I’m not usually a scaredy cat, but I have to admit yesterday while slamming through heavy rains on the highway, I did have the chance to do some mental calculations. So why is it so bad to do the speed limit (65-70 mph) while there are heavy rain? By heavy, I mean rain so think that you can barely see the headlights of a car 2 seconds ahead of you. 
Well there is some math to it, but the short of it, is that there were probably 1 in 10 chances of not making it there to the desitnation. Which is why I decided to eat those Doritos on Thanksgiving Day because I was going to enjoy all the time I had left (we made it fine BTW, but as you will see below that doesn’t prove that much).
Here’s the analysis:
1. Stopping distances. There are a couple of components, but the first thing is how to measure distances. I do this by counting off seconds between the car ahead of you passing something and you passing. Most folks mentally allow about two seconds of distrance. But here is the math…

Perception Distance. This is the distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it. The perception time for an alert driver is about 3/4 second. At 55 mph, you travel 60 feet in 3/4 second or about 81 feet per second.

Reaction Distance. The distance traveled from the time your brain tells your foot to move from the accelerator until your foot is actually pushing the brake pedal. The average driver has a reaction time of 3/4 second. This accounts for an additional 60 feet traveled at 55 mph.

Braking Distance. The distance it takes to stop once the brakes are put on. At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 390 feet to stop. It takes about 4 1/2 seconds.

Total Braking Distance. At 55 mph, it will take about six seconds to stop and your vehicle will travel about 450 feet.


So even on dry pavement with perfect visibility, it takes six seconds to really stop. Now the reason there aren’t more accidents of course is that the car you are about to rear end also has its stopping distance, so you have much more than six seconds as both of you are slowing. But its good to know that if you don’t see a stationary object (like a stalled car), you need six seconds to see it in the best conditions at 55 mph. 

2. Hydroplaning. This is the effect where in heavy rains, the tires actually lose contact with the pavement. Many people think that this is just a temporary condition as when you hit a puddle, but when you brake hard, you can find out that what is fine when you are gliding along really doesn’t work when you try to stop. Dynamic hydroplaning is the worst, but this happens at high speed (72 knots for aircraft) where the tire can’t break through and there is a thin layer. That’s hard to measure the effect, but call it an increase of at least. It is hard to estimate but most folks think the coefficient of friction drops by half to 1/10 compared with dry. So in the example above stopping time goes from 4.5 second to 9 to 45 seconds. Take the best case and you need something like 12 seconds to stop which is a long way. The scary part is that on the wet, if you lock your brakes then you actually take longer to stop than if the wheels are turning.

3. Energy. The energy of a collision increases by the square of velocity, so that means a collision that’s a fender bender at 5 mph, becomes quite a wreck if you hit at 20mph. In fact this is a 4x increase in speed and more like 16x increase in energy. So even a small increase in relative speed dramatically increases damage. That means that if you stop from say 70mph and the actually collision is a 15mph, then if it is raining and the collision is at 30mph, you get a mountain more damage. There are many variables, but in a controlled NTSC test.

4. Deceleration. And of course the question is how fast is the energy dissipated. In a typical chain reaction collision, the key is how quickly the cars stop. Do they hit and then roll and hit a concrete object. The actual force is equal to the square of the deceleration. So if you hit say a concrete barrier at 10 mph vs. 30 mph and go to a complete stop in the same number of milliseconds, the actual force increase by 9X. Put another way. Going from 60mph to zero in four feet (the crush space of a car) generates 30Gs.

5. Safety system failure. The safety systems in a car (air bags, crush space, seat belts) all have a limit that they can protect you against. The standard is 60G. You routinely get 50G what happens at 30mph into a concrete wall because some parts of your body (like your head) aren’t restrained until they hit the air bag.

6. Statistics. They say there are old pilots and bold pilots but no old, bold pilots. Thsi is because if you keep trying low probability catastrophic events, eventually, it will get to you. So for instance, if you travel in heavy rains routinely and say there is a 2% chance of accident each time, by the time you’ve made 30 round trips, you have a 70% chance of being in an accidnet. (This is because the probability of *not* being in an accident is 98% and so after the second trip, the probalbiy of no accident is 98% * 98%, then 98% * 98% * 98% and by the 30th trip, 98% to the 30th power is 0.7!). Obviously, the probabilities of an accident are low, but if you keep trying, they come up rapidaly.

So, to answer the question, there is much to think about when you are at the 65 mph speed limit on a rainy day and can’t see 2 secondsin front of you where a 10 car pile up may be ahead of you as an immovable barrier. Which is a long way of saying, this is why expert recommend slowing down by 30% (this reduces your energy in a collision by 50%) when you are in heavy rain conditions….

I’m Rich & Co.

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