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Saturday, May 15, 2010

Aggressive Driving: Physics of a Car Crash

In your dreams, you may be the greatest, most-skilled, driver on the face of the earth --- with instantaneous, robot-like reflexes to match and able to decide lightning fast whether a car should stop, slide or turn in any direction. In reality, the laws of physics dictate what will happen to aggressive drivers and their cars in a car crash.

Unfortunately, your competence as a driver in your dreams, does not transfer over into reality. You are only human, with a limited set of speeds (given enough training) and your car is a car: a heavy piece of machinery that is bound by the laws of science.

Do The Math

Imagine that an aggressive driver is traveling at 40mph (in a 30mph zone!) --- he runs a red light (which, assuming he survives, he will claim was yellow) while another driver, crossing with the right-of-way, accelerates to the speed limit.

The aggressive driver, running at 40mph is covering 58.8 feet per second. The right-of-way driver going 30mph (the speed limit) is covering 44 feet per second. Combined, they are approaching each other at a rate of 70mph, or 102.9 feet/second.

Human Reaction Time

Assuming each is an average driver, --- not the one with the robot-like reflexes mentioned above --- your reaction time is about ¾ of a second. That means that, when each car comes into the view of the other, it will take about ¾ of a second before each driver is even aware there is an emergency. In that time, the aggressive driver will travel an additional 44.1 feet and the other driver will travel an additional 33 feet before fully realizing there is a problem. Then each driver will have to assess the driving scene and make a decision on how to best avoid the collision either by braking or steering out of the way. Then they will have to put that plan into action. Assuming that the aggressive driver decides to slam on the brakes, it will take up to 1.5 additional seconds to decide what to do and fully apply the brakes. That means he will travel another 73.5 feet before his car even begins to slow.

There isn’t enough time! They are going to crash into each other!

What laws of physics will apply as they crash into one another?

Newton's Law of Motion

The first law of motion is: an object in motion stays in motion. To put it simply: a 3,000 pound car traveling at a 40mph CANNOT just stop abruptly. It will take 120 feet before the speeding behemoth comes to a complete stop. Imagine if both drivers try to stop --- and all they get is 10 mph lower than their current speeds --- lowering their approach rate to 50 mph just before the cars crash into each other.

Using an online crash calculator (http://hyperphysics.phy-astr.gsu.edu/Hbase/carcr.html#cc2 --- yes, I cheated a bit!) and assuming that both cars each weigh 3000 pounds, the force of the collision would look like this:


  • Weight – 6000 lbs

  • Speed – 50 mph

  • Crash Force – 501,779 lbs / 250 tons



I probably wouldn't call that crash a fender-bender. What's far scarier --- assume that one of the passengers is a 100-pound girl who decided that day to be “cool” and not wear a seatbelt. The force of her body flying into the dashboard is going to look like this:


  • Weight – 100 lbs.

  • Speed – 30 mph

  • Crash Force – 3,010 lbs / 1.5 tons



A force that great is going to be fatal to her.

However you vary the speed, driving skills and car weights --- there's no denying that the laws of physics will be the one to follow in a car crash. Aggressive drivers aren't superhuman --- they are bound to the same laws of physics rules as the rest of us --- they are just a bit more impatient. The faster an aggressive driver goes, the faster he gets into inescapable jams --- the faster he also gets to the hospital.

Learn more about how to prevent aggressive driving through a 1-hour Dangers of Aggressive Driving course offered 100% online.

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Posted by NSC - Traffic Safety at 2 Comments Links to this post

2 Comments:

Anonymous Crystal Jamison said...

Crash force is something that I have never heard of. There are SO MANY factors in an accident , speed, weight, human reaction time, the laws of physics. Now all of this factor into the "Crash Force". We all need to SLOW down!

June 11, 2010 at 12:59 PM 
Anonymous Christina said...

A car is a dangerous thing, and knowing the science behind it makes me want to drive just a wee bit safer.

July 1, 2010 at 7:10 AM 

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