Almost every driver has experienced this scenario: the app warns you about a speed camera ahead, you look at the speedometer, and the same question arises—is the speed shown on the dashboard the same speed the camera will record? And why, the following week, did a speed camera appear on the same stretch of road that no app had warned you about?
The two answers lie in how these devices measure and what the law requires the traffic authority to disclose. Understanding this changes the way you drive: instead of relying on alerts, you start reading the road—and the app becomes a reinforcement, not the only defense.
What does the radar measure and how does it arrive at that number?
No speed measuring device photographs speed: it calculates it. The method changes depending on the type of equipment, and that's what explains why some remain stationary on the same pole for years while others change corners every day.
Ties on the asphalt
Traditional fixed speed cameras, what everyone calls "speed traps," work with inductive loops—those rectangular strips cut into the asphalt before the equipment. When the car passes over the first loop, the system records the instant; when it passes over the second, it records again. The distance between them is known and fixed, so the calculation is simple: distance divided by time. If the result exceeds the speed limit, the camera triggers and records the image that proves the infraction.
Radio wave and laser
Equipment that isn't fixed to the asphalt uses a different approach. Radar itself emits a radio wave towards the vehicle and measures the frequency difference of the returning wave—the same effect that makes an ambulance siren sound higher-pitched as it approaches. Laser models do something similar, using a beam of light to measure the change in distance in fractions of a second. These are the devices that fit on a tripod on the sidewalk or inside a patrol car.
The types of meters and where each one is usually located.
The regulations separate equipment by installation method, and each category behaves differently in its day-to-day operation.
- Fixed controller. The classic speed camera, installed on a gantry or pole, monitors the maximum speed limit at that point. It stays in the same place, has a known location, and is the type that apps accurately identify.
- Fixed reducer. The electronic speed bump, with a panel showing the speed of approaching vehicles, exists to force drivers to slow down at a specific point—such as a school, intersection, or pedestrian crossing.
- Portable. The one mounted on a tripod on the sidewalk or supported on a parked vehicle, operated by a uniformed officer. Its location changes according to the day's operation.
- Mobile. Installed inside a vehicle that may be in motion, it measures the speed of those passing by. It is, by nature, the most difficult to predict.
The tolerance is already factored in.
Every speed measuring device has a margin of error recognized by Inmetro (Brazilian National Institute of Metrology, Quality and Technology), and this is automatically deducted before the system decides if an infraction has occurred. The rule in effect is straightforward: on roads with a speed limit of up to 100 km/h, 7 km/h is deducted from the measured speed; above 100 km/h, 7% is deducted from the measured value.
In practice, on a 60 km/h road, a speeding ticket starts at 68 km/h (measured speed). On an 80 km/h road, it starts at 88 km/h. On a 110 km/h highway, it starts at approximately 118 km/h. A word of caution: this margin exists to account for equipment error, not to be used as an extension of the speed limit—and your car's speedometer often shows slightly higher speeds than the actual speed, which consumes some of this leeway without you realizing it.
How much does it cost to go over the limit?
The speed limit is not calculated based on additional kilometers driven, but rather on a percentage range calculated from the road's speed limit.
- Up to 20% above: Medium-level infraction, fine of R$ 130.16 and 4 points on the driver's license.
- Between 20% and 50% and above: Serious infraction, fine of R$ 195.23 and 5 points.
- More than 50% above: Very serious infraction with multiplied fine, R$ 880.41, 7 points and initiation of proceedings to suspend the right to drive.
The jump between the second and third lanes is the detail that catches the distracted driver: on a 60 km/h road, 90 km/h is already the most dangerous lane. Anyone who wants to check what they got caught in their license can find a step-by-step guide on the subject. How to check a speeding ticket using your cell phone..
Why does the app notify some and not all?
Here's the part that most app lists don't explain. The authority with jurisdiction over the road is required to publish on its website a list of speed measuring devices in operation, and in the case of fixed devices, this list includes their installation location. This is where the apps' database comes from: public information, updated by the authority, indicating equipment that doesn't move.
With laptops and mobile devices, there's no such pre-set list—their positions change depending on the day's operations. What the app shows in these cases comes from the collaboration of other drivers, who mark the location as they pass. It works well on busy avenues, where someone has just reported, but fails on empty roads or during times of low traffic, when no one has passed before you to let you know.
What has changed in the signage?
Signs reading “Electronic Enforcement” or “Radar ahead” are no longer mandatory with the 2020 regulations. What remains mandatory is the signage indicating the speed limit: each fixed speed camera must have a sign indicating the maximum permitted speed associated with it. The logic behind this is that the driver should know the speed limit of the road, not the exact location of the equipment.
Two guarantees remain in place. The equipment cannot operate hidden—no concealed cameras behind trees or disguised poles—and the agent operating the portable meter must be uniformed and visible. Furthermore, every meter undergoes metrological verification by Inmetro, with a maximum period of twelve months between calibrations; equipment with expired calibration is one of the most common grounds for appeal.
Warning is legal; blocking is a very serious offense.
There's a costly misunderstanding. An app that shows the location of speed cameras uses public data and alerts users about speed limits—this is permitted and is what the most widely used navigation apps in the country do. A completely different matter is... anti-radar device, That device interferes with the radio wave of the equipment or disrupts the reading of the license plate.
This device is covered under article 230, item III, of the Brazilian Traffic Code: a very serious infraction, 7 points on the driver's license, a fine of R$ 293.47, and vehicle impoundment. Film, covers, and sprays that promise to "hide the license plate in photos" fall into the same category. The app warns; the blocker violates the law.
Three situations in which the app doesn't help.
Even the best app has blind spots, and they all appear precisely when you would most trust the alert:
- Newly installed equipment. There's a window between the app going live and the app updating its database. During this window, the radar has already issued a ticket, but the app doesn't yet know it exists.
- Inspection without fixed equipment. Officers on the road, video surveillance cameras, and roadblock operations are not allowed inside any radar base.
- The limit changes in the section. Roadworks, school crossings, and maintenance zones temporarily alter the speed limit. The app usually displays the registered limit, not the temporary one on the sign—and the sign is what counts.
How to heed the warning without committing another infraction.
There's no point in avoiding a speeding ticket only to receive a worse one. Holding or handling a cell phone while driving is a very serious offense under the sole paragraph of article 252 of the Traffic Code: 7 points on your license and a fine of R$ 293.47 — more expensive than the average speeding ticket you were trying to avoid.
The setup that solves the problem takes two minutes and is the same in any app: phone in a holder attached to the windshield or air vent, at eye level; audible alert on, so you don't have to look; volume above the radio volume; and route set before starting the engine. If you need to adjust anything, pull over.
Common questions
Does the car's speedometer show the correct speed?
It usually registers a speed slightly higher than the actual speed, by design, and the difference increases with tires that are not the original size. That's why the speedometer reading almost never matches the radar reading — and the difference works in your favor, not against you.
Does radar measure who is going too slowly?
On some roads there is a minimum speed limit indicated by signs, and driving below half the maximum allowed is an infraction under the Traffic Code. This is not the case with urban speed cameras, which monitor the maximum speed limit.
Does an electronic speed bump still issue a fine even if the speed limit is displayed on the dashboard?
Yes. The display is there to trigger a reduction in speed before the measurement point, but the equipment normally registers anyone who goes above the limit.
Does rain or a motorcycle in front interfere with the measurement?
The equipment is tested to operate under normal road conditions and undergoes periodic calibration. What can invalidate a citation is documented equipment failure or an expired calibration, not the impression that "it was a bad day.".
In summary
Fixed speed cameras are predictable because their location is public information; portable and mobile cameras are not, and no app completely solves this. The 7 km/h tolerance exists to cover equipment error and disappears quickly when the speedometer is already exceeding that limit. The cheapest way remains the most obvious: reading the road sign, which is precisely what the law guarantees will be there.
