Public information announcement for 'well equipped' cyclists in fog

[Peter Fox]

Just to remind people that reflectives much less effective in the fog
than they think. With no fog, light travels from (say) a car headlight
X feet to the retro-reflecting element then all of that which is
reflected travels another X feet back to the car driver. Job done.

But in the fog there is a double-whammy. Firstly only a part of the
original illuminating light ever reaches the reflector and then only a
fraction of that returned light gets back to the original source.

If on a trip of X feet 50% of a light beam is dispersed then this works
out at
* Arrives at reflector = 50%
* Is reflected (assuming perfect reflector) = 50%
* Gets back to source = 25%
(Inverse square law!)

I'm not sure what you're supposed to do with this information except
* keep reflectors clean
* have a bloody good rear light
* assume that, until really close, a following vehicle will only know
you're there by a single red light.
* stay at home in front of a blazing fire and leave freezing fog to
others more daft than yourself.
* and (see question below)

Also:
The same rule applies to fog banks and dipping into valley fog (day or
night) as going round bends in country lanes. Avoid (if possible or if
not accelerate) disappearing (round corner/into fog) just before a
following vehicle sees you otherwise they can catch up with you very
unexpectedly. (Obviously the following vehicle /should/ be taking the
possibility of something lurking round the corner/in the soup into
account, but it saves a bit of increased blood pressure.)

Question: Are red rear lights for bikes any use in daytime foggy
conditions? Or daytime any conditions? [There are 'obvious' answers
but what's the practical experience?]

--
PETER FOX Not the same since the porcelain business went down the pan
[email protected]
www.eminent.demon.co.uk - Lots for cyclists

 

[Rob Morley]

In article <[email protected]>
Peter Fox <[email protected]> wrote:
<snip>
> Question: Are red rear lights for bikes any use in daytime foggy
> conditions? Or daytime any conditions? [There are 'obvious' answers
> but what's the practical experience?]
>
I suspect that in really poor visibility an amber flasher might be more
effective than a constant red light, not because of the
physics/physiology so much as the psychology.

 

[Adam Lea]

"Peter Fox" <[email protected]> wrote in
message news:[email protected]...
> If on a trip of X feet 50% of a light beam is dispersed ...

Be careful. You have to take into account that light is preferentially
scattered forwards when it encounters clouds, so that light from a source
passing through fog will be scattered mainly away from the light source. I
can't remember the numbers exactly but it is something like 75-80%. This is
why clouds look darker when you are facing away from the sun compared to
when you are facing the sun.

Adam

 

[Peter Fox]

Following on from Adam Lea's message. . .
>
>"Peter Fox" <[email protected]> wrote in
>message news:[email protected]...
>> If on a trip of X feet 50% of a light beam is dispersed ...
>
>Be careful. You have to take into account that light is preferentially
>scattered forwards when it encounters clouds, so that light from a source
>passing through fog will be scattered mainly away from the light source. I
>can't remember the numbers exactly but it is something like 75-80%. This is
>why clouds look darker when you are facing away from the sun compared to
>when you are facing the sun.
>
>Adam
Aha! If you want to get technical:
(a) I said 50% _over a particular distance_ and an example.
(b) Ray scattering pattern depends on drop size
(c) Blurring of edges completely buggers up depth perception.


--
PETER FOX Not the same since the borehole business dried up
[email protected]
www.eminent.demon.co.uk - Lots for cyclists

 

[Peter Fox]

To correct my last erroneous message:
>(b) Ray scattering pattern depends on drop size
This is rubbish. Refractive index (which is pretty much constant)

--
PETER FOX Not the same since the statuette business went bust
[email protected]
2 Tees Close, Witham, Essex.
Gravity beer in Essex <http://www.eminent.demon.co.uk>

 

[Tony Raven]

Peter Fox wrote on 21/12/2006 10:01 +0100:
> To correct my last erroneous message:
>> (b) Ray scattering pattern depends on drop size
> This is rubbish. Refractive index (which is pretty much constant)
>

You've never heard of Rayleigh and Mie scattering then?

--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

 

[Roger Merriman]

Rob Morley <[email protected]> wrote:

> In article <[email protected]>
> Peter Fox <[email protected]> wrote:
> <snip>
> > Question: Are red rear lights for bikes any use in daytime foggy
> > conditions? Or daytime any conditions? [There are 'obvious' answers
> > but what's the practical experience?]
> >
> I suspect that in really poor visibility an amber flasher might be more
> effective than a constant red light, not because of the
> physics/physiology so much as the psychology.

the problems with the flashers particuly the amber as that could be
front or back, is it can be hard to judge the distance, particuly when
all you have is the light, such as in very dark unlight roads, or thick
foggy condistions.

while the flashers maybe well be seen before, in poor conditions they
confuse drivers abilty to judge the distance and speed.

roger

 

[Dan Gregory]

Roger Merriman wrote:

> while the flashers maybe well be seen before, in poor conditions they
> confuse drivers abilty to judge the distance and speed.

Wouldn't that confusion cause them to slow down?

 

[Al C-F]

Dan Gregory wrote:
> Roger Merriman wrote:
>
>> while the flashers maybe well be seen before, in poor conditions they
>> confuse drivers abilty to judge the distance and speed.
>
>
> Wouldn't that confusion cause them to slow down?

Depends. Do they drive according to their being able to see that the
way is clear, or according to their inability to see an obstruction?

 

[Roger Merriman]

Dan Gregory <[email protected]> wrote:

> Roger Merriman wrote:
>
> > while the flashers maybe well be seen before, in poor conditions they
> > confuse drivers abilty to judge the distance and speed.
>
> Wouldn't that confusion cause them to slow down?

it might well, but confusion when there is ton or so of metal is not
such a wise idea.

roger

 

[DavidR]

"Roger Merriman" <[email protected]> wrote
>
> while the flashers maybe well be seen before, in poor conditions they
> confuse drivers abilty to judge the distance and speed.

There is a big difference between 'poor conditions' and 'darkness'.

In poor conditions you are likely to be visible without a light but just
want something to be noticed by. In darkness the lights are the only points
of reference.

The other factor is that a cyclist might want a light just to compete with
the unnecessay levels of lighting from motor vehicles [*] and to attenuate
the whingeing noise about cyclists emanating from the persons within.

[*] Whether driving or cycling I find current car lighting way over the top
of what is needed to be helpful. So much of what is important in a street
scene just gets obliterated and a lot of information about the position &
trajectory of the cars themselves gets hidden.

 

[Roger Merriman]

DavidR <[email protected]> wrote:

> "Roger Merriman" <[email protected]> wrote
> >
> > while the flashers maybe well be seen before, in poor conditions they
> > confuse drivers abilty to judge the distance and speed.
>
> There is a big difference between 'poor conditions' and 'darkness'.
>

there is.

> In poor conditions you are likely to be visible without a light but just
> want something to be noticed by. In darkness the lights are the only points
> of reference.
>
fog or other weather will affect visabilty far more than simple
darkness, fog and others can reduse visabilty to a few feet at best.

> The other factor is that a cyclist might want a light just to compete with
> the unnecessay levels of lighting from motor vehicles [*] and to attenuate
> the whingeing noise about cyclists emanating from the persons within.
>
no one is sugesting, that bikes shouldn't have lights.

but what i sugesting is that flashing lights add to the clutter in a
urban area, and due to the flashing make ploting it's direction and
speed harder, and what it is. a steady light gives one more clues.

for a moment it can be easy to see a motorbike, coming at night only to
realise its a van/etc with one blown light.

> [*] Whether driving or cycling I find current car lighting way over the top
> of what is needed to be helpful. So much of what is important in a street
> scene just gets obliterated and a lot of information about the position &
> trajectory of the cars themselves gets hidden.

if you drive out of town and off motorways then your experance of car
lights may differ. though it is true that there is issue with lights
blinding. though how much is with folks not dipping, is debable.

roger

 

[Stevie D]

Peter Fox wrote:

> Question: Are red rear lights for bikes any use in daytime foggy
> conditions? Or daytime any conditions? [There are 'obvious' answers
> but what's the practical experience?]

If they are sufficiently bright that, in foggy conditions, they can be
seen from further away than the cyclist then yes, they are useful.

In normal daylight conditions, cycle lights will be near-enough
invisible and there is no point in using them.

--
Stevie D
\\\\\ ///// Bringing dating agencies to the
\\\\\\\__X__/////// common hedgehog since 2001 - "HedgeHugs"
___\\\\\\\'/ \'///////_____________________________________________

 

[DavidR]

"Roger Merriman" <[email protected]> wrote
> DavidR <[email protected]> wrote:
>> "Roger Merriman" <[email protected]> wrote
>> >
>> > while the flashers maybe well be seen before, in poor conditions they
>> > confuse drivers abilty to judge the distance and speed.
>>
>> There is a big difference between 'poor conditions' and 'darkness'.
>>
> there is.
>
>> In poor conditions you are likely to be visible without a light but just
>> want something to be noticed by. In darkness the lights are the only
>> points
>> of reference.
>>
> fog or other weather will affect visabilty far more than simple
> darkness, fog and others can reduse visabilty to a few feet at best.

I doubt we have had fog with just a few feet of visibility for over 20
years. The only real cause of genuinely reduced visibility these days is
likely to be heavy spray on fast, non-urban roads. Is that an issue for
cyclists?

>> The other factor is that a cyclist might want a light just to compete
>> with
>> the unnecessay levels of lighting from motor vehicles [*] and to
>> attenuate
>> the whingeing noise about cyclists emanating from the persons within.
>>
> no one is sugesting, that bikes shouldn't have lights.

I didn't suggest anyone is suggesting that.

> but what i sugesting is that flashing lights add to the clutter in a
> urban area, and due to the flashing make ploting it's direction and
> speed harder, and what it is. a steady light gives one more clues.

That assumies conditions where an outline is not visible. Outlines provide
much more information and the light does nothing more than attract
attention. Unfortunately it is in competition with bright vehicle
lighting - under modern streetlighting cyclists would otherwise not need
lighting at all.

Not that it matters if you want to use constant at all times. I flash when
light is poor but things are easily identified and use constant at night.

> for a moment it can be easy to see a motorbike, coming at night only to
> realise its a van/etc with one blown light.
>
>> [*] Whether driving or cycling I find current car lighting way over the
>> top of what is needed to be helpful. So much of what is important in a
>> street scene just gets obliterated and a lot of information about the
>> position
>> & trajectory of the cars themselves gets hidden.
>
> if you drive out of town and off motorways then your experance of car
> lights may differ. though it is true that there is issue with lights
> blinding. though how much is with folks not dipping, is debable.

At some point ambient lighting drops to a level that requires lamps bright
enough to provide illumination. It is when illumination is not required
that the masking caused by them is quite unnecesary.

 

[Roger Merriman]

DavidR <[email protected]> wrote:

> "Roger Merriman" <[email protected]> wrote
> > DavidR <[email protected]> wrote:
> >> "Roger Merriman" <[email protected]> wrote
> >> >
> >> > while the flashers maybe well be seen before, in poor conditions they
> >> > confuse drivers abilty to judge the distance and speed.
> >>
> >> There is a big difference between 'poor conditions' and 'darkness'.
> >>
> > there is.
> >
> >> In poor conditions you are likely to be visible without a light but just
> >> want something to be noticed by. In darkness the lights are the only
> >> points
> >> of reference.
> >>
> > fog or other weather will affect visabilty far more than simple
> > darkness, fog and others can reduse visabilty to a few feet at best.
>
> I doubt we have had fog with just a few feet of visibility for over 20
> years. The only real cause of genuinely reduced visibility these days is
> likely to be heavy spray on fast, non-urban roads. Is that an issue for
> cyclists?
>
um well no get out of town, on high ground, and visablity can fall below
20 feet, i drive though and walk though such every winter, such is life
in hilly areas.

spray is for most a part motorway curse which as you say is not a issue
for bikes.

> >> The other factor is that a cyclist might want a light just to compete
> >> with
> >> the unnecessay levels of lighting from motor vehicles [*] and to
> >> attenuate
> >> the whingeing noise about cyclists emanating from the persons within.
> >>
> > no one is sugesting, that bikes shouldn't have lights.
>
> I didn't suggest anyone is suggesting that.
>
> > but what i sugesting is that flashing lights add to the clutter in a
> > urban area, and due to the flashing make ploting it's direction and
> > speed harder, and what it is. a steady light gives one more clues.
>
> That assumies conditions where an outline is not visible. Outlines provide
> much more information and the light does nothing more than attract
> attention. Unfortunately it is in competition with bright vehicle
> lighting - under modern streetlighting cyclists would otherwise not need
> lighting at all.
>
outline may well not be visable or blend into the shadows, most roads
are not that well lit. i would not sugest riding unlit even on very
quiet and well lit roads. unlit walkers or bikes or even cars do blend
in very well with the rest of a urban area.

a lit bike/car says "i'm a moving object" people are used to that
convention, same as a solid white light is (or yellow in france) is
front and a solid red is the rear, flashing yellow are used more often
on things like roadworks.

yes bikes as do all traffic have to cope with the clutter of urban
lights, these means making sure your lit up so you can be seen, and
identified.

> Not that it matters if you want to use constant at all times. I flash when
> light is poor but things are easily identified and use constant at night.
>
in both cases be that a night time ride or ride though poor weather,
while the flashing will catch ones eye it will also make distance, speed
harder to work out.

> > for a moment it can be easy to see a motorbike, coming at night only to
> > realise its a van/etc with one blown light.
> >
> >> [*] Whether driving or cycling I find current car lighting way over the
> >> top of what is needed to be helpful. So much of what is important in a
> >> street scene just gets obliterated and a lot of information about the
> >> position
> >> & trajectory of the cars themselves gets hidden.
> >
> > if you drive out of town and off motorways then your experance of car
> > lights may differ. though it is true that there is issue with lights
> > blinding. though how much is with folks not dipping, is debable.
>
> At some point ambient lighting drops to a level that requires lamps bright
> enough to provide illumination. It is when illumination is not required
> that the masking caused by them is quite unnecesary.

lights are needed to provide light to see, and to allow others to see
you, the weather need not get that bad for lights to useful in this 2nd
role.

roger