# Re: Repeal of the National Speed Limit Law

```In article <3th052\$2l7@curly.cc.emory.edu>, lparker@curly.cc.emory.edu (Lloyd R.
Parker) says:
>
>So stopping distance is proportional to the square of the velocity, not
>the velocity itself.  And distance is what keeps you out of a collision
>(or gets you into one).

It's important to remember that we've been talking about relative velocities
and relative distances covered. When braking from 75 mph to 0 mph with respect
to a stationary observer, the stopping distance is pretty high. However,
when braking from a relative 10 mph to a relative 0 mph with respect to a
vehicle in front of you, the *relative distance* traveled is the same,
regardless of how fast the two vehicles are moving with respect to a
stationary observer. Or, to put it another way, if you are matching speeds
with another vehicle, you will always close the gap between the two by the
same amount, if you start at the same relative velocity and brake at the
same rate.

The reason why you should increase your following distance as you get to
higher speeds is that there is a linear increase in stopping TIME the faster
you go.

Suppose you're following a car with identical braking characteristics (I
realize that's an unwarranted assumption in the real world, but it will do
for an illustration) and the lead driver hits the brakes for a panic stop.
Assuming you're paying attention, you have a pretty much fixed reaction time,
which varies from one driver to another. In that time, assuming you were
pacing the lead car, a relative velocity builds between you and the leading
car. When you get on the brakes, there is now a relative velocity and you
are closing on the leading car by the RELATIVE VELOCITY. Once the other car
gets to 0 mph and stops decelerating, you have a chance to match velocities
again, which should take just as long as your reaction time in our simplified
example.

If the time it takes to stop with respect to the OTHER CAR is longer than the
time it takes to cover the distance between the two cars at their relative
velocities, then you have a collision.

So, since people almost NEVER leave an adequate following distance for a panic
stop, why don't we have more rear-enders? The reason is that drivers almost
never actually have to panic stop on the highway. Much more common is the case
where a driver perceives a threat, hits the brakes to slow by some amount,
say 10 or 20 mph, then either stays at that speed or accelerates again. The
drivers out there may not be leaving enough room for a 75-mph panic stop, but
they ARE leaving enough room to compensate for a 10 to 20 mph velocity change.

Michael Johnson, Relay Technology, Inc.
michael@maine.maine.edu, michaelj@relay.relay.com
"I will choose a path that's clear. I will choose Free Will." -- Neil Peart

```

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