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Re: Driving Pet peeves
Alain still doesn't get it. He whined to me privately about how "that's
physics, I'm talking about the real world" and went on about a bunch of
irrelevancies, such as:
- Different cars have different braking abilities (for lots of reasons)
- Different drivers have different reaction times
Never mind that physics is the most accurate way we have of modeling the
real world, neither of those points changes the fact that:
For a given pair of cars and drivers (i.e. you and that moron on your butt),
you speeding up or slowing down is not going to reduce the chances of a
collision, if the tailgater sticks to you. Note that we're talking about a
specific tailgater here. If a different tailgater gets on you, the numbers
change, but the basic principle does not. Here's why.
- The tailgater pretty much has a consistent reaction time, on the order of
less than a second generally. Let's say it's 0.5 sec, a convenient number
for doing the math. It might be more or less, it doesn't really matter.
- If you hit the brakes, the tailgater will always have the same velocity
relative to your vehicle when THEY START BRAKING, because you will be
decelerating at a constant rate (say 32 ft/sec/sec) for the period of that
reaction time. So let's say that by the time they get on the brakes, there
is a relative velocity of 16 ft/sec between the two vehicles. This will be
true whether you were going 90mph before you hit the brakes or 50 mph. You
are not going to come to a stop before the tailgater can get on the brakes,
if you are traveling at highway speeds.
- If the tailgater is really close, they are going to hit you before they even
have a chance to get on the brakes. Again, your initial velocity does not
change this. This might even be the most likely outcome, if they're three
feet off your bumper.
- If the tailgater doesn't hit you before they get on the brakes, then for a
given pair of cars both braking hard, there is going to be a fixed amount of
time (given a fixed initial following distance) before the trailing car will
either match velocity with the leading car or hit it. The initial velocity
does not matter. What matters is the relative velocity of the two cars, and
if they brake at different rates, the relative deceleration of the two cars.
The closer the tailgater is, the smaller this time slice will be.
If you are moving really slowly, you might manage to stop before this time
elapses. Alternatively, you might take your foot off the brake and resume a
constant speed. This is probably the best thing (other than hitting the gas)
that you could do to avoid a collision at this point, because it gives the
trailing car a better chance to match your speed before it hits you.
Please note that I'm not saying that slowing down is a bad idea. There are
reasons why it is a good idea. However, the idea that slowing down is going
to reduce the chance of a collision, if you have to suddenly brake hard, is
a fallacy. The only way this would be true is if you slowed down so much that
the tailgater was perforce maintaining a safe following distance. Since they
are probably going to come even closer to you as you slow down, you'd probably
have to come almost to a dead stop. And because they are coming closer to
you, you are actually INCREASING the chance that they will hit you if you
have to brake hard.
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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