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An important task we have to do as we move around the world is avoid running into things. One way we do that is to try to determine when an object is likely to hit us, called the time-to-collision. One piece of information we use is the size of the object. Larger objects are judged to be closer to collide with the viewer than smaller objects are, even if the larger object is farther away. This is known as the size-arrival effect (DeLucia, 2013). Numerous studies have shown that perception of collision is affected by the relative size of objects (Brendel et al., 2012; DeLucia, 2013; Hahnel & Hecht, 2012). You can see an illustration of this effect here.

The size-arrival effect results in an illusion that smaller objects are less likely to collide with the viewer. This finding has a number of unfortunate consequences for driving and transportation safety. For example, drivers may underestimate the likelihood of collision when turning when a smaller oncoming vehicle is approaching because it is perceived as being farther away. Indeed, both data from actual accidents and experimental simulations show that crashes between motorcycles and cars can occur when the car intrudes on the motorcycle’s path, with the car driver thinking the motorcycle is farther away (Brendel et al., 2012; DeLucia, 2013).


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

On the Illustration tab, you can adjust these parameters:

Move/Stop: starts or stops the motion of the two objects.
Try New Relative Position: allows change to the relative position of the two objects, leaving all the other settings constant.
Speed: changes how fast the objects approach.
Relative Size: changes the relative size of the two objects. They can be the same size, or the right object can be larger.
Relative Position: the distance of the two objects from you. You can adjust this slider to try to make the two objects appear to be equally close.
Arrive Same Time: after you have tried a minimal number of times, use this check box to indicate if you believe the two objects will arrive at the same time. You will get feedback on how you did. The text after the checkbox will tell you how many runs of the animation you have left.


Pressing this button restores the settings to their default values and allows you to adjust speed and relative size. It also resets the counter before you can indicate if you have the objects arriving at the same time.