HOW FAR DOES IT REALLY TAKE TO STOP?
A Closer Look !

By: Jack Murray CLI (ret) CFE,CCDI
Executive Director, International Council of Accident Investigators and Reconstructionists

One question that frequently comes up in investigating motor vehicle accidents is – “did the driver have room enough to stop before the collision?”

In my experience, too many times inexperienced investigators and insurance adjusters rely on the simple slide to stop formula, as a means of determining, whether or not the driver had room to stop. This formula consists of 3 things, speed of the vehicle, coefficient of friction (aka drag factor) and a constant (30). If appropriate, you can also factor in the degree of slope of the highway, (positive or negative).

The Basic Slide To Stop Formula:
Distance required to stop equals the square of the speed divided by
the coefficient of friction times the constant.


Normally the coefficient of friction is taken from published charts for various surfaces, primarily the one prepared by the Traffic Institute of Northwestern University and widely republished in various accident investigation texts.

If just looking up a coefficient of friction on a chart was all that entered into the process this would be fine and would indeed be a simplistic way of answering the question of distance required.

However, the reality is that all the published suggested coefficients of friction are from tests that were done over 40 years ago. Road surfaces, even in the same categories, have changed a great deal in that time. A good example being that most interstate highways surfaced with asphalt, have particles of glass in them to increase night time vision distance. This also changes the drag factor of the surface.

The only way to accurately determine a coefficient of friction, or drag factor, is to use a meter such as those produced by Vericom. While this is a basic tool for a reconstructionist it’s not something you’ll find in every investigators tool box.

Commercial vehicles create a situation where the weight of the vehicle and the nature of the braking system can be major factors in determining braking distance.
The second factor and by far the most important one is perception, reaction time, commonly referred to as PR/T. Perception Reaction Time is a four step process:

1. The driver sees an object.

2. The driver realizes it is a hazard.

3. The driver makes a decision as to what to do e.g. brake, accelerate, swerve etc.

4. The driver actually does what he decided to do. e.g. lift their foot from the accelerator and hit the brake.

For many years, all the major accident reconstruction schools taught that “normal” perception reaction time was 1.5 – 2.0 seconds, this was a one size fits all.

More recent studies, such as those done by Dr. Paul Olsen at the Traffic Institute of The University of Michigan, have shown that PR/T may vary from 2.5 to 5.0 seconds. Olsen’s study was performed by a method commonly referred to as “trapping.” Two vehicles connected by electronic means, get in front of and behind a target vehicle, that does not realize it is part of the study. When the first vehicle applies it’s brakes, it is recorded electronically in the second vehicles receiver and then when the target vehicle responds by applying their brakes this is also recorded electronically and the difference between the two times is the approximate PR/T.

This is partially a common sense issue, do we really believe that a 76 year old woman, with bad vision, has the same perception reaction time as a 21 year old woman in perfect health and 20/20 vision.

A very detailed paper on the subject was authored by Marc Green, Human Factors Science and Department of Ophthalmology, University of West Virginia Medical School. Mr. Green reviewed 50 different studies on the subject and basically came to similar conclusions.

Visibility- Even though you see an object you might not immediately recognize what it is because of the lack of light, and or the background.

Weather – heavy snow, or rain, or bright sunlight in the line of vision all influence how soon you see the object and are able to determine what it is.

Realizing a hazard exists – you might see a cow on the roadside, but not immediately discern that it is going to cross the road in front of you, or even worse, stop in the middle of the road.


How quickly you react to a situation can be influenced by a number of things, including:

Mechanical Distractions – cell phone usage, other people in the vehicle, radio or CD playing, not to mention the possibility of TV receivers, lap tops, palm pilots, raspberries etc.

Cognitive Distraction - the persons mental condition at the time – we currently have a case where a salesman had just gotten off the phone with his boss and had been informed that he had just lost a major account. It is not unreasonable to believe that this would cause one to be distracted.

Outside environment – The nature of the roadway can also be a factor in reaction time. A driver going down winding mountain road is more likely to be playing closer attention to the roadway than a driver traveling over a straight interstate highway. The level of traffic flow can also be a factor, heavy bumper to bumpier traffic tends to make one pay closer attention than light, or moderate traffic, with reasonable spacing of vehicles.

Physical Impairment – this would include one who is physically tired, for example, someone who has been driving for an extended period of time or who did not have sufficient rest the night before. Other considerations are people with allergies, sinus head
aches, colds, minor aches and illnesses, like the typical weekend warriors.

Driver Expectation - Personal observations of professional drag racers typically show them achieving a PR/T of .49 to .52 seconds. However, this is a situation where the driver is a professional, they know the green light is coming, they are ready for it and they are driving a vehicle finely tuned to respond to their foot on the gas. This is a diametrically opposed situation from a driver going down a highway, and suddenly a horse bolts out of a field and runs across the drivers path. Drivers may expect a traffic light to periodically change so that is not really an unexpected event.

What factors influence perception time?

Emergency situations – Our reference above to the horse crossing the road or a car running a red light are emergency situations which in addition to requiring immediate response can introduce the elements of surprise and fear into the equation. One would expect greater urgency to produce faster reaction, but this can actually lead to a slower reaction time as the driver has mixed emotions.

Complex emergency situations – This is where a driver is faced with more than one factor in reacting. For example, a car ahead of him is struck by another vehicle, not really posing a danger to his vehicle at first, but then one of the vehicles careens into his path and presents an immediate danger.

Substances in the body – Alcohol and drugs (both prescription and illegal) can and do effect perception reaction time, but there are no published studies that correlate a certain blood level with a certain degree of impairment. DOT regulations prohibit a driver with a blood alcohol level of .04, or more, from being behind the wheel of a commercial vehicle.

Age –As a general rule, a persons reaction time increases with their age, but a person may compensate for some of this, as they become more experienced and skilled drivers. A person who drives the same route everyday, may also compensate for some loss of speed in their reaction time, as they become more and more familiar with the route.

The tricky part is that what can be a total disaster for one person, may not be a big deal for another. One example I sometimes use, in testifying in court, is the example of the captain of the football team who takes a cheer leader to the prom. He drinks a six pack and is fine, she drinks two mixed drinks and vomits all over herself.

When we arrive at an approximate PR/T and this is always best given in ranges e.g.: from 2-3 seconds etc. we need to determine how far the person traveled during the period of PR/T.

We do this by taking the speed in miles per hour and multiplying that by 1.449, to get the feet per second. For example 30 miles per hour equals 43.47 feet per second. With a PR/T of 3 seconds we have 130.4 feet traveled before the braking process starts.

After we’ve determined the braking distance and the approximate perception reaction time we add these two together and then allow another .5 seconds for the physical time required for the action decided upon to actually start, e.g. the braking process starting, this can be substantially longer in commercial vehicles with air brakes.

We also have to consider the responsiveness of the vehicle. My son drives a Porsche 992, I drive a Chevy Monte Carlo, there is no comparison in the response time of these two vehicles, let alone if you’re dealing with a Chevy Suburban, or a Ford Expedition.

Braking vs steering: Reaction time for steering is slightly faster than reaction time for braking, because of the positioning of the hands and the lack of need to shift the foot from one pedal to another. This can be as big a difference as .3 seconds.

When all of these factors are considered, we have at best a reasonable expectation of how much distance is required for a vehicle traveling 30 miles per hour to come to a complete stop. Obviously PR/T is a very subjective judgment and subject to many variables.

The best you can do is determine a minimum distance required, allowing for all the known measurements and setting reasonable parameters for the rest.

NAIS CERTIFIED ACCIDENT INVESTIGATOR (CAI)
Your Instructors: Jack Murray, M.B.A., C.L.I., C.F.E. And Jay Murray, J.D.

Accident Investigation Seminar, Austin, Tx.- Feb. 24th-25th, 2006

Copyright: 2006, Jack Murray
All rights reserved.
NAIS use with permission