# Re: Repeal of the National Speed Limit Law

```michaelj@relay.relay.com wrote:

: >}The average car's brakes are far less effective
: >}at stopping from 75 than at 55.

: If by this you mean that it takes longer to stop from
: 75 than from 55, then that is correct. Isaac Newton
: explained it rather thoroughly. On the other hand, if
: by this you mean that it takes longer to slow down by
: a fixed amount, say 10 mph, then you are incorrect. It
: takes the same amount of time to slow from 75 to 65 as
: it does to slow from 55 to 45.

Um,  I don't think so.  Since slowing the car down involves
dissapating the kinetic energy of the car, and KE = 1/2 mv^2.
That is why stopping distances increase exponentially, not linearly
with speed.

So:  75*75 - 65*65 = 1400
55*55 - 45*45 = 1000

Ignoring the 1/2 and assuming the mass stays constant:

(1400-1000)/1000 = .4 ->  40%

Therefore it you have to dissapate 40% more energy to slow
from 75 to 65 than from 55 to 45.  And assuming that you
have the same constant energy removal rate (no brake fade)
in both cases, you would also travel more than 40% farther since
your average speed is higher.  Since brake fade is a real
problem in most cars, that will only make things worse.

To test this, go out in your driveway and go from 10mph to a
dead stop.  Pretty quick, huh ??  Now go out on the open road
and slow down from 70 to 60.  Took a more time, didn't it ??
Also, check out how much farther you travelled while you were
slowing down.

Not meant as a flame or personal attack, but stuff like this
is one of the best arguments *not* to raise the speeed limit.
There are too many people who have no idea of how to deal with
the incerased stopping distances and how the car is going to
behave.  I don't really think that the majority of drivers are
stupid, but they are certainly woefully undertrained and
undereducated.  Hell, half of them don't really seem to be
able to cope with 55 on a daily basis.

Oh well,
Jeff	jet@sun.soe.clarkson.edu

: to happen than having to come to a full stop on the freeway. Vehicles in front
: of you do NOT suddenly come to a complete stop instantaneously. More likely,
: they will apply the brakes to avoid some hazard, then resume their previous
: speed. That only requires that you slow by as much as they did. How fast you
: are going has no effect on how long it takes you to do so.

: Michael Johnson
: Relay Technology, Inc.

```

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