I never have liked memorizing stuff, just for memory's sake, especially things like vehicle stopping distance. This is sort of like the continuing mistake in mathematics education, where we teach the fundamentals of algebra, but don't explain the redeeming value of the knowledge (If X, Y, and Z are from the set of real numbers, such that X + Y = Z, then Z - X always equals Y. Yes, ALWAYS ...). We require people to be able to know stopping distances for a driver's test, but then don't do much with the knowledge. It is supposed to help us evaluate the safe speed to travel under the observed circumstances, but that application is rarely explained (if a brick wall suddenly appears less than 200 feet in front of you when you are going 60 mph, you are probably gonna hit it - so either slow down or don't drive where brick walls suddenly appear). Stuff like stopping distance is just great fodder for trick questions on multiple choice tests.
Anyway, here are some easy ways to get to the numbers. The critical first important number to remember is 1.47. This is the factor for converting miles per hour to feet per second (5280 feet per mile / 3600 seconds per hour). The other important number is 30 feet per second per second (also the number of tacos Jack suggests you buy when you are hungry, which is off the subject, but makes 30 easy to remember, at least for me).
Showing posts with label Safety. Show all posts
Showing posts with label Safety. Show all posts
7/23/2009
Reaction Distance
The old rule of thumb is to take the speed you are traveling, divide it by 10, and add the result to the speed, to give the reaction distance. For example, if traveling 30 miles per hour, the reaction distance is 3 + 30 = 33 feet. At 60 miles per hour, reaction distance is 66 feet.
This comes from an assumption that the typical reaction time – from the time you realize that stopping is necessary until the brake pedal is fully depressed – is 0.75 seconds. At 60 miles per hour, the vehicle is moving at 88.2 feet per second (60 x 1.47), so the reaction distance is 66 feet (88.2 x 0.75).
This comes from an assumption that the typical reaction time – from the time you realize that stopping is necessary until the brake pedal is fully depressed – is 0.75 seconds. At 60 miles per hour, the vehicle is moving at 88.2 feet per second (60 x 1.47), so the reaction distance is 66 feet (88.2 x 0.75).
Braking Distance
This is a little more complex. There is calculus involved in actually developing some of the necessary data, so approximations have to be used, at least by some of us. Real braking distances depend on the size and weight of the vehicle, the type of road surface, tire and vehicle condition, whether the road is wet or dry, whether the roadway is flat or not, whether the vehicle is fitted with anti-lock brakes or not, and the skill of the driver. "Average" stopping distances are often reported for a typical passenger car or small truck on dry flat pavement.
There must first be an assumption for the deceleration rate that the vehicle and driver are able to achieve. If you watch a car show like Motor Week on PBS, they routinely test braking distance and it is common to see about 130 feet necessary to stop a vehicle traveling at 60 mph. If you back calculate the deceleration rate indicated by this distance, it is almost 30 feet per second per second (29.9 fpsps), on dry flat pavement in a new vehicle with a professional driver and typically with ABS. This is best case, big time.
So, to calculate the braking distance from initial speed, the time to reach a complete stop is the initial speed divided by the deceleration rate. 60 mph x 1.47 equals 88.2 fps, and that speed reduced at the rate of 29.9 fpsps requires 2.95 seconds. A simple estimate of the average speed during braking is half the initial speed. So, if braking begins at 60 mph, the average speed across the braking event is 30 mph. 30 mph x 1.47 is 44.1 feet per second, and 44.1 fps x 2.95 seconds results in the vehicle travelling 130 feet to a full stop. See, it works.
There must first be an assumption for the deceleration rate that the vehicle and driver are able to achieve. If you watch a car show like Motor Week on PBS, they routinely test braking distance and it is common to see about 130 feet necessary to stop a vehicle traveling at 60 mph. If you back calculate the deceleration rate indicated by this distance, it is almost 30 feet per second per second (29.9 fpsps), on dry flat pavement in a new vehicle with a professional driver and typically with ABS. This is best case, big time.
So, to calculate the braking distance from initial speed, the time to reach a complete stop is the initial speed divided by the deceleration rate. 60 mph x 1.47 equals 88.2 fps, and that speed reduced at the rate of 29.9 fpsps requires 2.95 seconds. A simple estimate of the average speed during braking is half the initial speed. So, if braking begins at 60 mph, the average speed across the braking event is 30 mph. 30 mph x 1.47 is 44.1 feet per second, and 44.1 fps x 2.95 seconds results in the vehicle travelling 130 feet to a full stop. See, it works.
Stopping Distance: Summary
Total stopping distance is reaction distance plus braking distance. So, from 60 mph, reaction takes 66 feet and braking takes 130 feet, for a total of 196 feet. Total elapsed time is 3.7 seconds (0.75 seconds reaction, plus 2.95 seconds to brake).
The calculations reveal why attentiveness, physical capability, and skill are so important to safe driving. Reaction time can easily double from 0.75 seconds to 1.5 seconds (neither one is very long), with just the slightest distraction, adding another 66 feet to total stopping distance. The deceleration rate depends on the skill of the driver to apply maximum pressure on the brake pedal without skidding and maintaining it throughout the stop (or properly using ABS brakes), and also on rarely ideal conditions, recognizing that few circumstances other than driving uphill make braking distance shorter than the best case. If the deceleration rate is 20 feet per second per second rather than 30, the stopping distance increases from 130 to 195 feet. The problem with ABS brakes is that we often release pressure on the pedal when it begins to pulsate, negating the benefits of the anti-lock function - the rule is hit 'em hard and stay on 'em.
If you see differences in reported stopping distances, those typically result from differences in the assumed reaction time and the deceleration rate that is used in the calculations, since the other components of the calculations are constants or reasonable approximations. There seems to be better general agreement on the reaction time than the deceleration rate.
Or, instead of all this you can just memorize that 60 mph's stopping distance is 200 feet. It's more if you are going faster and less if you are going slower. Close enough for a multiple choice test, anyway.
The calculations reveal why attentiveness, physical capability, and skill are so important to safe driving. Reaction time can easily double from 0.75 seconds to 1.5 seconds (neither one is very long), with just the slightest distraction, adding another 66 feet to total stopping distance. The deceleration rate depends on the skill of the driver to apply maximum pressure on the brake pedal without skidding and maintaining it throughout the stop (or properly using ABS brakes), and also on rarely ideal conditions, recognizing that few circumstances other than driving uphill make braking distance shorter than the best case. If the deceleration rate is 20 feet per second per second rather than 30, the stopping distance increases from 130 to 195 feet. The problem with ABS brakes is that we often release pressure on the pedal when it begins to pulsate, negating the benefits of the anti-lock function - the rule is hit 'em hard and stay on 'em.
If you see differences in reported stopping distances, those typically result from differences in the assumed reaction time and the deceleration rate that is used in the calculations, since the other components of the calculations are constants or reasonable approximations. There seems to be better general agreement on the reaction time than the deceleration rate.
Or, instead of all this you can just memorize that 60 mph's stopping distance is 200 feet. It's more if you are going faster and less if you are going slower. Close enough for a multiple choice test, anyway.
2/09/2009
THINK
One of the best safety initiatives I can remember was a very simple one. It consisted of green stickers with white letters on them, with only one word on them: THINK. These stickers were everywhere. On vehicle dashboards, doors, file cabinets, over shop doors, on bulletin boards. Clear reminders that we all need to use our heads, before we do anything.
This was a program that was used 20-25 years ago in a small operating organization. What they didn't know is that they had stumbled very wisely into the essence of behavioral safety management, long before the consultants had grubbed onto the concept. In safety, we can try to teach people to follow procedures and rules, and we won't be any more successful at it than Moses was back in his day. But, when we help people know how to make good decisions and guide their own actions - through THINKing - then, accidents, injuries, and unwanted circumstances begin to truly depart the organization.
It's a good memory.
This was a program that was used 20-25 years ago in a small operating organization. What they didn't know is that they had stumbled very wisely into the essence of behavioral safety management, long before the consultants had grubbed onto the concept. In safety, we can try to teach people to follow procedures and rules, and we won't be any more successful at it than Moses was back in his day. But, when we help people know how to make good decisions and guide their own actions - through THINKing - then, accidents, injuries, and unwanted circumstances begin to truly depart the organization.
It's a good memory.
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