It’s just common sense. The faster you go, the longer it takes you to stop.
BUT, how much longer? This week, we’re going to look at the relationship between stopping distance and speed, and then throw in the human factor.
I’ve investigated hundreds of collisions that occurred only because the driver was simply going too fast. They didn’t understand the relationship between speed and how long it takes to stop. Then, for those of us that tow an RV or drive a heavy class A or C, you have to throw in the added kinetic energy that those vehicle/RV combinations have.
WARNING: CRASH GEEK CONTENT BELOW!
I’ll confess that I’m a metrican, so the following calculations will be in metric.
Some laboratory type examples:
- Car traveling 50Km/h (~31mph) will take about 14.6m to stop in dry pavement. This assumes all brakes are working.
- Same car traveling 70Km/h (~43mph) will take 27.5m to stop (almost exactly double!)
Who would have thought that an increase of 20Km/h (12mph) would have doubled the stopping distance? For those that want to know, the formula is:
d = s²/254µ
d = distance in meters
s = speed in Km/h
µ = coefficent of friction of roadway
Now, the human factor. Based on research done by a guy named Paul Olson, the 85th percentile (85/100 people) perception/reaction (P/R) time for a driver that is faced with an unexpected roadway hazard is 1.5 seconds. Interestingly the 95th percentile number is a longer 1.6 seconds.
“Now, HOLD ON A MINUTE! You’re trying to tell us that it takes a second a half for a driver to react to a hazard? I can do it faster than that!”
A little further explanation may be necessary here. Say I told you that I was conducting an experiment to measure your reaction time. I’ll set up a light bulb and a device like a “Jeopardy buzzer”. When the light comes on, hit the button, and I’ll measure your reaction time. Think you could do it faster than a second and a half? Of course you could; and here’s why:
- I told you I was running an experiment to check reaction time — you’re ready
- I told you what the stimulus was going to be (the light-bulb coming on)
- I dictated your response (hit the button)
All of those things will serve to reduce the amount of time it takes you to react. Driving isn’t like that. There is a myriad of possible responses to a roadway hazard. You can brake, steer, honk the horn, flash you lights, something else, or a combination of those possibilities. Then, once you decide what to do, you have to execute that decision — and this all happens unexpectedly.
During the P/R phase of a pre-collision event, the vehicle is usually traveling at what is assumed to be a constant speed. Now you take that 1.5 seconds, and convert that 50Km/h speed into meters/second (50Km/h = 13.88m/s or around 45 feet/sec), and you see that while the driver is deciding what to do and doing it, the vehicle will travel (13.88 x 1.5 = 20.8m). Then add the 14.6m it takes to stop from that speed, and you see that the total stopping distance is 14.6 + 20.8 = 35.4m, or over a hundred feet. This is why, especially in urban areas, drivers need to pay attention to their driving and obey speed limits. Distances to crosswalks from their “crosswalk ahead” signs, proximity of crosswalks to curves, radius of roadway curves to be negotiated in all kinds of weather, and speed limits are all factored using this type of logic by the roadway engineers.
Where I live, the speed limits in School Zones is 40Km/h (25mph) instead of the usual 50km/h (31mph). Is that 10Km/h (6 mph) really going to make that much difference if the worst happens and a child is struck? The answer is YES. Research says that if a pedestrian is struck at 60Km/h (35mph), there is roughly a 85% mortality rate. Decrease that speed to 40Km/h (25mph), and the mortality rate drops to about 15%. I won’t go into anatomy and why this is so, but I’ll simply say that it based on the ability of the body to protect itself from injury. The higher speeds create forces of injury the body simply cannot survive. Aortas tear, and brain injury is common.
Speed limits are in place for a reason, and it’s best to abide by them. Speed isn’t the most common cause of crashes, inattention is — at least based on my experience. But, if you are unfortunate enough to be in a collision, the faster you are going, the more severe the impact is going to be – for you and the other person. Slow down, pay attention, and enjoy the ride.
Next week, I’ll be talking about safety innovations is vehicles. Which ones should you spend your money on, and which may not be the best “bang for the buck”. See you then!
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