A well intentioned friend sent the following “helpful” e-mail. This is one of my pet peeves. Not that it was sent to me, but that the original author and apparently some of the people involved in the story are guilty as victims of bad science. My rebuttal/analysis follows.

> Read this, this could save your life!!!!


> A 36 year old female had an accident several weeks ago and totaled

> her car. A resident of Kilgore, Texas, she was traveling between Gladewater

> & Kilgore. It was raining, though not excessive, when her car suddenly began

> to hydroplane and literally flew through the air.


> She was not seriously injured but very stunned at the sudden

> occurrence!


> When she explained to the highway patrolman what had happened he

> told her something that every driver should know -NEVER DRIVE IN THE RAIN

> WITH YOUR CRUISE CONTROL ON. She had thought she was being cautious by

> setting the cruise control and maintaining a safe consistent speed in the rain.

> But the highway patrolman told her that if the cruise control is on and your car

> begins to hydroplaned – when your tires loose contact with the pavement

> – your car will accelerate to a higher rate of speed and you take off

> like an airplane. She told the patrolman that was exactly what had occurred.


> We all know you have little or no control over a car when it begins

> to hydroplane. You are at the mercy of the Good Lord. The highway patrol

> estimated her car was actually traveling through the air at 10 to 15 miles

> per hour faster than the speed set on the cruise control.


> The patrolman said this warning should be listed, on the drivers


> ICY along with the airbag warning.


> We tell our teenagers to set the cruise control and drive a safe speed,

> but we don’t tell them to use the cruise control only when the pavement is dry.


> The only person the accident victim found, who knew this (besides

> the patrolman), was a man who had had a similar accident, totaled his car and

> sustained severe injuries.


> If you send this to 15 people and only one of them doesn’t know

> about this, then it was all worth it. You might have saved a life.


While not using cruise control under adverse weather conditions is certainly good advice… to be true, this story requires cars which don’t exist and/or laws of physics not found outside the Cartoon Network.

To my knowledge, no stock model car on the road has any sensor that tells you how fast you are actually going relative to the world around you. The car (and by implication, the cruise control) only knows some rotational velocity (typically the engine, drive shaft, or wheels) which it converts to the speed of the car. This conversion assumes the tires are in contact with the road, the clutch (or torque converter) is locked, etc. Cruise control’s entire aim is to keep that rotational velocity constant. To simplify it, cruise control only tries to keep the wheels spinning at a constant speed.

So let’s assume you’re running along at this constant speed and hit a slippery spot. By definition, this slippery spot is a point where your tires lose traction with the road. Without traction there is no way to transfer the rotational power of the wheels into vehicular velocity. What with Earth having an atmosphere and the consequent drag it produces, at the point your wheels start to slip, you actually begin to SLOW DOWN.

And what’s the cruise control doing during all this? Well, it’s just happily humming the wheels along at the speed it was set at. It would never attempt to accelerate the car (that’s not it’s job), and would actually “let off on the gas” because the wheels are no longer under much of a load. It probably “thinks” you’re going downhill.

Granted, given that your wheels are now slipping, the cruise control will exacerbate the slip because it will keep spinning the wheels so that you can’t get your traction back. This will result in a loss of control (both car and bladder) and will often create the sensation of acceleration or flight – but it’s just fear you’re feeling. This is why cruise control on bad roads or in bad weather is a bad idea. But if this car actually zoomed faster and took off like an airplane it was because she pushed the button to ignite the after market JATO (jet assisted take off) packs her husband had installed.

If the car was equipped with traction control, the computer would have kicked out the cruise control and brought the wheels back into control. Anti-lock brakes are not a factor unless you step on the pedal. Apparently she didn’t as that would have disengaged the cruise control and “take-off” would have been aborted.

I must conclude that in an attempt to impress the cleavage laden accident victim, Barney forgot that the badge on his chest didn’t qualify him as an authority on driving, cars, physics, engineering, or apparently even common sense.

Send THIS to 15 people!! Tell them to encourage their kids to take science in school. As for cruise control, I would never tell a kid or other inexperienced driver to use it – ever. It slows your reaction time to any situation. If you can’t drive without it, you shouldn’t be using it. It’s not a crutch.

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