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 Post subject: THE BRIGHT FLASH
PostPosted: Fri Aug 06, 2004 23:48 
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hi all,

i've mentioned this before but thought it worthy of another. Approaching a camera on the other side of the dual carriageway (CW) a few times and someone going the other way set it off. I happened to be literally blinded for about 5 secs then tracers all the way home, how safe is that supposed to be. Oh it's also interesting to note how the cameras are set to flash oncoming drivers on other side of CW exceeding the limit though no penalties issued.

Andrew


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 Post subject: Re: THE BRIGHT FLASH
PostPosted: Mon Aug 09, 2004 17:44 
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andys280176 wrote:
Oh it's also interesting to note how the cameras are set to flash oncoming drivers on other side of CW exceeding the limit though no penalties issued.
:shock: :shock: I've not heard about that before. You sure? I've never been flashed by Gatsos this way, and I usually go past at... er, let me put it like this, I should have been flashed more than once if they operated this way. :wink: Or does this relate to those reversible Truvelos? (None near me luckily).

About the power of the flash on scameras. I found some technical details for Gatsos. Now that doesn't actually give any sort of standard output for the flash like in joules or anything. But it does tell us that the lens is focused at 20m and that its maximum aperture is f3.8. Any photographers reading this will know where I'm going here.

There's a very basic way of describing the flash power of a camera flash (built in or separate flashgun) called a guide number, or GN. It's really used as a rule of thumb for working out correct exposures for flash photography, but it is related to the actual output of the flash. The GN is the distance to the thing you want to photograph mutiplied by the aperture you want to use, allowing for whatever the film speed is. So for example, if I was standing on the pavement trying to take a flash photo of a car passing 20m away using f3.8, I would need to use a flash with GN 76 (assuming ISO100 film). This is what Gatsos have to do, and GN76 is a bloody powerful flash. You really really wouldn't want to be looking that way at close range when the thing goes off.

That's a worst case situation, as it assumes they always use ISO100 film. If you use a faster film speed (ie a more sensitive film) you can shoot further or use a weaker flash. So it does depend on the film in the Gatso too, but too fast would mean an unacceptable loss in quality. If it was me I'd hedge my bets and use medium speed most of the time, say ISO200 or 400. That would mean Gatsos wouldn't have to use a flash any more powerful than you could buy for a few hundred quid in any camera shop, which are about GN40 give or take (and if anyone would give me a few hundred quid I'll be happy to go buy one and demonstrate it :D ). I'd still be happier if it wasn't pointing at me while I'm driving though.

On top of that, we have something called the inverse square law. This says that light intensity from a point source, such as a flash, is inversely proportionally to the square of the distance. To put it another way, double the distance is quarter the intensity, half the distance is four times as much. In Gatso terms that means the flash is 4 times as bad at 10m than it is at 20m. At 5m it would be like me firing my flashgun in your face from about 2 feet. :cool: would be handy, but then you'd get pulled for driving with them at night :roll: . Andy, were you fairly close to the thing when it went off?

Of course, the other thing is how sensitive the human eye is to a flash going off, especially at night when you've adjusted to dark conditions. Andy, you don't say when you were flashed but I'm guessing it was at night. I think it's safe to say that when our eyes have adjusted to darkness flash will appear much stronger than at the same distance in daylight. How much I have no idea. Anyone know any eye experts who could go into this? Mad Moggie, any where you work?

One final point. All this is just about how much (or not) flashes from Gatsos can dazzle drivers. It's just as important to consider how much of a distraction they are. Even if it's not strong enough to actually affect your visions, some people might insticntively look for the source of the flash. A momentary distraction, sure, but sometimes a moments distraction is enough to create a dangerous situation.

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 Post subject:
PostPosted: Tue Aug 10, 2004 12:05 
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Gatsobait,

it was at night and ye these cameras-in my area anyway-are set to flash at oncoming traffic as well but I've noticed it is a lesser flash than a proper sting. What had happened to me was someone else going in the opposite direction set it off and I was on the outside lane of my side just about 20 meters away so I got a good blinder, 2 big flashes.

Andrew


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First, it is ridiculed.
Second, it is violently opposed.
Third, it is accepted as being self-evident."
-- Arthur Schopenhauer


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 Post subject:
PostPosted: Tue Aug 10, 2004 23:35 
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Gatsobait wrote:
This is what Gatsos have to do, and GN76 is a bloody powerful flash. You really really wouldn't want to be looking that way at close range when the thing goes off.

On the A38 at the bottom of Haldon Hill near Exeter is a Gatso set to catch the downhill, Exeter-bound traffic. At that point, it's a slight left-hand bend for the downhill traffic / right-hand bend for the uphill traffic.

A few years ago, that camera was positioned on the central reservation. I was in the outside, uphill lane driving a vehicle with a "commanding driving position" (i.e. the gatso wasn't much above my eye level) just after dusk. Just as I was about to draw level with the Gatso, some speeder in the opposite direction set the thing off. I was temporarily blinded -- all I could see was the image of the flash. The blindness probably only lasted five to ten seconds, but it seemed like an eternity. I consider I was lucky not to crash.

Thankfully, they've moved that Gatso to the left-hand side of the Exeter-bound carriageway, although goodness knows how many letters and phone calls it took before they realized their mistake.

I do wonder about the many Gatsos mounted in the centre of the road so that they can easily change the direction of entrapment. IMO, these things are an accident waiting to happen, particularly since they're at the eye level of the drivers of the largest vehicles on the road.

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 Post subject:
PostPosted: Wed Aug 11, 2004 10:49 
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willcove wrote:
IMO, these things are an accident waiting to happen, particularly since they're at the eye level of the drivers of the largest vehicles on the road.
I drive something low and sporty, so that hadn't occurred to me. Thinking about it the level of the roofline would prevent me seeing the flash directly unless I was fairly far away, in whcih case it wouldn't be so much of a problem (inverse square law again). Driving something like a tall MPV or 4x4 would make it much worse, and being in the cab of an HGV would probably be worst of all. I know the sort of reversable Gatso you mean. I don't know if it's still there, but there used to be one on the Great Chertsey Road near Twickenham, and there is one on the A30 near the airport. Same sort of situation. Dual carriageway with the Gatso in the central reservation. Potentially an HGV in the outside lane could get within a few meters of a Gatso just as it zaps someone going the other way. Perhaps 20+ tons under the control of a driver who's suffering a temporary reduction of vision. :shock: Great.

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 Post subject:
PostPosted: Sat Aug 21, 2004 09:45 
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All camera should be facing towards the line of traffic they are monitoring.

In my world anyway, as a Biker :P

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 Post subject:
PostPosted: Mon Nov 01, 2004 23:46 
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Location: Treacletown ( just north of M6 J3),A MILE OR TWO PAST BEDROCK
Thanks Gatsobait for the gatso flash figures, pity we couldn't get instantaneous power output -

- we've just had a spate of new gatsos planted locally and actually been in the passenger seat of a Transit when one fired into us (fortunately we were level with it) and wouldn't like to get it in the face .-wasn't one of these.

The new ones have all been planted on nominal 2 lane "towny" residential roads

anyone know the facts/figures on minimum SAFE distance to a Gatso - at night?
Might give the local Scamera mob a red face if their "SAFETY" DEVICE was questioned on the grounds of safety? :oops:
Might be next newspaperheadline is "SAFETY? " CAMERA SHOOTS PARTNERSHIP IN FOOT?
One has been put where normally the width is about 3 cars+, but normally being residential there's a few parked cars forcing you toward it.


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