[anb]

Originally Posted by libranof1987

Shouldn't mass (weight) be a big part of the equation. After all, braking is a form of Force. And F = m * a.

The equation assumes a certain value for the deceleration "a". For producing the same value of "a", a heavier vehicle has to apply more braking force since braking force is proportional to the mass of the vehicle.

[dhanushs]

Originally Posted by libranof1987

Shouldn't mass (weight) be a big part of the equation. After all, braking is a form of Force. And F = m * a.

I think the most important factor here is the coefficient of friction, and factors that affect it.

The better the car is, you get a better Co-Efficient of Friction, which means, wider tyres, better quality rubber, better reaction to brake pedal force etc etc..

Any formulae for braking, which is primarily a function of friction, without including a co-efficient of friction, (which again is variable for different cases), doesn't make sense.

[drmohitg]

I think the easier way to educate drivers is to instill the concept of a safe gap rather than ask them to calculate braking distance in various conditions. In the UK, the standard practice is to always maintain a 2-second gap to the vehicle in the front under dry conditions, 4s in wet conditions, and 10s in ice/snow.

The figures may not be absolutely possible in our Indian cities but can be easily followed on our highways where so many times you see people tailgating etc.

[supermax]

Originally Posted by libranof1987

Shouldn't mass (weight) be a big part of the equation. After all, braking is a form of Force. And F = m * a.

The formula is used to calculate braking distance for cars one can legally drive with a B license, and that comes with a weight limit of 3.5 tons. I'm sure they use a different formula for trucks or other vehicle categories.

[supermax]

Originally Posted by drmohitg

I think the easier way to educate drivers is to instill the concept of a safe gap rather than ask them to calculate braking distance in various conditions. In the UK, the standard practice is to always maintain a 2-second gap to the vehicle in the front under dry conditions, 4s in wet conditions, and 10s in ice/snow.

The figures may not be absolutely possible in our Indian cities but can be easily followed on our highways where so many times you see people tailgating etc.

Safe gap is just one aspect of it. The real reason why we are taught these, is to let us know just how much braking distances can be, in variety of situations. Gap is with a car/vehicle in front, but what about on an empty highway? People blindly overshoot speed limits, with a false sense of security, thinking that their cars are awesome and can stop on a dime. It is this misplaced confidence that contributes to highway fatalities.

[SS-Traveller]

Interesting thread.

Originally Posted by supermax

...a theory test...

The formula for calculating braking distance

There is a simple formula for calculation of braking distances, in ideal conditions (dry and perfect road, a car with perfect tires). This formula is thus:
1. Divide the speed by 10 (or take away a zero).
2. Square the result of 1. (original speed divided by 10 and multiplied by itself).
3. Multiply the result of 2 by 0.4.

If the speed of the car is 120 km/h, the braking distance under ideal conditions for this would be 12x12x0.4 which is 57.6 meters, or 189 feet.
For a car to stop from 70 km/h, the theoretical braking distance therefore is 7x7x0.4 which is 19.6 meters, or 64.3 feet

Stopping distance vs braking distance

Braking distance is the time taken by the car to come to a stop, after the brakes are hit. Stopping distance is the total distance a car rolls, before it comes to a stop, and this includes the reaction time to the incident that requires the braking.

Stopping distance = reaction distance + braking distance

Originally Posted by supermax

In India, nobody cares about educating license aspirants about the seriousness of maintaining at least 3 seconds of separation between cars on a highway...

No one is capable of calculating these theoretical braking distances and stopping distances while driving, which leaves us the only option, that of leaving a safe gap from the car ahead. This space is called the crash avoidance space, or CAS.

Some say it's 2 seconds, and some say it's 3 seconds. What I was taught was:

For dry tarmac roads, with good tyres
- Till 60 km/h, leave a CAS of 2 seconds;
- Above 60 km/h, it's 3 seconds;
- Above 120 km/h, CAS should be 4 seconds.

Add 1 second each for:
- Night-time driving
- Tired driver
- Poor tyre condition (but still legal tread)

Add 2 seconds each for:
- Wet roads
- Rain
- Debris such as leaf and gravel

Add 5 seconds each for:
- Waterlogging / water film on road
- Snow / ice / gravel / sand

Based on your stopping distance / braking distance formula, I ran up a chart where we calculate the gap between vehicles at a given speed, with 2- and 3-second CAS. It certainly leaves adequate space for total stopping distance, with a margin of error on the safer side (which will account for the variations in braking distance between different cars and tyre sizes). It also explains why the CAS needs to progressively increase with rising speeds.



So the theoretical calculations are perhaps not the best way of figuring out stopping distance, but a simple counting of one-thousand-and-one -- one-thousand-and-two is enough to figure out if the CAS is sufficient. Why one-thousand-and-one -- one-thousand-and-two, one may ask. This is because the average human can pronounce 4-5 syllables in a second. One can choose to say anything repetitively, like one-banana -- two-banana or one-supermax -- two-supermax!

[SS-Traveller]

Originally Posted by supermax

...what about on an empty highway? People blindly overshoot speed limits, with a false sense of security, thinking that their cars are awesome and can stop on a dime. It is this misplaced confidence that contributes to highway fatalities.

Again, for an empty highway, the CAS is 5 seconds to the farthest visible static object on the road. Here's further explanation:

Originally Posted by SS-Traveller

Corollary 2: Drive at a speed appropriate to a static visibility limit of 5 seconds for that lane.

Attachment 2274526

This means that your speed around the curve in the pic above should be enough for you to arrive at that arrow (the furthest visible point around the curve when you spot it) 5 seconds after you spot it. This is also called

static CAS

.

[supermax]

Originally Posted by SS-Traveller

Interesting thread.

No one is capable of calculating these theoretical braking distances and stopping distances while driving,

What's obvious to one is not necessarily so for others. I assumed that nobody would think this is something they need to do on the road, but I was wrong. The whole post is about information from preparing for a driver's license, and some of it is even checked out on skid pad to validate it. This is something to ponder over, not while driving certainly, but when at home, to try and understand braking distances better. Many are totally unaware of real world breaking performance and hold dangerous misconceptions. This post was brought on by a Skoda Slavia review by a fellow BHPian who had felt that he'd slowed his Slavia from 122 km/h to 0 within 15-20 feet.. It's definitely not something that people should be working out while on the road, but something to ponder over before hitting the road. If reading this makes people a bit more aware of braking performance and if it dispossesses them of dangerous misconceptions, I'm hopeful that they will be more hesitant before they overspeed dangerously, while feeling a false sense of security.

Cheers,
-S

[Guite]

I know about the original post in another thread. I have put my 10 cents in that thread as well. So I can fairly understand the reason for this new thread, and I appreciate it. Most of the discussion here is getting lost on pedantic: that all cars have different braking distances. I believe the purpose of this thread is to promote safety by making people aware that real life stopping distances are much longer than we assume.

[charanreddy]

The spirit of the post is being lost and too much emphasis is based on the boundary conditions like super cars, disk vs drum brakes etc. For most people a rule of thumb works better than accurate calculations fit for one’s car. It can be more easily remembered during a panicky situation. Wanting to have accurate data by car is like an ultra slim fit jeans. Only works for for one person / body type. Having a reasonable ball park number is like a regular fit jeans. Fits most people and use cases ( just trying to make my point with an anology. ‘over fitting’ vs ‘under fitting’)


For example, If travelling at 100 kmph, I will try and adhere to the 200 feet gap. If the vehicle in front is moving , of course this gap can be lesser. This helps me take informed decisions while driving.

[kiranton]

I feel the only formula that works for braking in time is using your common sense. ie keep a safe distance from the car in front.

[Ice]

Love the math!

I remember my family driver telling me that nothing teaches you about braking distance like a small accident.

Experience allows us to imbibe the math and make it a way of life, almost subconscious.

[ajayc123]

It is not very intuitive to reach a conclusion that the braking distance is proportional to the square of speed. Otherwise, it wouldn't have needed a Newton to formulate the famous second law of motion that we study during school.

For me, I feel that while it is important to watch out for the distance of the vehicle in front of you, it is equally important to keep an eye on the distance that the driver behind you is maintaining.

While you may be maintaining good distance from the car ahead of you as per the guideline, in braking situation, you may get rear-ended.

So usually, I would quickly let such spirited tailgating geniuses overtake me to eliminate such risk.

[StopUnderrides]

Originally Posted by SS-Traveller

Attachment 2337348

- The square of velocity multiplied by 0.4 formula for braking distance works for trials with elite drivers, perfectly maintained cars in ideal conditions. This is when those drivers know they will be tested on the stopping distance. Most of us can’t achieve that under any practical condition. I use a 0.6 multiplier instead of 0.4.
- The perception + reaction time is more like 1.5 seconds rather than 1 second

It’s necessary to maintain a 3 second CAS on Indian highways even at speeds as low as 40kph

[apachelongbow]

Braking distance is immaterial. The required bis a presence of mind, great hand, eye and foot coordination, and training to simultaneously see, brake and turn. If you can anticipate and then brake plus turn away from the object you will be safe.