Re: Repeal of the National Speed Limit Law
Andrew Gough (firstname.lastname@example.org) wrote:
: In article <email@example.com> firstname.lastname@example.org (Jeff Thompson) writes:
: > 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.
: > 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.
:Except you also need to consider that wind resistance is proportional to v^3.
:So one gets substantial help from wind resistance when slowing from 75 to 65
:mph. I would attempt to quantify this like you did, but that would require
:solving a differential equation, which I've refused to solve even since I was
:scared silly as an undergraduate when I solved on and got a Bessel function.
I believe that the wind resistance is higher, but I remain skeptical
about how much of an impact is has. Wind resistance is roughly 2.5
times higher at 75mph than at 55mph, but I'm not sure how much overall
impact it has. I know that it takes longer for my car to coast down
from 75-65 than if I hammer on the brakes. I admit that I left it out,
but I'm not going to do the differential equation either ;)
:I would postulate that people learn empirically how their car behaves in
:braking from various speeds by trying it. After a while, they get a "feel"
:for how their car behaves in most situations.
But if you raise the limit, there are going to be a lot of
experimenters out there. All on the same road together. Maybe
it will be OK. I seem to be more optomistic today. :)