Reading a number plate - self test

[Maroochydore]

I was just reading that Cleveland Police Matrix Team will start voluntary random roadside vision screening.

Drivers will need to read a number plate from a distance of 20 metres (as per the law).

The standard height for characters on a number plate is 79mm.

So my question is: if someone wanted to check their eyesight at home and couldn't find a clear distance of 20metres+ to put up a test piece, would checking a character of 40mm height from 10 metres be as accurate or is there a formula that allows for the height/distance ratio to be calculated?

This is not personal, my optician says I exceed the standard required based on a Snellen chart test. I also realise the easiest way to check is to walk down the street and measure 20 metres but I'm more interested in the ratio question.

[servodude]

It would be probably equivalent (it is from trigonometry)

If your eyes work properly the image projected on your retina will be the same size in either case

- but because eyes and optics are a bit mad I do think it would be possible in some circumstances for you to be able to read one but not the other

[servodude]

meant to mention it's the tan of an angle stuff that means it should be equivalent

[Lootman]

I was just reading that Cleveland Police Matrix Team will start voluntary random roadside vision screening.

Drivers will need to read a number plate from a distance of 20 metres (as per the law).

That is a terrible way to measure visual acuity. I suspect that anyone challenging that in court armed with a report from their eye doctor would get off.

Maybe Cleveland needs the revenue and prefers a soft and easy citation rather than actually going after real criminals.

[DrFfybes]

So my question is: if someone wanted to check their eyesight at home and couldn't find a clear distance of 20metres+ to put up a test piece,

You don't need a test piece. all you need is a parked car and stand 4 car lengths away from it. As you are asking the question, I assume the first part of that task is relatively simple

Paul

[bungeejumper]

The twenty metre test is all very well, but as a (surviving) ex-biker I reckon it's the peripheral vision thing that probably matters just as much, or maybe more.

The number of times I've seen people with bottle-glass lenses or tunnel vision who can hardly see the pavement if they're watching the traffic ahead. Such as my uncle, who should have hung up his car keys ten years before he hit the vicar's car.

BJ

[Gerry557]

I focused on the word "voluntary"

So no need to worry

[mc2fool]

The twenty metre test is all very well, but as a (surviving) ex-biker I reckon it's the peripheral vision thing that probably matters just as much, or maybe more.

The number of times I've seen people with bottle-glass lenses or tunnel vision who can hardly see the pavement if they're watching the traffic ahead. Such as my uncle, who should have hung up his car keys ten years before he hit the vicar's car.

BJ

That's also part of the rules.

"Standards of vision for driving

You must be able to read (with glasses or contact lenses, if necessary) a car number plate made after 1 September 2001 from 20 metres.

You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.

You must also have an adequate field of vision - your optician can tell you about this and do a test." https://www.gov.uk/driving-eyesight-rules

Note the "also"s; all three apply. Also note that if you go for an eye test and your optician decides you're not fit to drive then they may be required to inform the DVLA. https://optical.org/en/standards-and-guidance/disclosing-confidential-information/vision-and-safe-driving-what-to-do-if-a-patient-s-vision-means-they-may-not-be-fit-to-drive/

[Maroochydore]

It would be probably equivalent (it is from trigonometry)

One rec for answering the question without going off at a tangent - see what I did there

[bungeejumper]

The number of times I've seen people with bottle-glass lenses or tunnel vision who can hardly see the pavement if they're watching the traffic ahead. Such as my uncle, who should have hung up his car keys ten years before he hit the vicar's car.

That's also part of the rules.

"You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.

You must also have an adequate field of vision - your optician can tell you about this and do a test." https://www.gov.uk/driving-eyesight-rules

Indeed. But the OP's question was about self-tests. Which I think we agree are not good enough!

BJ

[mc2fool]

That's also part of the rules.

"You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.

You must also have an adequate field of vision - your optician can tell you about this and do a test." https://www.gov.uk/driving-eyesight-rules

Indeed. But the OP's question was about self-tests. Which I think we agree are not good enough!

BJ

Well a 20 metre self-test would be good enough for a definitive fail. If you really can't read a number plate at 20m in plain daylight, and you still can't do so even after taking a few steps forward, you really need to think about whether you should be driving! For more marginal cases, yeah, it probably isn't good enough.

In answer to the OP's "ratio" question about "checking a character of 40mm height from 10 metres" instead, Wikipedia says "Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters.", so 40mm from 10m would substitute it seems. https://en.wikipedia.org/wiki/Snellen_chart

[GrahamPlatt]

Sheesh. It’s not a high bar, is it?
For interest’s sake I’ve just had a go.
20m with a tape measure, turn & look (unknown plate).
Both eyes - easy. Left eye - easy. Right eye - hmmm. Try pinholing it. Yep, can now read.
Mind you, that was _without_ my specs (short-sighted).

[servodude]

It would be probably equivalent (it is from trigonometry)

One rec for answering the question without going off at a tangent - see what I did there

Going off at a tangent would have been a sin, cos this is DAK

[bungeejumper]

Going off at a tangent would have been a sin, cos this is DAK

Yea, may I be forgiven, for I have many times cosined most grievously.

BJ

[Lootman]

Sheesh. It’s not a high bar, is it?

It is really not. If you want a laugh take a look at the vision requirements for the US. There are of course 50 different sets of them but many require only 20/70 vision and some go out to 20/120. And especially states with a lot of old folks like Florida.

If you have poor eyesight you can probably find a US state where you can legally drive. That said a (cursory) vision test is part of the license renewal process every 5 years there and so in that sense it is tougher there compared to the UK where your license is never renewed in person.

note that if you go for an eye test and your optician decides you're not fit to drive then they may be required to inform the DVLA. https://optical.org/en/standards-and-guidance/disclosing-confidential-information/vision-and-safe-driving-what-to-do-if-a-patient-s-vision-means-they-may-not-be-fit-to-drive/

Yes although if you were really worried about that you could always go overseas for your annual eye test. Same with any other medical issue.

That said I always ask my eye doctor if I am OK to drive. She tests my peripheral vision as reading characters is not typical driving. She has said I am fine to drive, although she advocates caution when driving at night, which is when most vision shortcomings manifest themselves.

[stewamax]

Going off at a tangent would have been a sin, cos this is DAK

Yea, may I be forgiven, for I have many times cosined most grievously.

You need a good tanning or even a coshing.

[servodude]

Yea, may I be forgiven, for I have many times cosined most grievously.

You need a good tanning or even a coshing.

I'm not sure what your angle is, and I'm not trying to be obtuse, but my reflex is to think that sounds a bit hyperbolic?

[UncleEbenezer]

It would be probably equivalent (it is from trigonometry)

If your eyes work properly the image projected on your retina will be the same size in either case

- but because eyes and optics are a bit mad I do think it would be possible in some circumstances for you to be able to read one but not the other

Only if you assume both are equally in focus. Which is precisely what they're not if you're short-sighted.

Reading a number plate depends on many things. The light, the colour contrast and general clarity, the cleanliness or otherwise. And surely the test itself dates from the era of Mr Toad?

[servodude]

It would be probably equivalent (it is from trigonometry)

If your eyes work properly the image projected on your retina will be the same size in either case

- but because eyes and optics are a bit mad I do think it would be possible in some circumstances for you to be able to read one but not the other

Only if you assume both are equally in focus. Which is precisely what they're not if you're short-sighted.

Reading a number plate depends on many things. The light, the colour contrast and general clarity, the cleanliness or otherwise. And surely the test itself dates from the era of Mr Toad?

All true
- but adjusting font size for test distance in a Snellen chart is standard practice in optometry and they probably still explain to kids how to measure trees (or buildings these days?) to get the principle of a clinomemter, so the original DAK proposition should normally be sound

As you allude to it's the other questions that it raises that has been bouncing around in my head:
in what circumstances would it not hold?
- it would give twice the opportunity for stuff to get in the way - and excluding gross stuff like cars/people that will include environmental conditions that could affect the reflected light
- could the inclination angle of the head make a difference? it would probably if it were a half size number plate - or the the test card were at the same lowered height off ground

and would your eyes be doing the same thing?
- I think so given the Snellen chart stuff - but I really can't be sure (all the optic stuff I've had to play with has been with lenses that weren't aging bags of jelly being pulled in to shape by muscles!)

perhaps this is going off topic though?

[quelquod]

Only if you assume both are equally in focus. Which is precisely what they're not if you're short-sighted.

I doubt very much if there are (m)any short-sighted people whose focus sharpens significantly after around 10m though - opticians routinely measure distance vision at lesser distances and near vision at around half a metre.