Exploring the Unsensical Rule of Thumb for Following Distance

[jonnyk]

Hi,
Dont know whether the same rule of thumb applies internationally but here in Germany it states that the follow distance should be equal to half of the speed in kmh in meters. I thought about it and it doesn't seem to make any sense whatsoever. I've heard people say it's because the braking distance increases with speed. Well apparently they forgot the braking distance of the one driving infront also increases. In fact thinking more abt it actually the exact oppsoite is true. It takes longer for a car to brake from say 200-150 kmh than from say 50-0 kmh. So the driver behind wld be much safer if he followed closer at a speed of 200 kmh. For now say the front brakes and in 2 secs reaches 170 kmh, the impact speed wld be 30 kmh. Whereas whilst following closely at speed of 50 kmh the same 2 secs with same braking power cld make the car stop compleetly and the impact be 50 kmh which is much worse. Are the lawmakers nuts here or is there another reason behind this "rule of thumb"? Thanks.

 

[Danger]

So the driver behind wld be much safer if he followed closer at a speed of 200 kmh

Following this logic, it would be totally unsafe to travel on an unoccupied road.

 

[rcgldr]

It's not the braking distance but reaction time. For any given reaction time, as speed increases, so does the distance covered in that reaction time.

 

[jonnyk]

Hi "jeff",

It's not the braking distance but reaction time. For any given reaction time, as speed increases, so does the distance covered in that reaction time.

JK- The relative speed btw front car and the one behind traveling at 200 kmh when the front one brakes is actually LESS than it would be if the cars were traveling at 50 kmh IF the cars are considerably close to each other. It is this relative speed which is important because now in the 1st case the car at the back has more reaction time actually than the second provided in both cases the follow distance is the same.

 

[jonnyk]

Hi "danger",

Following this logic, it would be totally unsafe to travel on an unoccupied road.

I think i explained this in detail. What i meant is following a car at close distance WHICH IS ALSO traveling at 200kmh so both te car behind and infront are traveling at 200 kmh. Now if the front car brakes it's speed would not drop from 200-0 in a sec. Thts impossible. INFACT it wld take even longer for a car at 200 kmh to drop to 150 kmh than it wld take for a car to drop from 50 mh to 0 kmh BECAUSE KINETIC ENERGY INCREASES BY THE SQUARE OF VELOCITY. So now if the car behind is following very closely the relatzive impact speed btw both cars is considerbaly less than it wld be if two cars traveled at 50 kmh at the same distance and the one infront suddenly hit the brakes. Got it?

 

[YellowTaxi]

If you watch a formula 1 race on tv you'll see that the cars don't stay the same distance apart when they apply their brakes.

The time separation between 2 cars remains exactly constant, but the distance between them definitely closes when they brake before going through a curve.. I can't think of a better explanation than that.

 

[russ_watters]

JK- The relative speed btw front car and the one behind traveling at 200 kmh when the front one brakes is actually LESS than it would be if the cars were traveling at 50 kmh IF the cars are considerably close to each other. It is this relative speed which is important because now in the 1st case the car at the back has more reaction time actually than the second provided in both cases the follow distance is the same.

Huh? Relative speed isn't related to folowing distance or absolute speed, it is a function of reaction time and deceleration. Apply some math:

You are following another car at any speed and distance. The car in front of you slams on the brakes and decelerates at 5m/s/s. It takes you 1 second from the time he starts to apply the brake to when you start to apply the brake. Now, he is traveling 5m/s slower than you. In other words, you are traveling toward him at 5m/s and you will keep traveling toward him at 5m/s until both of you come to a complete stop.

From here you can calculate the following distance required to avoid hitting him by multiplying by the time required for you to stop. And that's where speed comes in. For example, if you are traveling at 20 m/s it takes 4 sec to stop and therefore you must leave 20m between you to avoid hitting him.

 

[DaveC426913]

In other words, you are traveling toward him at 5m/s and you will keep traveling toward him at 5m/s until both of you come to a complete stop.

Russ, are you sure about this? It seems to me that, once you apply your brakes, your relative speed will return to zero and will stay so until you stop.


The only time relative speed is > 0 is after he has applied his brakes but before you have. The result is that you will come to a stop the same distance from the other car as you were when you first applied the brakes.

See attached diagram.

 

[rcgldr]

you will keep traveling toward him at 5m/s until both of you come to a complete stop.

Being a bit nit picky here, but once the other car is stopped, your closing speed will decrease from 5 m/s to 0 m/s. Assuming constant decleration, your average speed in that last second is 2.5 m/s. However during the first second before you started braking your average rate of closure speed was also 2.5 m/s. You still need the 20 m following distance. Assuming constant rate of deceleration, the other car moves 40 m while stopping, and your car moves 20 m before you brake, then 40 m while stopping, for a total of 60 m. ... or you could just consider it's 2 seconds at average closure speed of 2.5 m/s and 3 seconds at 5 m/s = 20 m.

once you apply your brakes, your relative speed will return to zero and will stay so until you stop.

Once you apply your brakes, relative deceleration is zero, relative speed is constant until other car stops, then your relative speed to the now stopped car decreases as you also come to a stop. Referring to your diagram, note that the vertical distance, which represents the relative speed, remains constant during the brief period of time on your diagram when both cars are decelerating at the same rate. It would be easier to see this if the timing was closer or the initial speed was higher (resulting in a taller graph).

 

[pantaz]

All of you are missing jonnyk's main point -- he's talking about impacting the leading vehicle, not avoiding it. So, if you want to reduce the damage caused by solely by the collision speed, then yes, following more closely would work. However, in the real world, that ~200 km/h collision would likely send both vehicles sliding and spinning into surrounding vehicles, trees, light poles, bridge abutments, and so on.

 

[DaveC426913]

All of you are missing jonnyk's main point -- he's talking about impacting the leading vehicle, not avoiding it. So, if you want to reduce the damage caused by solely by the collision speed, then yes, following more closely would work.

He says nothing of the sort.

He wants to know why the lawmakers would opt for increasing the distance. The lawmakers obviously want to avoid collisions.

Are the lawmakers nuts here or is there another reason behind this "rule of thumb"?

 

[russ_watters]

Being a bit nit picky here, but once the other car is stopped, your closing speed will decrease from 5 m/s to 0 m/s. Assuming constant decleration, your average speed in that last second is 2.5 m/s.

Oops, yeah.

However during the first second before you started braking your average rate of closure speed was also 2.5 m/s. You still need the 20 m following distance. Assuming constant rate of deceleration, the other car moves 40 m while stopping, and your car moves 20 m before you brake, then 40 m while stopping, for a total of 60 m.

Lol, yeah, my 3 math errors canceled out: the two instances of one second at 2.5 m/s and it is 5 seconds from whe other car hits the brakes until you stop!

 

[russ_watters]

He says nothing of the sort.

Meh, reading it again, it turns out he did. Frankly, it was too confusingly worded for me to get it the first time:

...and the impact be 50 kmh which is much worse.

But the logic behind it was bad:

It takes longer for a car to brake from say 200-150 kmh than from say 50-0 kmh.

That's wrong. So is the belief that the reasoning of lawmakers is to lessen the severity of collisions: the reasoning behind the law is to avoid them.

 

[DaveC426913]

All of you are missing jonnyk's main point -- he's talking about impacting the leading vehicle, not avoiding it.

He says nothing of the sort.

Meh, reading it again, it turns out he did.

OK, he sort of did.
But it was part of his assumption that the lawmakers are trying to lessen the damage from an impact. Since we've disabused him of that notion, so too have we disabused him of the need to state or defend that line of reasoning. We can presume in his absence that he will retract it.

 

[Danger]

The thing that's been bothering me since the initial post, which I expected someone else to mention, is that Jonnyk seems to expect that the leading vehicle will decelerate in a civilized manner. If it gets hit head-on by another car or truck (or even a moose if it's a small car in my region), it's stopping right now. I always leave enough space to deal with that situation.

 

[russ_watters]

The thing that's been bothering me since the initial post, which I expected someone else to mention, is that Jonnyk seems to expect that the leading vehicle will decelerate in a civilized manner. If it gets hit head-on by another car or truck (or even a moose if it's a small car in my region), it's stopping right now. I always leave enough space to deal with that situation.

That's a rediculous amount of following distance and it would surprise me greatly if you actually left that much.

 

[Lsos]

If OP is talking about lessening the impact, then obviously if you're very close then the collision should be less severe than when you're far.

In fact, if you're EXTREMELEY close, as close as the front and rear section of a car for example (which are welded together), then the impact will be practially zero no matter how hard you brake (reasonably).

 

[jonnyk]

Hi "russ"

Huh? Relative speed isn't related to folowing distance or absolute speed, it is a function of reaction time and deceleration. Apply some math:

You are following another car at any speed and distance. The car in front of you slams on the brakes and decelerates at 5m/s/s. It takes you 1 second from the time he starts to apply the brake to when you start to apply the brake. Now, he is traveling 5m/s slower than you. In other words, you are traveling toward him at 5m/s and you will keep traveling toward him at 5m/s until both of you come to a complete stop.

From here you can calculate the following distance required to avoid hitting him by multiplying by the time required for you to stop. And that's where speed comes in. For example, if you are traveling at 20 m/s it takes 4 sec to stop and therefore you must leave 20m between you to avoid hitting him.

JK- Yes and if the front guy were braking from 40 m/s using the same brake force hed be deccelrating at 2.5 m/s/s. Thus now if i also travel behind him id be approaching him with 2.5 m/s. Ofcourse my braking wld also reduce so the time to brake 2.5 m/s wld be the same as 5 m/s in the first case but shld an impact happen in both cases, the latter case with faster speed is LESS HARMFUL.

 

[jonnyk]

Hi "pantaz",

All of you are missing jonnyk's main point -- he's talking about impacting the leading vehicle, not avoiding it. So, if you want to reduce the damage caused by solely by the collision speed, then yes, following more closely would work. However, in the real world, that ~200 km/h collision would likely send both vehicles sliding and spinning into surrounding vehicles, trees, light poles, bridge abutments, and so on.

JK- Yes this is basically my point. As for the sliding issue why wld tht happen IF the vehicles behind on impact also immediately hits full brake. The both cars wld act as ONE UNIT and normally come to a stop.

 

[jonnyk]

Hi "russ watters"

That's wrong. So is the belief that the reasoning of lawmakers is to lessen the severity of collisions: the reasoning behind the law is to avoid them.

It's wrong that a vehicle take more time to brake from 200-150 kmh than from 50-0 kmh given the same braking force appleid in both cases? Tht can't be since from 200-150 kmh much more KE has to be taken away as compared to 50-0 kmh.

 

[jonnyk]

Hi "dave",

OK, he sort of did.
But it was part of his assumption that the lawmakers are trying to lessen the damage from an impact. Since we've disabused him of that notion, so too have we disabused him of the need to state or defend that line of reasoning. We ce"an presume in his absence that he will retract it.

JK- You don't even avoid the impact more if you keep a greater following diatance btw the front car if both are traveling at higher speeds.That is nonsense. The following distance should be the SAME AT ALL SPEEDS RELATIVE TO THE GROUND NOT relative speeds ofcourse. Lemme give you another example. I am following a car at 200 kmh and so is the car also traveling at 200 kmh. I keep a 100m distance btw it. NOW the front car has enough distance to brake suich that a higher relative speed btw him and me is reached. I miss his braking for some reason and collide with him at a dangerous speed. As the car rushes towards me I am so nervous i can't avoid collision here either.

 

[jonnyk]

Hi "danger"

The thing that's been bothering me since the initial post, which I expected someone else to mention, is that Jonnyk seems to expect that the leading vehicle will decelerate in a civilized manner. If it gets hit head-on by another car or truck (or even a moose if it's a small car in my region), it's stopping right now. I always leave enough space to deal with that situation.

JK- This is totally another issue. In this way i might as well have a direct impact myself. This assumes that the latter driver is less decent than myself. This is also not the main reason given by people around here that I've heard. All I've ever heard is "braking diatnce of the car increases".

 

[jonnyk]

Hi "lsos"

If OP is talking about lessening the impact, then obviously if you're very close then the collision should be less severe than when you're far.

In fact, if you're EXTREMELEY close, as close as the front and rear section of a car for example (which are welded together), then the impact will be practially zero no matter how hard you brake (reasonably).

You seem to be best getting my point. Hence youd also agree perhaps that following distance should be fixed independent of speed.

 

[A.T.]

Hi "lsos"
You seem to be best getting my point. Hence youd also agree perhaps that following distance should be fixed independent of speed.

Fixed to zero, I guess, to reduce the impact velocity onto the front car to zero. Why avoiding impact, if you can use the front car as your crumple zone!

 

[jonnyk]

Hi "A.T"

Fixed to zero, I guess, to reduce the impact velocity onto the front car to zero. Why avoiding impact, if you can use the front car as your crumple zone!

No you could have a constant following distance of 25 m all the way from 50kmh-infinity. My qs is if you say keep a following distance of 25 m if traveling behind a car also at 50 kmh why wld you INCREASE your distance by twice the amount, as German lawmakers say, between the front car as both of you now travel at 100 kmh? HOW does that help you? If anything that wld make the situation worse as I've explained. If at 25 m distance your able to react and brake asa the front car brakes and there's no collison at all WHY shld there be a collision at the same 25m following diatnce if both cars trvalled at 100kmh? SURE your brakin diatnce wld increase BUT THE BRAKING DIATNCE OF THE FRONT CAR NOW ALSO INCREASE SO IT EQUALS OUT. Got it?

 

[jonnyk]

Hi all,
In case anyone didnt understand here is a clarification again. German lawmakers say keep a following distance = (.5 X speed in kmh) meters meaning at 50 kmh you should have an fd of 25, at 100kmh an fd of 50m at 200kmh an fd of 100m and so on. This is what i called nonsense.

 

[DaveC426913]

Hi "lsos"



You seem to be best getting my point. Hence youd also agree perhaps that following distance should be fixed independent of speed.

OK, so you think that stopping distance is determined by how to lessen damage as opposed to how to avoid damage.

So that is a hypothetical argument, having nothing to do with reality then. So your question about "are lawmakers nuts?" is meaningless, since it presumes a falsehood.

Right?

 

[jonnyk]

Hi "dave",

OK, so you think that stopping distance is determined by how to lessen damage as opposed to how to avoid damage.

So that is a hypothetical argument, having nothing to do with reality then. So your question about "are lawmakers nuts?" is meaningless, since it presumes a falsehood.

Right?

German lawmakers say keep a following distance = (.5 X speed in kmh) meters, meaning at 50 kmh you should have an fd of 25, at 100kmh an fd of 50m at 200kmh an fd of 100m and so on. This is what i called nonsense basically. Also read my post previous to that and then comment please.

 

[Lsos]

Hi "lsos"
You seem to be best getting my point. Hence youd also agree perhaps that following distance should be fixed independent of speed.

I get your point, but it obviously can't work in the real world where we have human drivers with human limitations (the fore mentioned "reaction time"). Your point would work only under certain conditions...such as already existing trains, as well as the upcoming "road trains", both applications where the human factor is eliminated.

However, on current roads with current variables and human drivers, recommending a fixed following distance would be very foolish, unless you enjoy death.

http://news.bbc.co.uk/2/hi/technology/8349923.stm

 

[DaveC426913]

Hi "dave",



German lawmakers say keep a following distance = (.5 X speed in kmh) meters, meaning at 50 kmh you should have an fd of 25, at 100kmh an fd of 50m at 200kmh an fd of 100m and so on. This is what i called nonsense basically. Also read my post previous to that and then comment please.

Do you agree that the intent of the laws are to avoid accidents, and not simply reduce damage?

 

[DaveC426913]

Hi "dave",



German lawmakers say keep a following distance = (.5 X speed in kmh) meters, meaning at 50 kmh you should have an fd of 25, at 100kmh an fd of 50m at 200kmh an fd of 100m and so on. This is what i called nonsense basically. Also read my post previous to that and then comment please.

Your original post:

...the impact be 50 kmh which is much worse. Are the lawmakers nuts here or is there another reason behind this "rule of thumb"? Thanks.

Simply put: Yes, there is another reason behind the rule of thumb. The reason is that German lawmakers are trying to avoid collisions, not simply lessen damage.

This has been explained several times in the course of this thread.

By your logic, the best way to lessen damage is to reduce following distance to zero.

This too has been explained several times in the course of this thread.

 

[jonnyk]

Hi "lsos",

I get your point, but it obviously can't work in the real world where we have human drivers with human limitations (the fore mentioned "reaction time"). Your point would work only under certain conditions...such as already existing trains, as well as the upcoming "road trains", both applications where the human factor is eliminated.

However, on current roads with current variables and human drivers, recommending a fixed following distance would be very foolish, unless you enjoy death.

http://news.bbc.co.uk/2/hi/technology/8349923.stm

Ok not a fixed distance but would it make sense to increase follow distance with an increase in speed NOT TO MENTION IN PROPORTION to speed as German lawmakers suggest?

 

[jonnyk]

Hi "dave",

Do you agree that the intent of the laws are to avoid accidents, and not simply reduce damage?

But as i said keeping a greater distance btw the front car when BOTH you and the one infront are traveling faster won't be more effective in avoiding a collision either. The distance traveled within reaction time towards the back car by the front one does NOT increase if if it brakes and both travel faster BUT actually DECREASES because the front car is now slowing by the same amount of speed it wld have at lower speeds IN MORE TIME. So if anything keeping a greater distance while trvaelling at high speed is dangerous coz you might notice the car braking tooo late when it's already come 3m close to u. NOW how wld u avoid a collison. You most probanly can't n now the impact speed wld be heavy too. Got it?

 

[mgb_phys]

In the Uk you have to memorise the official stopping distances and parrot them out in your driving test (or at least you used to when I learned)
Of course nobody asked the candidate to demonstrate they have any idea what 73m is - you just have to know the figures.

They are listed as thinking distances + stopping distance. The fact that speeds are in miles but stopping distances are in metric is just one of those quaint details

Speed : thinking + stopping = total
30mph: 9m + 14m = 25m
40mph: 12m + 24m = 36m
60mph: 18m+55m = 73m
70mph: 21m + 75m = 96m

In more useful units this would be
30mph = 1.8 secs
40mph = 2.0 secs
60mph = 2.7 secs
70mph = 3.6 secs

So on a freeway you should be driving with 4secs between you and the car in front.

 

[Lsos]

Probably beside the point, but I'm not so sure a faster car decelerates any slower than a slower car. Perhaps if the brakes were inadequate, but otherwise, why would it?