Judging the speed of a car traveling towards you or away from you on the highway can be difficult due to a phenomenon known as the "motion parallax" effect. Several factors contribute to this difficulty:
Lack of reference points: When a car is moving directly towards or away from you, there are limited stationary reference points in your field of view to gauge its speed accurately. Our brains rely on surrounding objects and scenery to estimate speed, and when the car is moving directly in your line of sight, these reference points are scarce, making it challenging to determine the speed intuitively.
Smaller visual changes: As a car approaches or moves away from you, the visual changes in its size and position are relatively subtle, especially at a distance. Unlike a car passing you by, where you can observe a more significant shift in its position and size, the changes in perspective when it is coming directly towards or away from you are less apparent.
Limited depth perception: Our depth perception plays a crucial role in judging the speed of objects. When a car is coming directly towards you, it is moving primarily within your field of depth, making it harder for your brain to perceive its movement accurately. This can lead to an underestimation of its speed.
Relative speed differences: If you are also in a moving vehicle, it becomes even more challenging to judge the speed accurately. The relative motion between your vehicle and the oncoming car adds another layer of complexity, making it harder to discern the car's true speed.
Visual illusions: In certain conditions, visual illusions can further complicate speed estimation. For instance, the "autokinetic effect" may make a stationary light source, like a distant headlight, appear to move slightly, leading to uncertainty about the car's actual speed.
To mitigate these challenges, many drivers rely on the use of speedometers and follow road signs and traffic regulations to maintain a safe speed. Law enforcement uses radar guns and other speed measurement devices to accurately determine a vehicle's speed. These tools provide objective measurements, eliminating the subjective and potentially inaccurate judgments that can occur due to the motion parallax effect.