[MeganM (OP)]

Charles has asked:

How do humans (or animals) judge how quickly something is coming towards us?

Any ideas?

[Hayseed]

By the amount of pain it delivers.

[Bored chemist]

https://en.wikipedia.org/wiki/Optical_flow

[alancalverd]

It is interesting that a relaxed human eye focuses, not at infinity, but at somewhere between 20 and 60 feet. Within the range 0 - 60ft we are very familiar with space and time clues, even at an unconscious level of reading ground textures or comparison with familiar vertical objects like people, animals and trees. This makes evolutionary sense  as it is the range over which we can reliably throw missiles or defend ourselves against predators.

Beyond about 60 ft we can learn to interpret speed clues but the finesse is generally unimportant. Approaching objects occupy an increasing proportion of our field of view and an experienced driver responds to this by maintaining a constant subtended angle of the leading vehicle even if there are no road markings or textural clues. In fact our judgment of a non-zero closing speed is quite poor and is, I think, more based on the learned ballistic properties of footballs and cricket balls than on any actual perception, which is why badminton and squash are so difficult - the projectiles are very nonlinear!

An approaching light source is generally interpreted as faster than actual. This follows from part of BC's reference to optical flow: the apparent intensity of a point source increases with the square of its approaching speed. This was used in a number of night tank assaults in the 1940s. Instead of approaching with no lights, some vehicles were fitted 
with focussed arc lamps and flicker shutters. These not only illuminated but also confused the target forces by giving the appearance of a smaller force approaching very rapidly, with the unlit tanks virtually invisible behind the flickering curtain, and destroying the night vision of the enemy gunners.

In the absence of context, assessment of closing speed is almost entirely a matter of learning. "Dogfighting in the blue bowl" takes a lot of training and is greatly helped by recognition of the enemy's machine.

[Bored chemist]

It is interesting that a relaxed human eye focuses, not at infinity, but at somewhere between 20 and 60 feet.

Or not.
In my case it's about 15 cm or 6 inches in old money.
But what's an order of magnitude between friends.

Beyond about 60 ft we can learn to interpret speed clues but the finesse is generally unimportant.

And that's the reason why nobody can play tennis- the court is 76 feet long.

[alancalverd]

I too have an uncorrected far point  of about 20 cm, as a matter of choice. I had cataract surgery and asked for the replacement lenses to match my previous myopia so I can do close work and read in bed without specs, but those blessed with "normal" vision, which includes a functionally focussing lens, tend to relax at 20 - 60 ft. This has been a matter of concern particularly to aviation authorities and most particularly for airline pilots flying in the "blue bowl", who simply do not notice objects at "infinity" because there is no intervening detail to draw the eye away from the 20 - 60 ft region.

The speed of a tennis ball at 76 ft distance is fairly unimportant. If it's going away from me, that's all that matters, and if it is coming towards me at an initial speed of 90 mph or so, I know (from experience, though others have calculated it) that air resistance will slow it to about 60 mph before it bounces and around 10 - 25 mph after the bounce. There have been some interesting studies of cricket that lead to the conclusion that a batsman attacking a fast bowler doesn't have time to make a conscious decision once the ball has been bowled: the art is in anticipating the pitch and swerve from the bowler's eyes and action, so that the stroke is already initiated and his feet are moving before it leaves the bowler's hand.

[Monox D. I-Fly]

It is interesting that a relaxed human eye focuses, not at infinity, but at somewhere between 20 and 60 feet. Within the range 0 - 60ft we are very familiar with space and time clues, even at an unconscious level of reading ground textures or comparison with familiar vertical objects like people, animals and trees. This makes evolutionary sense  as it is the range over which we can reliably throw missiles or defend ourselves against predators.

Ah, I see... That's why when I see someone more than 18 meters away (damn feet measurement, I had to do mental arithmetic to convert it first) I often have a hard time interpreting whether that person is walking to or away from me...

[alancalverd]

Should have gone to Specsavers! I find the position of the face often gives a clue as to direction, and the movement of the legs suggests speed. Once you have focussed on a moving object in context, the contextual clues also help judge speed.  A person walking on the ground is surrounded by context and once you have identified the blob as a person, you have focussed (to some extent) on it.

Remember the 20 - 60 ft (or 18 m) distance is the focal distance of a relaxed average eye with no context  ("in the blue bowl"). Focussing is entirely subconscious as soon as an object is detected. The pilot's problem is one of chickens and eggs: it's difficult to detect a small object if you aren't already focussed at the appropriate distance. Not a problem in normal life but with closing speeds up to 8000 ft  (2400 m) per second and an eye-to-hand response time of about 0.3 s, 18 m focal distance is not a lot of use.