[neilep (OP)]

Hi,

I'm Neil ( a newbie !)

Whilst driving down my favorite bit of M1 Motorway tonight. I noticed the car in front of me kept slowing down and speeding up !

The car was a good 300 meters ahead....

Eventually , when we were out of the dreaded 40mph zone we both sped up to about 75mph !

It was clear to me that the car in front kept making changes to it's speed but it was very marginal.....and yes..I am sure I too was also making slight changes !!!!

I'll get to the point shall I ? ...

Even though the distance between us was a fair bit..I could easily tell if I was gaining or losing ground to the car in front...and there was that wonderful moment when you know you're traveling exactly the same speed too.. the degrees of miles per hour must have been something like a quarter of mile (ish) of an hour difference.....so..How sensitive is the human eye to perceiving movement like this ?

What's the slowest speed that we can perceive..


I'll stay in the slow lane so that you can overtake me with your answers.

[Seany]

I'll stay in the slow lane so that you can overtake me with your answers.

Tut tut tut! Now you're on your laptop, whilst driving, in the car, with a camera? []

[neilep (OP)]

I'll stay in the slow lane so that you can overtake me with your answers.

Tut tut tut! Now you're on your laptop, whilst driving, in the car, with a camera? []

.......and I'm juggling............blindfolded !!

[another_someone]

There are two ways you can easily see if another vehicle has a similar speed to yours.

If the vehicle is straight ahead of you, then you will be judging the size of the vehicle as it gets bigger or smaller - this only works if the vehicle is large enough/close enough to take up a significant percentage of your field of view - not easy when you are trying to judge the speed of a smaller vehicle (e.g. a motorcycle) at a great distance away from you.

The other way, if you are travelling on parallel tracks, is that you will judge the angle that the vehicle takes with regard to you (as you pick up speed, the angle gets closer to 90 degrees, as you slow down, the angle gets closer to zero degrees).  This only works if your tracks are exactly parallel, since you cannot judge the component of speed that is not parallel to your track.

[Seany]

I'll stay in the slow lane so that you can overtake me with your answers.

Tut tut tut! Now you're on your laptop, whilst driving, in the car, with a camera? []

.......and I'm juggling............blindfolded !!

Neil! Wait.. Look.. No!! Wait! LOOK AT!! NEIL!!!!!!!!!!!

*CRASH*

[DoctorBeaver]

A pome [sic] just for Neil

Two little piggies went for a drive
Out in the country on the A45
LOOK OUT! TOO FAST! PUT THE BRAKE ON!
Ooops, too late. Now they're bacon

[DoctorBeaver]

I'm not sure what the answer is to how slow a speed we can perceive but I've noticed that if something is moving very slowly I notice a sort of quantum movement. I.e. I don't actually see it moving but after a time it seems to have jumped forward very slightly. Has anyone else noticed this effect?

I assume it's because my eyes don't register such slow movement but my brain suddenly realises the object is a different size and makes the adjustment.

[paul.fr]

A pome [sic] just for Neil

Two little piggies went for a drive
Out in the country on the A45
LOOK OUT! TOO FAST! PUT THE BRAKE ON!
Ooops, too late. Now they're bacon

Stop "hogging" the topics  []

[DoctorBeaver]

But I like Topics. They've got a hazelnut in every bite  []

[that mad man]

The eye has a refresh rate a bit like a pc monitor from about 20hz to 60hz.

Anything below 20hz (I think) is seen as fairly stationary and single frames of film will not seem continuous but as a series of jerky images. Above 60hz it gets smoother, that's why a PC monitors refresh rate should be 60hz or higher as you don't notice the refresh rate (flicker) much.

Bee

[another_someone]

The eye has a refresh rate a bit like a pc monitor from about 20hz to 60hz.

Anything below 20hz (I think) is seen as fairly stationary and single frames of film will not seem continuous but as a series of jerky images. Above 60hz it gets smoother, that's why a PC monitors refresh rate should be 60hz or higher as you don't notice the refresh rate (flicker) much.

Bee

Not really a refresh rate - a response rate (although this is the eye and brain together).

If it was a true refresh rate, then you would have strobe effects (like when you see wagon wheels moving backwards on TV screens).

The minimum response rate simple means that the time it will take you to realise that there is a difference in speed (in fact, this is as fast as the brain will realise anything, whether it is visual or from any other sense) will at least be ¹/_20^th of a second, it does not tell you about how discriminating the eye will be about judging the change in speed.  Since the question was regarding judging differences in relative speed, what we are really talking how long it will take the eye to judge that an object is approaching or receding, and that depends on how long you wait to see that the object has gotten closer or further away from you.