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SS-Traveller 24th January 2013 19:46

ARTICLE: Guidelines & Tips for Safe Driving in FOG
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For those who have experienced it, driving in fog is THE toughest weather condition to drive in. The best of drivers quail when faced with dense fog, and for a novice, being surrounded by fog with visibility reduced to less than 50 metres is a situation that can cause panic attacks. Here we outline certain do's and don'ts to get you home safe through the fog.

Must-Read Articles:
This article will be divided into 6 parts:
  1. The physics and physiology of driving in fog

  2. Equipping yourself to drive safely in fog

  3. Driving in fog during daylight hours: Urban areas vs. highways

  4. Driving in fog at night: Urban areas vs. highways

  5. General tips and tricks

  6. How to effectively de-fog the windshield

The links above may be used to jump to the relevant post within this thread.

SS-Traveller 24th January 2013 19:47

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1. The physics and physiology of driving in fog

So what is fog? It's just a thick cloud of water droplets suspended in the atmosphere at or near the earth's surface. Since these droplets scatter the light that should have been directly transmitted to our eyes, we are unable to see as far as we should have on a clear day (or night).

In addition, whatever object we can see, is severely reduced in CONTRAST, making the object we see through the fog to be fainter and more indistinct. How much contrast is lost depends on
  • the distance of the object from your eye
  • coefficient of scatter of light in the medium (smaller the droplets of water, more the scatter)
  • whether it is fog or smog (smoke + fog). Opaque particles of dust and smoke make for poorer visibility
  • the wavelength of the light reaching your eye - blue scatters more than red

Our brain suffers from a certain amount of loss of perception as a result of this loss of contrast in the images it receives from the eyes. The primary effect is loss of perception of motion - as a result, we tend to judge motion to be slower or faster than it actually is, and/or fail to distinguish between a moving and stationary object (eg. a stopped car ahead of us can be perceived to be moving, or vice-versa).

Our own perception of the speed we are travelling at, is also reduced in fog. At high speeds on a clear day, our peripheral vision tells us how fast we are crossing a stationary object on the side of the road, without our needing to check the speedometer. In dense fog, this feedback mechanism reduces or ceases to exist.

In addition, our judgment of distances is also severely hampered in fog. A phenomenon called aerial perspective, causes us to perceive objects which are low contrast and indistinct as being farther away. We learn this because of an environmental regularity: light from distant objects, such as mountains that are far away, appear hazy/foggy, while the brain interprets closer objects to be clearer.

The Mandelbaum Effect also comes into play when staring into fog. "Accommodation" is the physiological process by which the eye brings objects of different distances into sharp focus. Fog causes the eye to mis-accommodate, resulting in blurred vision for more distant objects - the tendency of the eye is to approach "resting" accommodation under conditions of poor visibility. Since the focal length of resting accommodation is relatively short, a little over three feet, objects down the road will be out of focus, because our brains tell our eyes not to bother focusing that far away!

SS-Traveller 24th January 2013 19:50

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2. Equipping yourself to drive safely in fog

The primary requirements when you venture out to drive into fog are:
  • Your ability to see as clearly as possible
  • Allowing others to notice your presence as clearly as possible
  • Making your intentions understood to others, about your next manoeuvre

Run through the following list to ensure you are well prepared:

Windshields & mirrors: Make sure your front and rear windshields as well as all window glasses and ORVMs are cleaned and polished to a perfect streak-free state. Once you are in fog, it is probably too late to stop to clean them on the road - the risk of someone ramming into you if you are wrongly parked is rather high. I use a solution called Lenshine, which is a proprietary lens cleaner for spectacles, to clean the inside surfaces of my windshields. Products like Rain-X can also be used.

Wipers: Make sure your wiper blades do not streak / skip and leave marks on the glass when used with windshield washer fluid.

Washer fluid: Check that the windshield washer tank is topped up with either a proprietary washer fluid, or a concoction of soap and water. The soap is required to break down grease and dirt. I prefer to use drinking water into which I add about 10 ml of dish-washing liquid soap like Pril.

Eyewear: Your spectacles (especially if you use prescription lenses) should not have tinted/polarized/photochromatic lenses. With visibility already severely compromised in fog, you don't want to make it worse by allowing your eyewear to reduce whatever little light reaches your eye. Make sure that the lenses are also sparkling clean.

Fog lamps: If your car is equipped with front and rear fog lamps, make sure these work. In case fog lamps are not supplied as OE with the car, it would be a good idea to install them ASAP if you live in fog-prone zones like North India. Ensure that all of your indicator lamps are functioning as well.

Torch: Keep a powerful torch in the car. It can be used for signaling, and also when you need to survey the road ahead that you can't view from the car at night. Check that the batteries have sufficient charge remaining.

Rules and regulations governing use of fog lamps:
Must read:
1. Fog lamps? What use are they?
2. How to use fog lamps
3. There's no fog, but can I use fog lamps?
4. About rear fog lamps

This is India, and our rules are there to be broken, on the excuse that one does not know what rules apply! Fitment of front and rear fog lamps is optional according to our rules, and this is what is mentioned:

The rules governing the use of fog lamps are clarified internationally, and restrictions on their use are listed below:

On behalf of Team-BHP, I make an earnest request to all drivers:
Please do not use fog lamps in clear weather conditions, and/or in any manner that distresses other drivers on the road.

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DO NOT use your hazard lights in reduced visibility conditions while driving. Use them only when your vehicle is stopped, regardless of the weather conditions. The purpose of these flashing lights is to warn other drivers about the existence of your vehicle as a hazard to others driving past it, OR to warn others of a hazard ahead that you can see, but which may not be visible to them.

Also read: Avoid using hazard lights during fog.

In the situation that you have a car that is not equipped with fog lamps (and you do not wish to spend the paltry sum required to fix fog lamps anytime in the future), but you are travelling on a fog-bound road, you can switch on your hazard lights, only until you are sure that a vehicle coming up behind you has noticed you and slowed down sufficiently.

Under no circumstance should you continue to use hazard lights while another car is continuously following you. This may lead to "moth-effect", especially at night, and your vehicle would be the worse off if the vehicle behind you just happens to be a truck with an inebriated driver at the wheel (inebriated drivers suffer from greater "moth effect").

Also read: 'The moth effect' endangers motorists.

In addition, if you incorrectly activate your hazard lights, you are handicapped in your ability to signal your intention to turn and / or make lane changes.

SS-Traveller 24th January 2013 19:52

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3. Driving in fog during daylight hours:

Urban Areas:
Rule #1: SLOW DOWN. You do NOT have to match the maximum speed limit in fog. Driving at 30 km/h in a 50 km/h zone is not a crime. However, ensure you do not slow down too much, as that makes your vehicle more likely to get rear-ended by a much faster moving vehicle in these poor visibility conditions. Still, going slow and being rear ended is safer than going fast and hitting a pedestrian or solid obstacle that you didn't see up ahead.

• Even if it is daytime, visibility is severely compromised in fog. Even if you cannot see, make sure that you are seen. Always turn on your headlamps ON LOW BEAM. Do not use high beams - they cause unnecessary irritation to oncoming traffic.

• You can flick the high beams to warn errant drivers who may be straying into your path. DO NOT try to blind them and make things worse.

• Use the horn liberally to warn unmindful pedestrians and other road users. Severely foggy conditions reduce audibility and deaden sharp sounds, making them appear to originate from further away than they actually are.

• During the daytime, it is not really essential to turn on your fog lamps in urban areas, either front or rear, unless visibility drops to less than 50 metres. Just the headlights on low beam, and the regular tail lamps are sufficient for you to be noticed.

• Stick to the main roads in your city with high volume of traffic, and avoid empty lanes, especially in areas you are not familiar with. You will notice that fog intensity is lesser in zones where traffic flow is high, automatically improving visibility and safety. It is also easier to follow your lane by keeping a watch on the lane markers, which are usually more prominent on main roads.

• Remain alert to bicycles and motorized two-wheelers that cut across your path without warning. Without the intervening windshield, they can see you better, and assume that you can see them too. But you can still have an accident since you failed to notice them. Remember, our peripheral vision becomes almost zero in fog.

• Maintain twice the usual distance behind the vehicle you are following. Beware of sudden obstructions and panic stops by the vehicle ahead. You could ram into him due to the "moth effect".

Rule #1: Estimate the visibility range. Reassess constantly. You need to slow down and assess how far you can see an object that is not showing any lights in the fog. A tree trunk on the side of the road, or a parked truck with its lights off - gauge the distance at which you can manage to make out the obstruction. This distance can be considered the maximum braking distance you have, and will consequently determine your speed on the highway (I normally convert every 2 metres of visilbility to every km/h of speed - so a visibility range of 50m means a max speed of 25 km/h).

• Keep those lights on. Run your front and rear fog lamps along with dipped headlamps. BUT remember to switch them off once visibility increases.

• Be alert, and keep an eye on the RVMs. Visibility range changes very rapidly on the highways. Hitting sudden pockets of dense fog when driving along clear stretches, can disorient you and cause you to go out of control. Be prepared to slow down rapidly, but don't panic-brake - you don't want the car following you to start clambering inside your boot. Let that tailgater overtake you when the visibility is better.

• If you think you are unable to make headway in the fog and are feeling claustrophobic, wait for a "sweeper" vehicle to come along. A truck with its lights on is a better bet to follow, than an LMV whose driver thinks he is the next best thing that can happen to F-1.

• Worst case scenario: Pull over and stop for the fog to clear. But before you take your wheels off the tarmac, make absolutely sure there is solid ground there. Rain and fog often go hand-in-hand, making the ground soft and wet. Pick a straight and safe stretch of road to stop beside. Avoid stopping on curves.

SS-Traveller 24th January 2013 19:54

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4. Driving in fog at night:

Urban Areas:
It is much easier to drive on a well-lit city road in dense fog, than it is on the dark highway. The yellow high mast sodium vapour lamps that are commonly used in our cities help to improve visibility as well as minimize glare.

Your high beams scatter the light back into your eyes in fog. Because of the brightness ahead when you stay on high beams, you might assume that you can see better. This is a fallacious assumption, and the glare from your own lights will reduce your ability to spot objects ahead.

The basic rules of driving in fog during daylight hours, which have been outlined in the previous posts, also apply to driving in fog at night.

To reiterate:
Switch on low beams as well as front & rear fog lamps
Keep your windshields and mirrors clean. Use wash-wipe and defoggers liberally
Drive slower than the speed limit if necessary
Switch off fog lamps when fog lightens. Switch them on again when you enter zones with dense fog
Do not brake sharply unless it is unavoidable
Watch out for sudden lane-cutting by smaller poorly lit vehicles & 2-wheelers
Be very wary of unannounced lane changes and turns by the car ahead
Use the horn liberally to warn others of your presence
Turn on hazard indicators when parked in areas where there is heavy traffic flow next to your car
Do not fall prey to the "moth effect"

If you haven't driven in fog at night on the highway accompanied by an experienced person previously, avoid doing this unless it's a national emergency! Even then, remember that it is THE most hazardous type of driving.

Traffic on highways is extremely minimal on a foggy night - laden trucks generally do not venture out due to the extreme risk involved. If you have been inadvertently caught in fog on the highway at night, try to stop somewhere safe, and wait till the fog lifts or till daybreak (driving in fog during daylight hours is easier than driving at night, although fog generally intensifies just before sunrise).

Any vehicle travelling reasonably confidently on a fog-bound highway at night is most likely to be a local driver who knows this stretch of the highway like the back of his hand. Use him as a "sweeper" vehicle if you must, but if you keep following him without looking around, you'll reach his home and he won't serve you dinner just because you showed up! :D Or even worse, when he makes a mistake and hits something ahead, you have a chance of taking up residence inside his boot.

Look for lane markers or central dividers on highways, which you can follow. If the lane markers / dividers cease to be visible, stop and reassess the path you need to follow further ahead.

Keep a powerful torch in the car. Get out on foot and survey the road if necessary, to make sure you don't land in a ditch.

In extreme fog (visibility less than 10 meters), I have had to get a co-passenger to lead me to the side of the highway on foot at one time. I had completely lost my bearings.

In patchy fog conditions (areas of clear visibility followed suddenly by zones of dense fog) on the highway, trucks have much better visibility and continue to drive at constant speeds. Conversely, drivers of low-slung sedans have sudden loss of visibility. This is because of the truck driver's high seating position, which allows him to see clearly by his own lower-placed headlights, without the scatter dazzling him. But the sedan driver's headlights are at his eye level, and the scatter from the beams blind him to an extent, and force him to slow down. In such conditions, it is wiser to maintain a safe distance behind such a "sweeper" truck and follow him at constant (but slower) speed, rather than overtaking him in the clear areas, only to have him catch up with you in the next fog-bound area.

SS-Traveller 24th January 2013 19:57

5. Some general tips and tricks

• Fog density can vary suddenly. Be very watchful.

• Always run the AC and front (and rear, if so equipped) defoggers at all times in fog. Forget about FE!

• Moisture and dirt buildup on the outside of your windscreens will be gradual, while you imagine that fog is becoming more dense. Run the wash-wipe at regular intervals of a minute or two. Now you know why the wipers should be good and the washer bottle full before you venture out into fog!

• Keep your window wound down partially. It has two advantages:
• You can hear better when other cars are honking around you. Fog deadens sound and make them appear to come from far away.
• You can look out from the window and assess the density of fog. It will help you to understand whether the fog is really getting denser or your windows and windscreen have fogged up from the inside or outside.
• Don't keep staring straight into the fog ahead, hoping to sight any obstruction instantaneously. The Mandelbaum Effect we talked about earlier, will set in, and your eyes will adjust focus onto the edge of your bonnet. Instead:
• Make a conscious effort to look around to the left and right
• Blink frequently
• Poke your head out of the window and look ahead
• Rub your eyes even if you don't feel like it
• Switch your vision rapidly between your meter console and the road
• Flick your headlamps off and on rapidly once or twice, or try flicking them on to high beam quickly once or twice. The pupils of your eyes will contract-dilate-contract quickly as a result, and you'll be able to focus better
• Use the painted line on the left edge of the road as a guide if you are having trouble following the road ahead.

• If you are forced to follow another vehicle which has its hazard lights on in fog, try not to stick to the same lane behind him. It may be risky to pull out and overtake him, but at least you can drive in the adjacent lane at a safe distance - this will reduce the dazzle to your eyes, and you are less likely to run into his rear bumper if he brakes or changes lanes suddenly.

• Yellow plastic stuck on your fog lamps or headlamps is useless. So are yellow bulbs. Been there, done that, and enough research has been done to prove that doing this has no proven advantages. Read this article as further proof.

• Do not wipe the inside surface of your front windscreen with your bare palm. The sweat and oils on your skin will streak the glass, and it will fog up more rapidly as well as scatter the light of oncoming cars. Use the defogger with AC to prevent moisture buildup. If you already have moisture deposit on the inside, use a microfibre cloth, old newspaper, paper tissue or a clean & dry handkerchief to clean the moisture in unidirectional strokes. Avoid circular rubbing on the glass surface - this makes oncoming headlights dazzle you more.

• After wash-wipe sequence, run the wipers without washer fluid for a few times. This will ensure the glass is clean and dry, and will not "flare" the light from car headlights approaching you.

• Keep your GPS device turned on ONLY IF you are sure that the area is well-mapped as per your device. Keeping the GPS running helps to predict upcoming curves and turns that you might miss in the fog, both in the city and on highways. On the other hand, a bad map can guide you onto non-existent roads & bridges, or roads that have been recently realigned, and the consequences can be disastrous in fog.

• If you are unfortunate enough to have been involved in a multi-car highway pileup in dense fog, don't jump out of the car and run across the road. Remain in the car and turn on your hazard lights. After that, make sure you can open the door(s) and get out without getting run over, then leave the car to go to a safer spot. Your car's bodyshell is usually strong enough to offer you better protection if another car hits you again, rather than when you are on foot.

• Above all else, have patience and keep a cool head. Panic and road rage don't make the fog go away. Even if you cannot see the road, it's still there. It's just that you have to calmly find it and stay on it.

To conclude, driving in fog may not be entirely avoidable for people who live in North India and cannot stop going to work just because it is winter-time. At the same time, adequate precautions and conscious actions must be the order of the day when driving in fog, mainly because the natural & subconscious instincts that we use to drive on a clear day (or night), do not work well when driving in fog.

Drive safe, unless you can avoid driving in fog!

Rehaan 24th January 2013 21:27

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
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Superb article SS-Traveller!

There was tons of interesting information in there, especially with regards to sight & sound in fog. Thanks a millions for taking the time to share this.

Two things i wanted to add:

1) Fog lights are usually placed fairly low. This enables them to light up maximum road and minimum fog. When you light up fog, most of what it does is scatter and reflect light causing glare. I've also heard that in some situations, fog is less dense just at ground level (perhaps the temperature of the earth has some part to play in this?). I can't find any solid back-up for this statement though.

2) Here are 3 screen-grabs from a video I have of me driving in very mild fog at night. Each picture is virtually the same location with low-beams (on the left image) and high-beams (on the right image). This helps illustrate why high-beams should be avoided:

Notice how much of the high-beam reflects back as glare
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The glare kills the visibility of whats beyond it. Look at the green shrubs that are at the base of the signpost in both images to compare
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Once again, high-beam glare on the right prevents you from seeing as far into the distance
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SS-Traveller 24th January 2013 23:05

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
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Originally Posted by Rehaan (Post 3024098)
Superb article SS-Traveller!
There was tons of interesting information in there, especially with regards to sight & sound in fog. Thanks a millions for taking the time to share this.

And thank you for the time you took to edit it and make it more presentable!

1) Fog lights are usually placed fairly low. This enables them to light up maximum road and minimum fog. When you light up fog, most of what it does is scatter and reflect light causing glare.
It is also important to understand the different beam patterns that our headlights generate vs. our fog lights. I have tried to present a rough illustration...
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...where it is obvious that the fog lamps have a widely spread out and not far-reaching beam pattern, which tends not to be reflected back by the fog into our eyes - hence, we can see the surface of the road close to the car better in fog. But this does not allow us to drive very fast, since the reach of the lights is not at all good.

A good resource to learn more about fog lamps and their design & limitations:


I've also heard that in some situations, fog is less dense just at ground level (perhaps the temperature of the earth has some part to play in this?). I can't find any solid back-up for this statement though.
Yes, a fog bank can well lie about 6" - 1' off the ground - I've seen this quite often in the early parts of the night, just after sundown, when (I suspect) the ground is warmer and prevents the moisture from condensing into droplets close to it; but the air being colder as well as moisture-laden, fog forms at a higher level.


Here are 3 screen-grabs from a video I have of me driving in very mild fog at night. Each picture is virtually the same location with low-beams (on the left image) and high-beams (on the right image). This helps illustrate why high-beams should be avoided:

Notice how much of the high-beam reflects back as glare
Attachment 1042050

The glare kills the visibility of whats beyond it. Look at the green shrubs are at the base of the signpost in both images to compare
Attachment 1042048

Once again, high-beam glare on the right prevents you from seeing as far into the distance
Attachment 1042049
clap: That's a perfect set of illustrations! (Exactly what the doctor ordered... :p )

rajeev k 24th January 2013 23:34

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
Very informative post and T Bhp is now a days on the educative mode which will augurs well for the community which learns and follows unnecessary and unwanted elements too quickly.
Regarding fog lamps, today fog or no fog, fog lamps always remain on. That is the style followed by almost all, at least in Kerala. Here some suggestions were mooted by some safety committee with regard to keeping the headlamps of two wheelers on during day time to get them noticed by drivers of bigger vehicles as a preventive methodology against accidents.No sooner this news appeared the so called literati started this practice and that too in full blast of the high beam. But luckily that practice ceased.

Octane_Power 25th January 2013 00:03

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
Rehaan, I have experienced the same phenomenon.
Once I was in Lonavala at around 3 am and it was the densest fog i have ever come across at night. It was also my 1st fog encounter while driving.
When my beam was low I could manage well to drive and as I switched to high beam I noticed that my light itself reflected more from the water particles and blinded myself.
Very helpful article SS-Traveller. Thank You Sir.


Nonstop-driver 25th January 2013 09:40

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
Thank you SS-Traveller Sir for this piece! Now of course I can point my office guys to the thread for reference.

Alas, that I did searched Traffic Police web-sites for such advisory and couldn't find anything on the Delhi Traffic Police site. They don't say anything, neither in support, nor aginst it.

The only place I can find this, where they advise against using hazard light in foggy weather is the Chandigarh Traffic Police Website (http://www.chandigarhtrafficpolice.o...ing_in_fog.php). Kudos to them!:thumbs up

GTO 25th January 2013 09:58

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
Truly a bible for those who frequently drive in fog. Thanks for sharing SS-Traveller. Your driving experience just shines through this article.

Luckily, we don't face foggy driving conditions in Mumbai. It's only when you visit hill stations like Mahabaleshwar that you encounter fog in our neck of the woods.

I learned a lot from your article this morning. Our Advice articles are comprehensive (as yours has proven) and they do take time to read & understand. But knowledge & practice is what separates good drivers from the bad :thumbs up

ajmat 25th January 2013 10:19

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
A very useful well written article. A couple of points

RainX, I would avoid on windscreen since it causes the wipers to smear in my experience

Washing up liquid in the washer bottle - Use very little or avoid since it also is slightly abrasive

R2D2 25th January 2013 10:22

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG
Excellent post SS-Traveller. Thanks a lot. It will be most useful for all TBHPians but mostly for those in the north parts of the country and in the mountains where fog and bad visibility are endemic during the winter.

ghodlur 25th January 2013 11:14

Re: ARTICLE: Guidelines & Tips for Safe Driving in FOG

:thumbs upTruly hats off to you guys for dishing out the informative threads which I believe every driver should be aware of. Fantastically put, every word in the thread carries so much weight when one has to navigate through the foggy weather.

Although some cars do have the front & rear fog lamps (or atleast the slots for the same), they are either not utilized or wasted in the sense some of the lamps have no or broken glass which makes the fog lamps ineffective. How would one rely completely on the Inner rear view mirrors when the outer RVM's are misted due to the fog, is it considered safe to drive with the window's rolled down? How's the road grip in places where heavy fog is witnessed, what should one be careful of? Also since the road signs/direction boards are pretty much invisible during the fog, how does one tackle this? Sorry too many queries.

One suggestion to Mods: Compile the recent threads on safe driving and publish them as a handbook.

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