How Do Police Detect Your Speed?

Speed detectors are fascinating tools law enforcement uses to make sure that vehicles follow the speed limit. But how do these devices determine the speed of moving vehicles? Let's dive into the world of speed detection, exploring how both radar and lidar technologies work.

How Does Radar Work?

Radar technology has been a fundamental tool in traffic enforcement for more than 50 years. The principle behind radar is relatively straightforward — it involves using radio waves to detect the speed of an object, such as a car.

At its core, a radar speed gun is composed of both a radio transmitter and a receiver. The transmitter sends out electromagnetic waves that travel through the air. When these waves encounter an object, like a moving vehicle, they bounce back, or echo, to the radar device.

The fascinating part of radar technology is how it uses these echoes to measure speed. This is where the Doppler shift comes into play. Similar to how the pitch of a train's whistle changes as it approaches and then departs from you, radio waves change frequency when reflecting off moving objects. If a vehicle is moving towards the radar, the frequency of the returning waves increases. If it's moving away, the frequency decreases. By measuring this change in frequency, the radar gun can determine the speed of the vehicle.

The operation becomes slightly more complex when the radar is used from a moving vehicle, such as a police car. In this case, the device must also account for the speed of the police vehicle itself to accurately calculate the speed of the target vehicle.

What About Lidar?

While radar uses radio waves, lidar (which stands for Light Detection and Ranging) uses light to achieve similar results but with potentially higher accuracy. This newer technology employs a laser, usually infrared light, to measure vehicle speed.

A lidar gun emits a series of light bursts that travel to the target vehicle and reflect back. The device then measures the time it takes for each light burst to return. Since light travels at a known constant speed (the speed of light), calculating the distance to the vehicle is straightforward. To determine speed, lidar systems rapidly send multiple bursts and measure the change in distance over time. By doing this several hundred times in less than a second, lidar devices can accurately calculate the speed of the vehicle.

One of the advantages of lidar over radar is its precision. The narrow beam of a laser allows it to target a specific vehicle more accurately, which is particularly useful in heavy traffic where traditional radar might get confused by multiple reflecting objects.

Both radar and lidar technologies provide law enforcement with reliable methods to measure vehicle speeds, which is crucial for maintaining traffic safety. Understanding how these devices work not only satisfies curiosity but also highlights the remarkable ways in which physics and technology come together to serve our communities. As technology advances, we might see even more accurate and efficient speed detection methods in the future, but for now, radar and lidar remain indispensable tools in the pursuit of safer roads.

References: How Radar Detectors Work | How Does A Speed Camera or Radar Gun Work?