Pilar 5: Competencia leal

A second simulation matrix was used to evaluate potential scenarios in a subsequent vehicle impact that resulted in severe pelvic injury. As noted in Chapter 4 the MADYMO pedestrian model has a fairly limited set of standard injury measurements.

Equipping the pedestrian model with more of the occupant model’s injury sensors would be very useful when correlating injury during accident reconstruction. In this instance APF (Abdominal Peak Force) would have been a useful indicator of force applied to the abdominal and pelvic regions. As it stands, it is entirely possible to equip the pedestrian model with sensors but there is no validation data available for this injury measurement.

5.10.1 Introduction

Subsequent to the initial vehicle-pedestrian collision, the vehicle went over a bank at the edge of the road and caused fatal injuries to the pedestrian. As noted previously, this case study was based on an incident that was determined to be a homicide and not an accident as maintained by the accused. Some of the unusual vehicle orientations are best examined with this information in mind.

The injury pattern and the evidence available suggested that the vehicle landed on the pedestrian a few metres down the bank, shattering the pedestrian’s pelvis. It also appeared likely that the vehicle passed through a fence slightly further downhill and the lack of injuries on the pedestrian consistent with impact with the fence suggest

external injuries indicated that the pedestrian had passed underneath the vehicle feet-first.

The likely impact location and the fence location were used as constraints to investigate the vehicle speed and probable pedestrian location/orientation prior to impact as the vehicle left the road using a series of MADYMO simulations. Parameter exploration with an example vehicle can be seen in Figure 5.56. Figure 5.57 shows the fence broken by the vehicle on its downward travel.

Figure 5.56: Testing with Exemplar Vehicle at Top of Bank.

5.10.2 Vehicle and Environment Modelling

The same vehicle and pedestrian models from the previous modelling scenario were used as starting points for the models for this scenario. The vehicle model was modified from Case Study 2 – Part 1 to include an underside, complete with engine-sump and exhaust. The suspension modelling was enhanced to provide better accuracy over the undulating terrain. The pedestrian model’s shoes were removed because (i) they were removed in reality during the on-road collision, and (ii) clearance issues between the underside of the vehicle and the ground were identified, causing high ankle forces.

Survey data was used to create surfaces that approximated the roadside bank and fence. In the absence of readily available published values for roadside verges the ground stiffness was taken to be 2 kNmm^-1 which is approximately 75% that of Chadborne et al’s (1997) lowest value.

5.10.3 Simulation Matrix

The test matrix used the following variables:

• Pedestrian orientation:

o Standing

o Lying on vehicle bonnet o Lying on ground

• Pedestrian placement:

o At edge of road o At top of bank o Partway down bank

(pedestrian placement and orientation combinations can be seen in Figure 5.58)

• A vehicle speed range of 0.56 ms^-1 (2 km/h) (automatic transmission creep) to 3.9ms^-1 (14 km/h) (maximum attainable speed for vehicle over available distance) in 0.56 ms^-1 increments. No vehicle acceleration was applied because (i) there was no evidence of acceleration/deceleration on the grass verge, and (ii) once the vehicle was over the bank gravity provided the dominant

Figure 5.58: Various Pre-Impact Pedestrian Placements

5.10.4 Simulation Overview

Very few of the simulations indicated an injury pattern that matched the injuries inflicted. Some scenarios were proven unlikely for reasons other than injury correlation; when the pedestrian was placed prone on the road-side and vehicle was travelling at low speed, the vehicle became jammed on top of the pedestrian and failed to proceed down the bank.

It was determined from time and distance travelled that the most likely pre-impact pedestrian position/orientation placed the pedestrian on their back, feet towards the top of the bank, part way down the bank. Other pre-impact scenarios suggested events that were inconsistent with the actual injuries inflicted, including the pedestrian contacting the fence before the vehicle. From time and distance analysis it was determined that the vehicle speed as it went over the bank was approximately 3.3 ms^-1 (12 km/h). A simulation sequence showing this result can be seen in Figure 5.60.

For a full summary of the results from the simulation matrix conducted please refer to Case Study 2: Lamar, in Appendix III.

Injury correlation was then used in an attempt to confirm the speed range.

5.10.5 Pedestrian Abdominal and Hip Injury Correlation

A simulation matrix was created with the vehicle given an initial speed at the top of the bank of between 1.5 to 4 ms^-1 in 0.5 ms^-1 increments. A vehicle speed of 1 ms^-1 was not feasible as the vehicle failed to successfully negotiate the top of the bank at this speed or less. 4 ms^-1 was determined to be the maximum speed achievable by the vehicle in the space available.

Figures 5.61 to 5.63 show the results for abdominal and hip force. Although the pathology report refers to a pelvic injury it would appear that in the MADYMO multibody model the hip joints are the closest measurement location for such an injury. This is not an unreasonable approach as the hip/pelvic group is often considered as a whole in injury analysis.

Abdominal Force During Off-Road Pedestrian Run-over

0 1000 2000 3000 4000 5000 6000

1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (milliseconds)

Force (N)

1.5 m/s 2 m/s 2.5 m/s 3 m/s 3.5 m/s 4 m/s

Figure 5.61: Abdominal Force during Off-Road Pedestrian Run-over

Based on the abdominal injury risk curve shown in Figure 5.11 it can be ascertained that all the scenarios indicated in Figure 5.61 pose a high risk of an AIS 3 or greater abdominal injury.

Right Hip Force During Off-Road Pedestrian Run-over

1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (milliseconds)

Figure 5.62: Right Hip Force during Off-Road Pedestrian Run-over

Left Hip Force During Off-Road Pedestrian Run-over

0

1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (milliseconds)

Figure 5.63: Left Hip Force during Off-Road Pedestrian Run-over

McElhaney et al (1974) refer to Messerer’s (1880) findings indicating a minimum pelvis anterior-posterior loading tolerance of 170 kg or approximately 1670 N for the

et al, 2003; King et al, 2004). If Messerer’s tolerance is applicable then it would appear that the results shown in Figures 5.62 and 5.63 indicate likely hip trauma although the result is far from conclusive.

The modelling for this scenario requires scrutiny. The actual accident site was noted to include rocks and mounds not included in the simulation. Such terrain features could well cause considerably higher loadings should they coincide with the pedestrian during the vehicle impact. Although the vehicle suspension was modelled any differences between the model and reality could easily result in major differences to the load applied to the pedestrian. As Snedeker et al pointed out, a finite element pedestrian model is the preferred choice for such modelling rather than a multibody model. The vehicle underbody was only roughly approximated. Any solid objects protruding below the plane modelled (such as engine sump, transmission) could well have caused a point-loading on the pedestrian in reality.

5.10.6 Case Study 2 – Part 2: Discussion

The simulation series was able to determine a possible pedestrian placement that may have resulted in the actual injuries incurred by the pedestrian. This was determined using a time and distance method of analysis.

Injury correlation indicated the potential for severe injury, but this could also be determined by simple inspection (i.e. having a vehicle land upon a person is liable to cause injury if there is insufficient clearance between the underside of the vehicle and the ground).

Injury modelling was unable to provide additional correlation in this instance because of:

• Potential inaccuracies in terrain modelling

• Potential inaccuracies in vehicle modelling

• The use of a multibody pedestrian model instead of a finite-element model

These issues stem from the relatively unusual form of vehicle-pedestrian interaction that occurred in this instance and highlight the relative inflexibility of the modelling

method used once situations outside of the typical on-road vehicle-pedestrian interaction are considered.