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What sense does it make to increase follow distance btw front car as speed increases?

  1. Nov 9, 2009 #1
    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 doesnt seem to make any sense whatsoever. Ive 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. Infact 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.
     
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  3. Nov 9, 2009 #2

    Danger

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    Following this logic, it would be totally unsafe to travel on an unoccupied road. :confused:
     
  4. Nov 9, 2009 #3

    rcgldr

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  5. Nov 9, 2009 #4
    Re: What sense does it make to increase follow distance btw front car as speed increa

    Hi "jeff",

    JK- The relative speed btw front car and the one behind travelling at 200 kmh when the front one brakes is actually LESS than it would be if the cars were travelling 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.
     
  6. Nov 9, 2009 #5
    Re: What sense does it make to increase follow distance btw front car as speed increa

    Hi "danger",

    I think i explained this in detail. What i meant is following a car at close distance WHICH IS ALSO travelling at 200kmh so both te car behind and infront are travelling 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 travelled at 50 kmh at the same distance and the one infront suddenly hit the brakes. Got it?
     
  7. Nov 9, 2009 #6
    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  8. Nov 9, 2009 #7

    russ_watters

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  9. Nov 9, 2009 #8

    DaveC426913

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     

    Attached Files:

    Last edited: Nov 9, 2009
  10. Nov 9, 2009 #9

    rcgldr

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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, 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).
     
    Last edited: Nov 10, 2009
  11. Nov 9, 2009 #10
    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  12. Nov 9, 2009 #11

    DaveC426913

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.


     
  13. Nov 9, 2009 #12

    russ_watters

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    Oops, yeah.
    Lol, yeah, my 3 math errors cancelled 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!
     
  14. Nov 9, 2009 #13

    russ_watters

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    Meh, reading it again, it turns out he did. Frankly, it was too confusingly worded for me to get it the first time:
    But the logic behind it was bad:
    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.
     
  15. Nov 9, 2009 #14

    DaveC426913

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  16. Nov 10, 2009 #15

    Danger

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    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.
     
  17. Nov 10, 2009 #16

    russ_watters

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    Re: What sense does it make to increase follow distance btw front car as speed increa

    That's a rediculous amount of following distance and it would surprise me greatly if you actually left that much.
     
  18. Nov 10, 2009 #17
    Re: What sense does it make to increase follow distance btw front car as speed increa

    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).
     
  19. Nov 10, 2009 #18
    Re: What sense does it make to increase follow distance btw front car as speed increa

    Hi "russ"

    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.
     
  20. Nov 10, 2009 #19
    Re: What sense does it make to increase follow distance btw front car as speed increa

    Hi "pantaz",

    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.
     
  21. Nov 10, 2009 #20
    Re: What sense does it make to increase follow distance btw front car as speed increa

    Hi "russ watters"

    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 cant be since from 200-150 kmh much more KE has to be taken away as compared to 50-0 kmh.
     
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