What technique helps to accurately judge the speed of a moving vehicle?

The technique of looking at the ground beside the vehicle is effective for accurately judging the speed of a moving vehicle because it provides a reference point that remains stationary. By focusing on the ground as the vehicle passes, you can more easily gauge how quickly it moves relative to that fixed point. This method allows you to visually assess the speed by observing the rate at which the vehicle travels past the reference line, making it easier to perceive speed changes without being influenced by the natural tendency to focus directly on the vehicle itself.

Counting seconds while the vehicle passes, while also a useful technique, might not give as clear of a perspective on speed compared to a stationary reference point, and other methods, such as estimating based on distance or staring directly at the vehicle, can lead to misjudgments due to motion blur or distraction from the vehicle's speed. Thus, looking at a fixed point on the ground beside the vehicle enhances your ability to accurately assess its speed.