevant when the measured speed is very close to the alleged speed—small margins matter in these cases.


Vehicle Interference


Dan Joffe, traffic lawyer at NextLaw, notes that multi-lane traffic creates the potential for vehicle confusion. Radar devices track the strongest reflected signal, which may come from a different vehicle than the officer intended to target.


Larger vehicles with flat reflective surfaces (trucks, buses, SUVs) often produce stronger radar returns than smaller cars. In traffic, the radar might lock onto a nearby truck while the officer believes they're targeting the car in front of it.


For lidar, the narrower beam reduces but doesn't eliminate this concern—especially at longer distances where even the narrow beam covers a wider area.


Radio Frequency Interference


Radar devices can be affected by external radio frequency sources. Jon Cohen notes that interference can come from:

- Nearby power lines or electrical equipment


- Other electronic devices


- Radio towers or broadcasting equipment


- Other radar units operating nearby


- Even some electronic accessories in vehicles

While modern radar units have interference rejection systems, these aren't foolproof. Strange or inconsistent readings can sometimes be traced to interference sources.


Calibration Failures


If a device isn't properly calibrated, all readings from that device are potentially unreliable. Calibration can drift over time, batteries can affect performance, and environmental conditions can impact accuracy.


The calibration requirements—annual certification, tuning fork tests, pre/post-shift verification—exist specifically to catch these issues. When calibration procedures aren't followed, the reliability of any reading is questionable.


Operator Error


Speed measurement devices require trained operation. Operator errors can include:

- Improper aiming or targeting


- Confusing targets in traffic


- Misreading the display


- Failing to verify the device is functioning correctly


- Using incorrect operating modes

Dan Joffe notes that disclosure review often reveals incomplete officer notes that don't adequately document the targeting process—which can raise questions about whether proper procedures were followed.


Environmental Factors


Both radar and lidar can be affected by environmental conditions:

- Heavy rain or snow can interfere with readings


- Fog can cause issues, particularly for lidar


- Temperature extremes can affect device performance


- Reflective surfaces (wet pavement, certain road signs) can create ghost signals

Multi-Path and Reflection Errors


Radar beams can reflect off buildings, overpasses, or large vehicles and create false readings. This "multi-path" effect is rare with modern equipment but can occur in certain environments—particularly near large metal structures or in urban canyons.


Why Understanding Errors Matters


Jon Cohen emphasizes that identifying potential errors doesn't guarantee a defense will succeed. However, understanding what can go wrong helps evaluate whether a particular case has defensible issues.


The prosecution must prove the speed reading was accurate. If reasonable doubt exists about the reliability of that reading, the charge may not stand.


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Every Case Deserves Analysis


No speed measurement technology is perfect. Every speeding ticket based on radar or lidar evidence deserves analysis of the specific circumstances, equipment used, and procedures followed. Understanding potential error sources is the starting point for that analysis.


This article is based on NextLaw's professional analysis of Ontario speeding legal procedures and is provided for informational purposes only. Every case presents unique circumstances, and outcomes depend on specific case facts and proper legal representation. https://www.nextlaw.ca/?p=33168