Abstract
Often, forensic human factors consultants are hired to work on a case after the incident area is altered, repaired, or destroyed. Photographs documenting the original incident area are critical in determining the causes of the injured parties’ fall. The case presented in this paper involves a trip and fall that occurred on the sidewalk of a large apartment complex. A human factors consultant analyzed the incident sidewalk post repair and was able to estimate the original condition of the sidewalk through the use of photograph scaling. In addition to an analysis of the human factors surrounding an incident, photograph scaling can be a useful tool in estimating elevation changes and determining the causes in a post repair trip and fall incident.
Keywords
Introduction
In 2019, Mrs. Z, an elderly woman, fell on an uneven sidewalk in her apartment complex sustaining significant injuries in upstate South Carolina. Shortly after Mrs. Z’s fall, the owners of the apartment complex repaired the incident area by replacing the two concrete sections bordering the joint to create a more level surface. Subsequently, the plaintiff’s counsel hired a human factors consultant to analyze the available information in accordance with applicable codes, standards, and safety literature to determine if a condition of defect in the sidewalk was the cause of Mrs. Z’s fall.
Summary of Related Case Fact
The fall occurred in 2019 in update South Carolina. The uneven surface portion of the sidewalk was located near the joint between two approximately 4 ft by 4 ft sections of sidewalk (see Figure 1 ). Mrs. Z exited her apartment and proceeded along the sidewalk leading to the parking lot and her vehicle. While walking, she encountered an uneven portion of sidewalk. Mrs. Z “fell from the sidewalk to the asphalt…” (Deposition of Mrs. Z, p. 32) sustaining significant injuries.
A view of the sidewalk on date of incident outside of Mrs. Z's apartment.
The apartment complex attempted two repairs to the incident area before Mrs. Z’s fall. The first repair occurred when contractors ground the edge of the sidewalk section to match the elevation of the adjoining section. While the ground sidewalk did temporarily make the sidewalk a more level walking surface, surrounding tree roots continued to grow under the sidewalk, further raising the incident section of concrete. The second repair attempted to bridge the height difference between the slabs by forming a ramp between the two sections ( Figure 1 ). Sometime after Mrs. Z’s fall, the property owners repaired the incident area for the third time by removing the original concrete and replacing the sections bordering the joint to create a flush and even walking surface ( Figure 2 ).
A view of the incident sidewalk after the final repairs were made.
Forensic Analysis
Plaintiff’s counsel retained a human factors consultant to provide insight into human perception and behavior. The defendants took the position that the sidewalk did not cause Mrs. Z’s fall, and she should have perceived the variable slopes and elevations in the sidewalk. A human factors consultant inspected, photographed, and documented the incident area in 2022 after the post-incident repairs were complete.
Estimation Measurements
Due to the subsequent repair, the two original incident sidewalk portions were destroyed before a site inspection could take place. Therefore, the human factors consultant conducted an additional analysis to estimate the relative changes in elevation using photograph scaling. The consultant measured the height of a curb located immediately to the left of the incident sidewalk during the site inspection (See Figure 3 ). This section of curb remained untouched during the repair and was present on both the incident date and the site inspection. The consultant chose this curb for the photograph scaling constant due to it being unchanged, a known height of 6 inches, and located within the same plane as the unknown elevations. The human factors consultant used the constant height of the known curb to estimate the heights, elevation change, and slope gradients of the unknown area as documented in the incident day photograph utilizing photograph scaling.
Within the incident day photo, the consultant measured each area of interest and then scaled the measurements to estimate the heights and slope gradients of the incident sidewalk. The scale factor was 0.04 inches such that every 1 inch in the environment measured an average of 0.04 inches within the incident photograph. The consultant assessed each estimate with the most conservative measurements and with the assumption that the sidewalk was as level as possible. The consultant measured each area of interest within the incident photograph 10 times and then averaged the measurements to calculate the tolerance. All estimated elevation measurements are reported in Figures 3 and 4 .
Estimated curb measurements of the sidewalk on the date of incident.
Estimated elevation heights of the incident curb.
The height of the incident portion of concrete located at the edge of the ground section estimated to be 6 ⅜ ± ¼ inches while the height of the patched concrete section estimated to be 4 ⅝ ± ¼ inches ( Figure 3 and 4 ). The horizontal distance between the edge of the ground down section and the edge of the patched section estimated to be 17 ⅛ ± ⅝ inches. Therefore, the overall change in elevation between the edge of the ground section to the edge of the patched concrete ramp estimated to be 1 ¾ ± ⅛ inches with a 1:10 slope gradient ( Figure 3 ).
In addition to the estimated changes in elevation among the incident curb, the consultant also calculated the slopes ( Figure 5 ). Alone, the patch that formed a ramp and bridged the height difference between the two sections of concrete had an estimated slope gradient of 1:11 and the ground section had an estimated slope gradient of 1:10 ( Figure 5 ). Additional estimated slope gradients are reported in Figure 5 ).
Estimated slope gradients of the incident sidewalk.
Applicable Standards
Through photograph scaling, the relative elevation changes and slopes of the incident area were calculated to estimate the condition of the sidewalk on the day of the incident. Here, the estimations were used to assess the walkway’s compliance with industry standards.
ASTM F1637-19 is a 2019 is an applicable consensus standard that gives guidance on the construction and maintenance of safe walkway surfaces such as the incident sidewalk. ASTM F1637-19 section 5.2.2 states that “changes in levels up to ¼ in. may be vertical and without edge treatment” and section 5.2.3 states, “Changes in levels between ¼ and ½ inches shall be beveled with a slope no greater than 1:2 (rise:run).” As shown in Figures 3 , 4 , and 5 , the change in level where Mrs. Z tripped, fell, and was injured was estimated with photograph scaling methods to have an overall change of 1¾ inches in violation of ASTM F1637-19. The ground section of concrete alone has an estimated elevation change of 1 inch and the malformed concrete ramp has an estimated elevation change of ¾ inches, both in violation of ASTM F1637-19.
A ramp was placed to serve as a transition between the elevation changes and concrete sections. However, the ramp was estimated to have a slope gradient of approximately 1:10 rather than the required 1:12 slope as required in ASTM F1637-19. Alone, the concrete patch that formed a ramp and bridged the height difference between the two sections of concrete had an estimated slope gradient of 1:11 and the ground down section had an estimated slope gradient of 1:10. The steeper and variable slope gradients likely contributed to Mrs. Z’s fall. It would have been practicable to take out and/or patch the malformed concrete and make the walkway surface flush in accordance with ASTM F1637-19. The sidewalk where Mrs. Z fell violated ASTM F1637-19 sections 5.2.1 through 5.2.4.
Analysis and Discussion
The human gait cycle consists of four phases: stance phase, toe-off phase, swing phase, and heel-strike phase (Cohen & LaRue, 2005). If the walkway surface violates a pedestrian’s expectation, such as an unexpected change in elevation during the swing phase of the gait cycle, then a fall is more likely to occur (Di Pilla, 2010).
Typically, when a pedestrian encounters an unexpected level change in the walkway surface, the pedestrian will misstep. Missteps occur as pedestrians place their foot down incorrectly most often due to a misperception of the walking surface’s depth (Cohen and Pauls, 2006). The incident sidewalk portion had an overall estimated change in elevation of 1 ¾ inches and a slope gradient of 1:11. Mrs. Z did not expect to encounter a sudden change in elevation. If she did, she could have adjusted her gait and limb placement to avoid a fall or avoid the hazard entirely.
Pedestrians’ expectations of the environment can influence their ability to detect and recognize walkway hazards. Pedestrians generally expect the walkway to be uniform and free of hazards (Cohen & Pauls, 2006). This expectation of walkways could be influenced by pedestrians’ numerous successful interactions with walkway surfaces that do not result in a fall. Pedestrians traversing in a familiar walking surface where there have been numerous successful walking interactions may have a lower risk associated with the walkway surface and may be “less vigilant in the searching for hazards on the walkway in which they are interacting with (Dixon, and Nathan-Roberts, 2017, p. 1255, as cited in Ayres, Wood, Schmidt, & McCarthy, 1998). Though Mrs. Z was generally familiar with the incident sidewalk, she had never fallen or stumbled before near the incident area. On the incident day, Mrs. Z likely did not expect a sudden change in elevation due to her numerous previous successful interactions with the general sidewalk in front of her apartment.
A pedestrian’s limb placement when navigating around hazards is dependent on pedestrians perceiving the presence of a hazard in the walking surface (Franchak and Adolph, 2010). If a pedestrian does not perceive a hazard such as a sudden change in the walking surface, they cannot successfully guide their foot placement to avoid a hazard or change their gait to compensate for changes. A pedestrian’s visual attention while walking is generally “directed forward in line with his intended walking route at a vertical angle… only slightly tilted towards the floor” (Zohar, 1978, p 667). Past research illustrates that a pedestrian’s effective visual field while walking is a cone shaped projection from the eye, covering approximately 10 degrees below the horizon (Panero and Zelnik, 1979). Mrs. Z most likely did not expect to encounter a sudden change in elevation while traversing over the incident area and was not looking down and at her feet for hazards. Rather, she was likely looking ahead toward her destination (i.e. her vehicle) or for other vehicles in the parking lot. While looking ahead, she encountered a sudden change in elevation of the walkway surface, causing her to misstep and resulting in her fall.
Details and objects that fall outside of a pedestrian’s field of view are “unlikely to be noticed and attended to unless they are made conspicuous by using properties such as blinking, motion, flickering, or high contrast in order to cause a shift in the pedestrian’s visual gaze” (Dixon, and Nathan-Roberts, 2017, p 1254). Most likely, Mrs. Z’s visual attention likely focused ahead on her vehicle and others in the parking lot rather than hazards located near her feet. Additionally, the hazard of a sudden change in elevation of the sidewalk surface was likely outside of Mrs. Z’s field of view. Therefore, in order for the hazard to be perceivable, it needed to draw Mrs. Z’s visual attention toward it. Due to the lack of visibility aids such as high-visibility yellow paint, markings, or warnings on the incident sidewalk, it is likely that Mrs. Z did not perceive the elevation change in the sidewalk before her fall.
Conclusion
The use of photograph scaling allowed the human factors consultant to estimate the incident area condition on the incident date after subsequent repairs. While this method can be useful by allowing consultants to estimate the condition of the scene on the incident day, there are many factors such as camera angle and resolution that affect the usefulness of incident photographs. In addition to an analysis of the human factors surrounding an incident, photograph scaling can be a useful tool in estimating elevation changes and causes in a post repair trip and fall incident.
