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Falls on Stairs      Slip and Falls     Trip and Falls     Equipment Safety     Warning Development and Testing

Highway Safety     Railroad Safety     Aviation Safety     Site Safety Audits     


The rise height is (sin 35.3)(12.5") = 7.2"
The run is (cos 35.3)(12.5") = 10.2"

Falls on Stairs

Falls down stairs are a major cause of serious injury and death. The reasons for these falls include such things as the person not realizing a step or stairway is present, excessive variation in riser heights or run lengths, inadequate lighting, nosings that are difficult to see and poorly designed, placed or missing handrails.

Unless a stairway is obviously uneven, people expect the rise height and run length (the tread in the direction of travel) of a stairway to be consistent. Research shows that small inconsistencies increase the chance of falling. But taking accurate measurements is not as straightforward as one may think.

The traditional way of determining rise height is to use a ruler or tape to measure the distance from the nosing down to the back of the adjacent tread. But treads often slope from rear to front. And rise height is defined as the height of one nosing above the next nosing – not the height of a nosing above the back of an adjacent tread. Any slope in the tread from rear to front invalidates the traditional way of measuring rise height.

Furthermore, run length is traditionally measured from the front of the step to the back. But if a nosing protrudes over the next tread, that protrusion eliminates a portion of the tread from being used as one descends the steps. Run length must take that protrusion into account. The traditional method of measuring run length does not yield accurate results.

Dr. Johnson has developed a method, first introduced by Jake Pauls, which determines rise height and run length with greater accuracy, even on carpeted stairs, than can be obtained by the traditional methods. This improved method measures the angle of the slope and the distance between each two adjacent nosings. Then the true rise height and run length, as experienced by the user and as defined by code, are calculated trigonometrically.

Dr. Johnson has investigated over 200 cases involving falls on stairs.

Slip and Falls

A person slips and falls when there is inadequate friction between the shoe and the walking surface. Usually there is a lubricant, such as water, on the surface or on the person’s shoe that reduces friction.

When investigating an instance where a fall has taken place it is important to not only measure the slip resistance of the clean and dry walking surface but also the surface with the lubricant on it.

The traditional type of device for measuring slip resistance, the drag sled, also known as a pull meter, measures friction on dry surfaces. But this type of device does not give valid readings when a lubricant is present. If a pull meter is placed on the surface covered with water adhesion between the bottom of the pull meter and the underlying surface occurs. When pulled the meter gives spuriously high readings because of this adhesion.

The device Dr. Johnson uses is called a Variable Incident Tribometer, or the English XL, and is manufactured by William English. The VIT does not allow a dwell time between the Neolite shoe and the wetted surface so that there is no artificially high adhesion.  Dr. Johnson is certified as an XL tribometrist (CXLT).

Dr. Johnson has worked on about 190 cases involving slips and falls.


Trip and Falls

A person trips and falls when there is a small, usually inconspicuous, rise in a walkway surface that the person expects to be smooth. When the toe of the foot catches this rise the foot is stopped in flight and momentum causes the person to stumble or fall forward. Because small vertical rises may not be readily visible they are more dangerous than larger rises.

Vertical rises can be made less dangerous if the top edge is beveled, thereby reducing the chance that a foot will be stopped in flight. The height of the rise and any beveling must be accurately measured to determine if it meets code.

The typical method for measuring a trip hazard involves a straight edge or tape measure. This method is fraught with problems involving not only eye parallax, but difficulty in recording the actual rise height as well as accurately measuring any edge beveling.

Dan Measuring a Trip Hazard    Trip Hazards

Dr. Johnson has devised a very accurate method of measuring the vertical rise as well as the height and angle of beveled surfaces.  He has been consulted on over a 170 cases involving trip and falls.

Equipment Safety

The design of equipment, from hand tools to large machines, should take into account the human factors issues involving user safety. Systems analyses should be conducted by the manufacturer to determine the likely uses and misuses of the equipment. Then methods should be devised to reduce or eliminate serious injuries associated with the equipment. Below are just a few of the human factors issues.

Guarding - An important human factors issue involves guarding to prevent users from inadvertently coming into contact with a hazardous location of the equipment. Conveyors, for instance, may have pinch points that are unguarded and which can cause serious, even fatal, injuries. Engineering controls should be instituted so that when covers are removed, thereby exposing these pinch points, the conveyor should be automatically deactivated. Warnings should be posted telling employees not to override these controls.

Vibration - Some hand tools vibrate when they perform the function they were designed for. Excessive exposure to this vibration can cause severe, even permanent, injury to the nerves and vessels in the hand and arm. Design of the equipment should be examined to determine if the vibration can be eliminated. Use of damping, such as is possible with gloves, may reduce injury severity. Warnings should be provided to alert users to the symptoms and dangers of vibration.

warning label for scissor lift

Scaffolds, Scissors Lifts and Other Raised Platforms - Raised work platforms produce the potential for tip overs and falls. Safety issues include the use of guards, moving a rolling scaffold or scissors lift on uneven or sloped surfaces, electrocution from overhead electrical sources and moving a platform too near a drop off or hole. While equipment design may eliminate some of these hazards, other issues, such as adequate lighting, floor maintenance and management oversight can result in fewer injuries and deaths.

Dr. Johnson has consulted on over 65 cases involving equipment design. In doing so he has designed, tested and produced warning labels that have been in use on scaffolds, scissors lifts and swing stages for over 15 years.

Warning Development and Testing

A warning can be useful in alerting the unwary person, or reminding the normal user, of a potential hazard. But for a warning to be effective it must meet certain criteria.

First, it must be perceived as a warning by the person in time to avoid the hazard. This means that the warning must be seen; it must not have worn off, been covered with grime or be in an area with inadequate lighting. The warning must be readable to the majority of expected users and be placed such that it is visible in time for the person to take appropriate avoidance behavior.

Second, it must be comprehensible. It must be printed with the appropriate type size, and the words must be legible and understandable to someone with the minimum education level of those expected to read it.

Third, it must describe the hazard, if not obvious, and how to avoid the danger associated with that hazard.  The avoidance instructions must be feasible and not require a person to perform actions at odds with the task the person must perform.

Fourth, the warning should attempt to persuade people to follow the instruction on how to avoid the danger and not be perceived as just a way to avoid legal action.

If graphics are used to communicate a non-verbal message the graphics must be shown to effectively communicate unambiguous information.

conveyor belt warning label  fishbowl breakage warning label

Dr. Johnson has developed and tested numerous warnings and safety instructions used on a variety of products including scaffolds, scissor-lifts and commercial aircraft. He has authored ANSI-approved standards related to the design and testing of warnings (see ARP577).

He has been consulted on over 200 cases involving warnings and has testified before Congress on the need to develop and test safety instructions in order to reduce injuries and death. He has authored peer reviewed articles on this topic.


car collision drawing

Highway Safety

Every highway collision involves inappropriate actions by one or more of the participants. Sometimes the inappropriate actions are design induced. At other times they result from driver or pedestrian inattention, excessive risk taking or physical abnormalities such as drug or alcohol intake, physical disease or deficits in perception and mentation that can occur with advanced age.

Auto-Pedestrian Collision - Some injuries occur because pedestrians do not realize how difficult they are to see. Other times, pedestrians appear to trust the apparent protection provided by a marked crosswalk. On the other hand, many drivers are not vigilant to, or disregard, pedestrians in both marked and unmarked crosswalks. When these instances coincide the chance of an auto-pedestrian collision is increased. Improved crosswalk lighting is one possible design improvement that can reduce these injuries.

Closing Rate Collision - A driver may find it difficult to perceive that a vehicle ahead is stopped, especially in the fast lane of a motorway. The rate of change in visual angle of a stopped vehicle at distance is so gradual that it may not be noticed until one is very close. Then there may not be an adequate distance in which to stop or an alternate available route and a collision becomes unavoidable. The placement of advance warnings (signs, road flares, signal persons) can reduce this type of collision.

Perception/Response Time (PRT)- This is the time it takes a driver to notice a hazard, recognize what it is, decide what action to take and to initiate that action. PRT is shortest when a hazard is seen, immediately recognized and there is only one possible avoidance maneuver, such as

applying the brakes. PRT is longer when the person is distracted (as when using a cell phone), has been taking alcohol, or when there are multiple avoidance maneuvers that need to be considered.

To fully understand how and why a highway collision has occurred many factors need to be considered. Dr. Johnson has been consulted on over 100 incidents involving highway collisions.


Railroad Safety

There are a number of human factors issues related to railway-auto and railway-pedestrian collisions. People have difficulty appreciating the velocity of a large object closing at high speed. One reason is that small objects appear to move faster than large objects traveling at the same speed; a B-747 landing seems to be going much slower than a small airplane even though both are at the same speed. An approaching train may seem to be moving at a slower velocity than it really is, which may be just one reason why someone ventures into its path.

Another reason is that the sight at a crossing may be obstructed, by vegetation for instance, so that a driver cannot see far enough along the track to detect an oncoming train.

A third reason is that at private crossings there are no active signals and the train may not be required to sound its horn. So a driver will have no active alarm to warn of a train that is close by and approaching at high speed.

A fourth reason is that a train may sound its horn at a distance of a quarter mile. The driver of a noisy truck or an automobile with windows up and radio on may not hear the horn.

And pedestrians on trestles (where they should not be, but sometimes are) may not have time to get off when they hear the horn just seconds before the train starts across the trestle.

Dr. Johnson has been consulted on over 15 incidents involving railway-auto and railway-pedestrian collisions.

broken RR crossing sign

Aviation Safety

Most commercial airline accidents (about 80%) result in no casualties and few are reported in the media. A small number (about 5%) are fatal to all on board and always reported. (This skewed media coverage probably causes people to think airplane accidents are usually fatal when they are not.) In the remaining 15% whether passengers survive or not often depends on what they know and do.

Passenger knowledge of safety procedures is often less than optimal. Most pay little attention to the flight attendant or video briefing, and few read the safety briefing cards where most of the important information is provided. And once an emergency takes place there is little or no time for passengers to inform themselves. Take the case of  “brace positions.”

Brace Positions - This is probably the most important information passengers should know if an unexpected emergency takes place on landing or takeoff. Surveys as well as actual incidents show that many passengers do not know how to get into the appropriate brace positions. The National Transportation Safety Board has reported that this important information can be life saving but it is seldom described by flight attendants. A relevant safety standard (see ARP1384) recommends that airlines show this in the video briefing, but this is seldom done. It is only depicted on the safety briefing cards, items that most passengers choose to ignore. If airlines were to make sure this information was covered in the safety briefings, and were more persuasive in getting passengers to attend, passengers’ ability to survive emergencies could be increased.

Flotation Devices - There are many other issues that affect passenger safety. These include knowledge on how to escape from a burning or sinking aircraft, and how to use any of the flotation devices, including life preservers which can be very difficult to put on in times of stress.

Overhead Bins - Even the issue of items falling from overhead bins is one that needs to be addressed. Passengers are not always told that items can fall and injure those sitting beneath them. Yet, there are designs that restrain items even after the bin is opened.

airline overhead bin instructions by Interaction Research Corp.

Implementation of improved designs and increases in the effectiveness of passenger education can result in a higher level of airplane passenger safety.

Dr. Johnson has worked in this area for several decades and has authored many articles as well as a book on the topic. He has been consulted on over 15 cases involving commercial airlines and passenger safety.

Site Safety Audits

Occasionally a company or other entity, such as a city, wishes to know if people who use its facilities are exposed to unnecessary hazards. The company may want to determine if there are walkway hazards, such as floors that can become dangerously slippery when wet, single step risers in unexpected places, poorly designed or maintained stairs or ramps that are too steep or which have surfaces without adequate slip resistance. These are some of the more common and unnecessary causes of injury.

If inadequacies are found the company will want to know what measures can be taken both in the short as well as the long term to reduce the chance of injury. For example, if a stairway is found to have excessive variation in risers or runs an immediate remedy might be to highlight each nosing so as to reduce the chance of a misstep.

measuring a handrail

If a handrail is found to be too large or too low to be easily grasped a remedy might be to add an additional, smaller handrail onto the existing one. If a surface is found to be slippery when wet an immediate remedy might be mat placement, or the application of a slip resistant finish to reduce the chance of a fall.

If injuries are likely to occur outside then placement of lighting fixtures or  highlighting of dangerous areas could be useful in reducing the chance of injuries.

If hazardous locations cannot be eliminated then barriers intended to keep the unwary person from encountering the hazard could be erected in order to prevent injury.

If all of these measures still result in a hazardous condition existing, then an appropriate warning for the general user, or the training of employees, could achieve a higher level of safety.

Dr. Johnson has conducted several site safety audits for both public and private organizations.

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