Low Speed Accidents

Analogies to Assist in Understanding Bodily Injuries Due to Motor Vehicle Collision

— John W. Ellis, M.D., Oklahoma City, OK

The expert medical witness is needed to assist the judicial system in understanding how motor vehicle collisions can cause permanent bodily injury. The analogies, explanations and chart in this article will assist in explaining in lay terms how factors such as collision speed, change in speed (Delta-V), change in time (Delta-T), inertia, momentum, human anatomy and physiology and body position can contribute to injuries.

"Low Speed" Is Not the Same for a Car and a Human Body

Because there may be little motor vehicle damage at impact speeds less than 20 m.p.h., such speeds might be considered "low speed" for motor vehicle collisions. However, such speeds cannot be considered "low speed" for the human body. The person not trained in human anatomy and the treatment of injuries may mistakenly assume that a "low speed " collision will cause little damage to the human body. This reasoning error occurs when one incorrectly assumes that the human body tolerates accident forces the same as machines. Ligaments and muscles are not metal and plastic. Also, contributing to this reasoning error is that one experiences less of a sensation of speed in an enclosed vehicle at 20 m.p.h. than one experiences on an open bicycle at 20 m.p.h. About 10 m.p.h. is fast for the human body. Joggers and runners have been clocked at speeds from 6.62 feet per second (4.5 m.p.h.) to 12.96 feet per second (8.9 m.p.h.)1. If a runner could run a 100-yard dash in nine seconds, the runner would have an average speed of 22.7 m.p.h. Jurors can understand that someone running less than 10 m.p.h. can be injured due to a sudden change in speed such as hitting a wall.

Analogy Comparing Change in Speed to the Height of a Fall

The expert medical witness should assist jurors in understanding the effect of change in speed (Delta-V) and change in time (Delta-T) in motor vehicle collisions by using examples and analogies.

The analogy of comparing "low speed" collisions to the forces involved in a cough does not correctly address the Delta-T which is longer in a cough than in a collision. The analogy of stepping off a curb is incorrect because it is a vertical deceleration which the human body can better tolerate and does not occur in most motor vehicle collisions. A better analogy to assist jurors in understanding the direction of forces is to compare the forces in motor vehicle collision to a person sitting in a chair pushed into a wall.

Analogies to Assist in Understanding Bodily Injuries Due to Motor Vehicle Collisions 

Although many jurors may not have been involved in motor vehicle collisions that resulted in injury, many jurors will have familiarity with falls. Another good analogy is to compare the speed one obtains when falling from various heights to various changes in speed in motor vehicle collisions.

A fall off a 3.3 feet desk results in a speed at impact of 10 m.p.h. A 10 m.p.h. change in speed (Delta-V) in a motor vehicle collision is equivalent to falling off a desk. Similarly, a 15 m.p.h. change in speed is equivalent to falling 7.5 feet - off a step ladder. A 20 m.p.h. change in speed is equivalent to falling 13.4 feet - off the roof of a one story building. A 25 m.p.h. change in speed is equivalent to falling 20.5 feet - off a two-story building. A 30 m.p.h. change in speed is equivalent to falling 30 feet - off a three-story building.

A less than 20 m.p.h. motor vehicle collision should not be considered a "low speed" in regards to the human body. A fall off a 7.5 foot ladder (10 m.p.h.) may fracture an extremity. Many who fall from the roof of a one story building (15 m.p.h.) sustain injuries. Most who fall from the roof of a two-story building (20 m.p.h.) sustain injuries.

Why Are There Injuries at Low Speeds?

The question is not whether there can be bodily injury in some "low speed" motor vehicle collisions but why is there not greater bodily injury. Single collagen fibers in ligaments and tendons are not extensible and begin to fail at 7% to 8% elongation². The suddenness of movement (Delta-T) and the large inertial forces in a motor vehicle accident can readily tear ligament, tendon, joint, disc and muscle fibers.

Jurors can understand that a motor vehicle occupant sitting in the seat with their back, buttocks and legs has some protection from the cushioned seat. Unfortunately, the occupant’s head, neck and upper back are not usually touching the back of the head rest or upper part of the seat. At impact there is sudden movement of the head, neck and upper torso that may tear and disrupt tendons, ligaments, joint capsules, discs, muscles, nerves and other structures. A race car driver may have fewer neck and back injuries than an occupant in a motor vehicle collision. A race car diver has better protection from a double harness and having their head in a helmet firmly cushioned against the headrest.

Understanding Bodily Injury in Motor Vehicle Accidents
A lack of medical training can result in the incorrect assumption that speeds that cause little damage to hard vehicle parts will cause little damage to soft human body parts.

Comparisons of Vehicle Impact Speed to Height of a Fall: 
10 m.p.h. at impact is equivalent to falling 3.3 feet off a desk
15 m.p.h. at impact is equivalent to falling 7.5 feet off a ladder
20 m.p.h. at impact is equivalent to falling 13.4 feet off a 1 story bldg.
25 m.p.h. at impact is equivalent to falling 20 feet off a 2 story bldg.
30 m.p.h. at impact is equivalent to falling 30.0 feet off a 3 story bldg.
40 m.p.h. at impact is equivalent to falling 54.0 feet off a 5 story bldg.

Ellis Clinic: Comparing Height of Fall to Speed Impact

Article and graphic ©2000. All Rights Reserved. 

1. Thompson, T. (1991, March/April). "Pedestrian Walking and Running Velocity Study."Accident Reconstruction
Journal, March/April 1991, page P28.

2. Canale, S. (1998). Campbell’s Operative Orthopedics, Ninth Edition, "Acute Traumatic Lesions of Ligaments",
page 1154. St. Louis, Missouri, Mosby.

3. Article and graphic © Copyright 2000. All Rights Reserved.