Saber
December 31st, 2007, 12:38 AM
Part Two - Perception Reaction
What is perception reaction? ~ The time it takes a person to perceive a hazard (or threat) and formulate then implement a course of action.
Basically in vehicle crashes this amounts to jamming on the brakes or attempting object avoidance (steering input).
The most common reaction to a roadway hazard upon very short notice is braking. One mistake often made during heavy braking is attempting to steer. A locked tire will not steer, so if you have you brakes locked up and are in a slide, you are going to go straight regardless of what you do with the steering (good ole inertia and Newton’s First Law). The invention of ABS relived this problem with static breaking. The tire never stays locked so during the intermittent braking the tire will steer. ABS creates a static tire patch on the roadway verse a dynamic patch which is sliding on the surface with locked conventional brakes.
So how does perception reaction play into a crash? It adds to the distance you will travel before you can stop. The average perception reaction time is 1.5 seconds. So going back to our velocity conversion formula from Part 1, at 55 mph you will travel 120.94 feet during an average perception reaction time. D = (S)(1.466)(T) ~ Distance is equal to Speed times (1.466) times time.
Let’s go back to Part 1, if you going 55 mph you are traveling at 80.63 fps/s and then we were distracted for 3 seconds so according to Newton we traveled 241.89 feet in those 3 seconds. Now we have redirected our attention back to the roadway and now see a hazard in the roadway, let say a deer. It will take us ‘on average’ a total of 1.5 seconds to perceive this and decide on a course of action and then implement it. So let’s say we jump on the brakes. From first taking our eyes off the road to our initial braking we have already traveled 362.83 feet and haven’t even started slowing down yet.
This brings us to stopping in time before our collision. Back to Newton’s First Law, once we are in motion we will stay in motion unless acted up by an external force. Now you’re thinking if I let off my gas I will come to a stop, eventually? Right? The answer is yes, you will come to a stop eventually due to gravity. An object in motion has Inertia and also has a certain amount of Kinetic Energy. Before an object (our vehicle) can come to a stop it has to get rid of all that energy. Now energy cannot be created or destroyed, it has to be transferred.
Which takes us into Part 3: Coefficient Of Friction (and how it pertains to stopping)
***One final point on perception reaction times; there have been several studies done to determine this time. Remember it is an average time not a specific time. Some people are faster while others are slower. The actually study that was performed came up with an average time of 1.6 seconds but was rounded to 1.5 seconds for ease of use in formulas. Lowering it to 1.5 actually gave the driver a benefit. Other things affect ones reaction time including but not limited to illness, fatigue, sleep deprivation and drugs/alcohol. Every calculation in reconstruction is truncated down to 2 decimal points. Speed calculations are always rounded ‘down’ to the nearest mile per hour. When testifying in a criminal case a police Reconstructionist can always say they gave the driver every benefit in his or her calculations. There are very few ‘actual’ speed formulas with most being minimum speed determinations and everything should be given with a range and not an exact speed.***
What is perception reaction? ~ The time it takes a person to perceive a hazard (or threat) and formulate then implement a course of action.
Basically in vehicle crashes this amounts to jamming on the brakes or attempting object avoidance (steering input).
The most common reaction to a roadway hazard upon very short notice is braking. One mistake often made during heavy braking is attempting to steer. A locked tire will not steer, so if you have you brakes locked up and are in a slide, you are going to go straight regardless of what you do with the steering (good ole inertia and Newton’s First Law). The invention of ABS relived this problem with static breaking. The tire never stays locked so during the intermittent braking the tire will steer. ABS creates a static tire patch on the roadway verse a dynamic patch which is sliding on the surface with locked conventional brakes.
So how does perception reaction play into a crash? It adds to the distance you will travel before you can stop. The average perception reaction time is 1.5 seconds. So going back to our velocity conversion formula from Part 1, at 55 mph you will travel 120.94 feet during an average perception reaction time. D = (S)(1.466)(T) ~ Distance is equal to Speed times (1.466) times time.
Let’s go back to Part 1, if you going 55 mph you are traveling at 80.63 fps/s and then we were distracted for 3 seconds so according to Newton we traveled 241.89 feet in those 3 seconds. Now we have redirected our attention back to the roadway and now see a hazard in the roadway, let say a deer. It will take us ‘on average’ a total of 1.5 seconds to perceive this and decide on a course of action and then implement it. So let’s say we jump on the brakes. From first taking our eyes off the road to our initial braking we have already traveled 362.83 feet and haven’t even started slowing down yet.
This brings us to stopping in time before our collision. Back to Newton’s First Law, once we are in motion we will stay in motion unless acted up by an external force. Now you’re thinking if I let off my gas I will come to a stop, eventually? Right? The answer is yes, you will come to a stop eventually due to gravity. An object in motion has Inertia and also has a certain amount of Kinetic Energy. Before an object (our vehicle) can come to a stop it has to get rid of all that energy. Now energy cannot be created or destroyed, it has to be transferred.
Which takes us into Part 3: Coefficient Of Friction (and how it pertains to stopping)
***One final point on perception reaction times; there have been several studies done to determine this time. Remember it is an average time not a specific time. Some people are faster while others are slower. The actually study that was performed came up with an average time of 1.6 seconds but was rounded to 1.5 seconds for ease of use in formulas. Lowering it to 1.5 actually gave the driver a benefit. Other things affect ones reaction time including but not limited to illness, fatigue, sleep deprivation and drugs/alcohol. Every calculation in reconstruction is truncated down to 2 decimal points. Speed calculations are always rounded ‘down’ to the nearest mile per hour. When testifying in a criminal case a police Reconstructionist can always say they gave the driver every benefit in his or her calculations. There are very few ‘actual’ speed formulas with most being minimum speed determinations and everything should be given with a range and not an exact speed.***