Force Science News #120:
New study yields unique grid for computing suspect speed in officer attacks
The latest time-and-motion study from the Force Science Research Center offers investigators and prosecutors a new tool to apply to officer-involved shootings and other threat encounters and presents trainers with a challenge in improving their students’ firearms skills.
The study’s core findings are captured in a unique grid chart that you can download and print out free by clicking here or typing http://forcescience.org/speedgrid.html into your browser. (If you print it out now, you’ll better understand the applications described in this article.)
This chart, called the Force Science SpeedGridTM, allows you to understand and demonstrate how to compute the speed at which a suspect is charging toward an officer with an edged weapon, for example, or running away after a confrontation.
“The chart expresses the speed in miles per hour,” explains Dr. Bill Lewinski, FSRC’s executive director. “This is a means of measuring and illustrating speed that is readily understood by civilians—jurors, for instance—and can help them appreciate the urgency that officers often face in force encounters.
“If you can show that a knife-wielding offender was running toward an officer at 15 mph, let’s say, it can help a layman better understand why that officer needed to shoot without delay before the suspect reached him.”
The chart has other possible applications, as well, Lewinski says. “It can help establish how fast an officer needed to move to escape a vehicular attack or help explain why an officer’s rounds struck a fleeing suspect in a particular location or at a particular angle or missed him altogether.
“In short, this is one more means by which reviewers can assess an officer’s actions and analyze the human dynamics involved in certain encounters. When an officer says, ‘The suspect was coming at me fast,’ you may now be able to explain just how fast.”
The study that resulted in the SpeedGridTM stretched across a 15-year period in which Lewinski conducted fitness evaluations of students entering the professional law enforcement program at Minnesota State University-Mankato, where he is a faculty member and where FSRC is headquartered.
Among other things, the testing involved the students sprinting for 50 yards and running for a quarter-mile to simulate a foot pursuit. This was done on a compacted sand-and-gravel track where each footfall left a visible impression.
“When I first started competing in track meets nearly 50 years ago, I noticed that people accelerated at different speeds,” Lewinski told Force Science News. “I also noticed that as each person accelerated, his or her stride changed. There is a fairly consistent style of movement that most people use as they drive forward. They start with short, quick steps, then take increasingly longer but quicker ones as they pick up speed.”
When testing the law enforcement candidates, Lewinski began periodically recording measurements of time, distance, and cadence of strides. “Each step a subject took connected to a different rate of acceleration and to a different length of stride,” he says. “Realizing this led to the idea of computing how fast the runners were actually traveling at the various stages of their acceleration.”
He estimates that he studied more than 1,000 individuals—“all kinds: short, stocky, fat, tall, lean, male, female, predominately in the age range of suspects officers are most likely to encounter in force situations.”
On average, he found that when youthful, vigorous, relatively fit people start to run, their first step takes about .35 of a second (not counting reaction time) and covers 1.5-2.0 feet. Their second step requires about .34 of a second and covers about 3.0 feet. Their third takes about .33 of a second and the stride lengthens to 3.5 to 4.0 feet…and so on. Typically, after 5 to 7 steps, people max out at about .25 of a second per 5.5- to 6.0-foot stride, which they can then maintain for a short period of time.
With the help of Dr. Bill Hudson, FSRC’s deputy director and head of the university’s Electrical and Computer Engineering Dept., Lewinski transformed his findings into a grid that allows you to convert stride length and speed into a miles-per-hour (mph) reading.
A suspect whose stride takes about 1/3 of a second and covers 4 feet, for example, is traveling 8.26 mph. If his stride has stretched to 7 feet and is down to 0.3 seconds, he’s moving at 15.91 mph. Sounds fast—and it is. But such a speed is not unusual for an “ordinary” person fueled by adrenalin. Olympic runners and some other rare superstars can accelerate to more than 27 mph. By comparison, a comfortable walking speed is 3 mph and a comfortable bicycling speed is 12 to 15 mph.
How do you know what stride length and time per stride to plug in to get a mph reading? Two possibilities:
1. Go with the averages, which are denoted in a box that’s included as part of the SpeedGrid TM. To illustrate:
Say you’re investigating an OIS in which an officer has shot and killed a fleeing suspect. The officer says she decided to shoot when the running offender, at a distance of about 25 feet, turned toward her and fired at her. But the suspect actually was hit at a point about 15 feet further away—and ended up shot in the back.
Using averages from Lewinski’s study, you can assume that the suspect was fully accelerated at the point the officer decided to fire. His stride would have been about 5.5 to 6.0 feet long and have taken about one-quarter second, so it was likely he was traveling at about 15 mph. The subject then covered the distance between the point that the officer decided to shoot and the point the bullet struck in just over one-half second – about the time it would take an officer to just align their weapon and fire.
“Knowing that, it’s easier to grasp that the suspect would have traveled more distance and could have turned his back to the officer between the time she decided to shoot and her bullet actually impacted.”
As another example, take a common training scenario: an edged-weapon suspect charges toward an officer from a distance of 21 feet.
Using averages, the attacker’s first stride is at about 3 mph. But accelerating, he can reach a speed of 12 mph or more and cover 21 feet in about 1.7 seconds in about 5 steps. Considering that the average officer requires about 1.5 seconds to draw and fire one round from a Level 2 holster (not even allowing for his initial reaction time), his disadvantage in this situation is made crystal clear.
2. In some cases, evidence at a scene may allow you to be more precise. “Often runners leave marks on a surface that you don’t have to be a tracker to see,” Lewinski says. “You may be able to see where they dug their toe in to start and then perhaps see where the front edges of their shoes bit into the surface at each stride.
“If someone hasn’t disturbed it, this kind of forensic evidence can often be detected on asphalt and other hard surfaces as well as on dirt, sand, and grass.” You can measure these markings to determine stride length.
If you’re fortunate enough to have video of the suspect running, you can clock the timing of his stride and then find mph on the SpeedGridTM. “If you don’t have an exact time measurement,” Lewinski says, “you can still plug in the average as an approximation.”
Here, the physical characteristics of the suspect may help you refine your calculations. “Large, lumbering people will likely have a stride time slower than average,” Lewinski points out, “and more agile, athletic body types may be faster.
“What you come up with won’t be exact, but you can get a good estimate that’s meaningful in the average person’s frame of reference.”
In some cases, the chart may be useful in measuring an officer’s speed of movement. In cases where officers are targeted in vehicular assaults, for example, it’s sometimes claimed that the officer could simply have stepped out of the way. But could he?
A car coming at an officer at just 10 mph covers nearly 15 feet every second. “Figure that the officer requires 1/3 of a second to react and then takes 2 lateral steps of about 1/3 of a second each, moving as fast as he can,” Lewinski suggests. “That means it takes nearly a full second for him to cover only about 3.5 feet, and that may not be enough for him to safely clear the oncoming car—unless he’s Superman.”
The ability to accentuate the speed at which people can move underscores a challenge to firearms instructors, Lewinski points out. “When you state a running suspect’s speed in miles per hour, it’s easy to understand why officers often miss when trying to shoot under those circumstances.
“In simulator scenes, the threat usually occurs in the middle of the screen, rarely or never running across it, even though lateral movement in officer-involved shootings is very much a real-world fact. Officers typically will miss in shooting at a suspect dashing across their line of sight because they usually aren’t trained to properly lead a target that can be running at 15 mph or more.
“Hopefully, the SpeedGridTM chart will motivate instructors to expand their training to include fast-moving targets and thereby improve both their officers’ hit ratios and personal safety.”
Written by Force Science Institute
April 9th, 2009 at 9:20 am
© 2017 Force Science Institute Ltd.