Neil's News

DMV Written Test

8 June 2011

Being more than the twice the age that most people get their driver's license, I figured it was time to start the process of getting mine. First step was inhaling the 100 page California Driver Handbook and passing the written test.

Most of the contents is common sense stuff. E.g. "Do not shoot firearms on a highway or at traffic signs."

Some of the information doesn't even pass the straight-face test. "In the United States, a vehicle is stolen an average of every 21 minutes." That means just 68 stolen vehicles per day. A quick check of the FBI's website returns an estimate that's 48 times greater. The driver's handbook probably meant to say "every 21 seconds" (2,410 results on Google) as opposed to "every 21 minutes" (1,860 results on Google).

More perplexing are passages such as this one: "When driving within 500 to 1,000 feet of a school while children are outside or crossing the street, the speed limit is 25 mph unless otherwise posted."

Read literally that means that at 1,000 feet from a school one has to drop to 25 mph, then 500 feet later one can accelerate back up to full speed as one passes the school, then one has to drop back to 25 mph for another 500 feet once one passes through the other side of the ring. Since this doesn't make a lot of sense, we can check out the actual law: CVC 22358.4. It turns out that the slowdown is only "when approaching". What a strange law, it does indeed appear to allow driving by a school at full speed.

Finally, here's another statement that caught my eye: "The force of a 60 mph crash is not just twice as great as a 30 mph crash, it's four times as great!"

Let's examine this one a bit more closely. First, Newton's second law states that Force = mass x acceleration. Mass is constant thus the claim simplifies to a claim of a quadrupling of deceleration. [Yes, I know mass is not constant. The faster car does weigh more than the slower car thanks to Einstein's Special Relativity. But if you are going so fast that red traffic lights blue-shift to green, you've got other problems.] Solving for the deceleration of a 30 mph and 60 mph crash is a function of the size of the car's crumple zones. Assuming one foot in both cases, that gives a deceleration of 30 G for a 45 millisecond crash at 30 mph and 120 G for a 23 millisecond crash at 60 mph. Yes, that's exactly a quadrupling of deceleration given a doubling of speed.

However the premise is incorrect. The above supposes that the crumpling done to the car is the same in both cases. Here's a photo of a car crash at 35 mph. As can be seen, there's still plenty of car left to crumple. Double the energy and there will certainly be more damage. Assuming that twice as much deformation occurs, then one gets a deceleration of 60 G for a 45 millisecond crash at 60 mph.

In reality one wouldn't get twice the deformation since the next stage of destruction (pushing the engine block into the driver's lap) is more work than folding up the hood. So the net force will rise non-linearly. But it won't reach the 4x stated unless both the car and the obstacle it hits are made of unobtainium.

My apologies for all the icky Imperial measurements. I feel like a different Neil Fraser.

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