Today, numerous cars and trucks record large amounts of electronic data that can be tapped into when determining the events leading up to a motor
Today, numerous cars and trucks record large amounts of electronic data that can be tapped into when determining the events leading up to a motor
Accident Reconstruction and Human Factors in Pedestrian Collisions Pedestrian collisions occur every day; in fact, a pedestrian is killed every 1.6 hours and injured every
Probability of Frontal Airbag Deployment in Vehicle Collisions Accident reconstruction experts often determine the impact severity of a vehicular collision in their forensic investigations. One
Accident reconstruction experts often determine the impact severity of a vehicular collision in their forensic investigations. One common collision severity index is Delta-v (ΔV). The accident reconstructionists at Collision Reconstruction Consulting define Delta-v as the instantaneous change in velocity of an object during a collision event.
Airbag deployment thresholds can be a useful metric of collision severity in accident reconstruction applications, along with other analytical techniques. The National Automotive Sampling System (NASS) has provided a publicly-available database of real world motor vehicle collisions, including more than 10,000 event data recorder (EDR) reports retrieved from airbag control modules. These reports typically indicate the airbag deployment status and the corresponding Delta-V (ΔV) of each recorded event.
Our analysis of the NASS EDR data revealed that the Delta-V threshold for a 50% probability of deployment event is higher for Toyota than for GM and Ford vehicles. In addition, SUVs and pickup trucks had higher deployment thresholds than sedans. An increase in Delta-V thresholds was observed for more recent vehicle model years. A higher Delta-V is required for frontal airbag deployment in underride collisions, in which a sedan contacted a vehicle with higher ground clearance (SUV, pickup truck, or van), compared to collisions with direct bumper-bumper engagement.
The statistical analysis indicates that in general, GM and Ford airbags deployed at lower Delta-V values than Toyota airbag systems. The Delta-V corresponding to a 50% occurrence of airbag deployment is 8 to 9 mph for GM, 9 to 10 mph for Ford, and 11 to 12 mph for Toyota. However, the Delta-V where 90% of collisions resulted in a deployment event converged at 18 to 19 mph across the three manufacturers.
Effect of Model Year
Reports were categorized by model years 1994-2001 and 2002-2016, where the transition period 2001-2002 corresponds to pretensioners and dual stage airbags being introduced in vehicles . From our analysis, there has been a 1 to 3 mph increase in Delta-V thresholds for deployment between the years 1994-2001 and 2002-2016.
Effect of Vehicle Type
The vehicle types studied are categorized by sedan, sport utility vehicle (SUV), and pickup truck. The findings indicate pickup trucks and SUVs generally have higher deployment thresholds than sedans. For example, the Delta-V corresponding to a 50% probability of airbag deployment is 7 to 8 mph for sedans, 9 to 10 mph for SUVs, and 11 to 12 mph for pickup trucks. This finding suggests size and weight of the vehicle may be factors in the deployment algorithm.
Effect of Impact Configuration
Underride collisions occur in cases where the front of a vehicle collides with the rear of a vehicle with higher ground clearance. As a result, the bullet vehicle’s upper structure components (i.e. hood, grille, condenser, radiator support, engine block, etc.) contact the target vehicle’s bumper system. While cars in direct bumper-bumper impacts experience primarily compressive forces, the events in an underride collision involve both shearing to the sheet metal and compression to the bullet vehicle. Consequently, underride crashes were considered separately from collisions with bumper-bumper contact to evaluate the effect of contact height as a potential contributor in the airbag deployment criteria.
Underride data appear to trend towards a lower probability of airbag deployment for a given Delta-V range as compared to all frontal collision data. In other words, for a given likelihood of airbag deployment, the Delta-V threshold is higher in underrides than bumper-bumper collisions. A 50% probability of deployment corresponds to a Delta-V of 10 to 12 mph for underride collisions compared to 8 to 9 mph for bumper-bumper collisions.
Collision Reconstruction Consulting analyzed the NASS EDR database and determined the 50% likelihood of airbag deployment corresponds to Delta-V of around 8 to 12 mph. Toyota vehicles had a higher Delta-V threshold than GM and Ford vehicles. The Delta-V thresholds increased by 1 to 3 mph between the years 1994-2001 and 2002-2016. SUVs and pickup trucks generally have higher deployment thresholds than sedans. A comparison of underride and bumper-bumper collisions suggests the Delta-V threshold for airbag deployment may be greater in underride collisions.
To download a complete text of this peer-reviewed SAE study, please visit:
Pedestrian collisions occur every day; in fact, a pedestrian is killed every 1.6 hours and injured every 7.5 minutes in traffic crashes.
Accident reconstruction experts at Collision Reconstruction Consulting are retained to determine what happened in pedestrian collisions by analyzing the physical evidence, that may include vehicle damage, hair deposited onto a car’s windshield, tire marks, rest positions, shoe scuffs, or a blood pool from the pedestrian.
There are three main stages in a vehicle-pedestrian collision:
1. Momentum transfer from vehicle to pedestrian;
2. Pedestrian separation from striking vehicle, initial contact with ground;
3. Pedestrian sliding and tumbling on the ground to rest.
The “throw distance” is defined as the total horizontal distance that the pedestrian is displaced from the point of contact to rest.
As you can imagine, the vehicle shape, braking activity, and impact configuration can all affect the trajectory of the pedestrian. The majority of vehicle-pedestrian collisions involve the pedestrian being either projected forward from the striking vehicle (forward projection) or wrapped onto the front of a striking vehicle (wrap trajectory). The type of engagement is dependent on the pedestrian’s center of gravity and geometry of the striking vehicle. For example, a wrap trajectory occurs when the pedestrian’s center of gravity is below the leading edge of the vehicle’s hood. The pedestrian is struck at the legs and the body will wrap over the hood. The pedestrian may ride on the hood before sliding off to the ground.
On the other hand, a forward projection occurs when the pedestrian’s center of gravity is above the leading edge of the vehicle’s hood. Common scenarios include the front a sedan/SUV making contact with a child, or a truck striking an adult. In this case, the pedestrian would be projected onto the ground in front of the striking vehicle.
Vehicle-Pedestrian Collision Reconstruction
From a liability context, a common question that gets asked is “Was the driver operating the vehicle at a safe speed?” A reconstruction expert can calculate the impact speed of the striking vehicle using analytical methods. Based on staged crash tests and real-world collision data, studies have demonstrated correlations between the impact speed and pedestrian “throw distance” for wrap or forward projection collisions. Accident reconstruction experts use these projection distance models to predict a range of impact speeds.
Event Data Recorders—Can Crash Data Be Recovered?
Many modern vehicles are equipped with Event Data Recorders (EDR), which can store valuable information about driver inputs leading up to a crash. Unfortunately, for accidents involving vehicles impacting pedestrians, the chances for data to be captured by the EDR is limited. This is due to passenger vehicles outweighing a typical pedestrian by a factor of 10 or more, so the crash impulse would not cause an appreciable speed change to the passenger vehicle to trigger an EDR event. This limitation is further compounded by recent legislation that requires vehicles manufactured after September 2012 to report crash data only for events with a “delta (ΔV) equal or exceeding 5 mph in the longitudinal direction over a 150-millisecond period.” The striking vehicle would need to be traveling at high speeds during impact in order to achieve a speed change of 5 mph or greater in pedestrian collisions.
However, there is a commercial tool, TIS Techstream, for Toyota, Scion, and Lexus vehicles where “crash-related” data can potentially be retrieved from the Vehicle Control History (VCH) for select vehicle models with model year 2013 and later. Unlike the Bosch EDR system, there is no ΔV trigger threshold requirement.
The VCH triggers based on system-related events (lane departure, pre-collision braking, or ABS activation) and driver-related events (sudden braking, sudden steering, sudden acceleration). The VCH stores data such vehicle speed, engine RPM, throttle opening, acceleration, and steering for 5 seconds prior to an event and 5 seconds after an event. For each event, the associated ignition cycle, date and time according to the GPS system, event triggered, and odometer are recorded.
When a car collides with a pedestrian, we expect the driver to respond by applying hard braking immediately after collision, thus triggering a VCH event. Pre and post-crash data from this type of event can potentially be recovered using the Techstream tool, whereas it would not be recorded by the Bosch EDR system.
Avoidance Potential and Human Factors
Another question asked when determining liability in a pedestrian collision is “Could the driver have avoided the collision given the circumstances in the actual incident?” To address this issue of avoidance potential, the accident reconstructionist will determine the time-distance relationship between the vehicle and pedestrian before the collision, and then consider scenarios for avoidance, either by braking to a stop or swerving. Accident reconstruction experts work with human factors experts to determine an appropriate response when evaluating driver avoidance potential.
Human behavior—the leading cause of motor vehicle collisions
Statistics show that human-related behavioral factors are the single most important cause across all traffic accidents. Human factors experts use scientific knowledge of psychological principles such as thinking, attention, and perception to understand how people interact with, and react to, the environment. Indeed, epidemiological analyses, case studies, naturalistic road observations, and simulator experiments are revealing the general trends about various factors that impact reaction time in the context of driving performance.
Human factors influence the probability of traffic accidents
Various factors influence driving capability by shaping perception, judgment, decision-making, and reaction time. Factors such as inexperience, advanced age, disability, and chronic disease can reduce driving capability for an extended period of time, while fatigue, temporary distraction, and acute stress or drug intoxication are typically associated with short-term reduction of capability. Also, perceptual and cognitive factors including driver expectation and roadway visibility may be involved in a particular traffic accident.
Driver expectation affects perception-reaction time and brake response time
Driver expectation influences the ability to react and apply the brakes. For example, the unexpected onset of brake lights or traffic lights will lengthen the perception-reaction time (PRT), while mean PRT will shorten if the driver is expecting for braking to ensue or the traffic light to change. The total PRT is highly variable and depends, in part, on both personal and situational factors such as drivers’ alertness, time to collision, road lighting, and traffic density. For example, there is no need to act quickly at a longer distance, slower speed, or both. Thus, both the precise traffic situation and the urgency of the situation need to be considered.
Components of drivers’ perception-reaction time in braking response
The total duration of the brake response time is composed of a number of components that are arranged in a sequence.
1. Perception time and mental processing: In this context, it is defined as the time required to detect an object and to initiate a motor response. This component can be subdivided into sensation (i.e., detecting an object), perception (i.e., recognizing the meaning of that object), and the response selection and motor programming (i.e., deciding on a response and nervous system programming of movement).
2. Time of motor movement: This is the time it takes to perform the programmed or required muscle movement (e.g., foot relocation time).
3. Device response or mechanical movement time: This is the time it takes for the physical device to respond (e.g., the time it takes to bring a vehicle to a complete stop due to physical forces such as gravity, speed, and friction).
To calculate total stopping sight distance, we need to account for the distance that the vehicle travels before the driver applies the brake due to PRT needed to respond, the time required for the brakes to engage when brake pedal is depressed, and the stopping distance once the brakes fully engage to provide their maximum braking efficiency. The total stopping sight distance created by human PRT depends on various individual and environmental factors, and also is contingent upon speed and braking capability of the vehicle. Thus, the time needed to respond to a stimulus while driving must be estimated correctly.
High “load” impairs drivers’ visual attention and driving performance
There are three separate subsystems of memory in humans: short-term memory, working memory, and long-term memory. As but one example, working memory is more heavily taxed if an individual engages in cell phone use while driving. This working memory load associated with performing different aspects of the task is referred to as cognitive load. Certainly not an exhaustive list, but below examples illustrate the load effect.
1. In-vehicle technologies: The use of displays or cell phones is distracting (even the use of a hands-free device, which does not require visual attention) because it increases the total amount of cognitive processing, which is known to interfere with driver’s attention, and therefore with driving performance.
2. Passengers: Conversing with other vehicle occupants is a source of driver distraction, which has the potential to degrade driving performance.
3. Road environment: High cognitive and perceptual load due to reduced visibility of the driving scene can influence visual attention and, as a result, impair detection of relevant targets such as pedestrian crossing the street.
Relevant human factors considerations
Human factors experts must use the evidence available to them and rely on peer-reviewed published data to investigate potential factors that could have contributed to, or caused, this accident.
1. Time of day: A shorter PRT is typically observed when traveling during daytime.
2. Topography & locality: A shorter PRT is typically observed when traveling in a residential area, as well as when approaching an intersection.
3. Conversing with passengers: This secondary activity is distracting because it competes for driver’s attention, which affects visual scanning and information processing capacities such as decision making and reaction time.
4. Complexity of conversation: The mental workload is high due to the nature of the conversation (i.e., discussing a physical location). This cognitive process requires mental imagery, and thus influences visual attention.
Understanding the relative positions between a vehicle and a pedestrian leading up to a collision can help parties determine liability. This can only be achieved through analyzing the physical evidence that’s been properly documented. Accident reconstruction experts take into account factors such as the vehicle shape, braking activity, geometry of pedestrian, impact configuration when reconstructing pedestrian collisions. Crash data is typically not recorded in passenger vehicles during pedestrian collision events, but a dealer diagnostic tool can be used to potentially recover crash-related data for newer Toyota, Lexus, and Scion vehicles.
Today, numerous cars and trucks record large amounts of electronic data that can be tapped into when determining the events leading up to a motor vehicle collision. The accident reconstruction experts at Collision Reconstruction Consulting routinely download and analyze crash-related data from event data recorders.
An Event Data Recorder (EDR) is also referred to as a black box. It stores electronic data that provides vehicle and occupant information, enabling forensic engineers to understand what happened in a motor vehicle incident. The features and benefits of a black box simplify the process of reconstructing an accident and arriving at objective conclusions. Accident reconstructionists can utilize black box data to collect details such as a vehicle’s speed and acceleration, the application of brakes, steering activity, and whether occupants wore seat belts. They can download information from the black box to evaluate what happened and understand the operation of the vehicle’s safety systems.
Generally, there are three places where the black boxes are mounted in the vehicles – under the driver’s seat, passenger front seat, or under the center console. When interpreting data, it is essential to consult an accident reconstruction expert with a certification in EDR data retrieval and analysis. Inspections often involve specialists connecting to the vehicle’s Diagnostic Link Connection port, after which they download information from a microchip onto a laptop.
EDR technology plays an increasingly integral role in facilitating objective physical evidence that helps collision reconstruction professionals make fair and ethical determinations. Today, many advanced modules record a wide range of operational data that forensic engineers use. There are two types of recorded crash events. The first is a non-deployment event that records data without employing airbags. Next, we have a deployment event that utilizes airbags. The data may consist of both pre-crash and crash data, which experts store and cannot override. However, note that the data is stored in the airbag control module of the vehicle, and when the module is replaced after a deployment event, the data is no longer be accessed in the vehicle. It is therefore important to preserve the vehicle until the event data recorder is downloaded.
Over 97% of vehicles manufactured today are equipped with an event data recorder. A list of passenger vehicles that are supported with EDR data can be downloaded here.
At Collision Reconstruction Consulting, we apply our expertise in accident reconstruction to help clients understand “what happened” and “how it could have occurred” in motor vehicle incidents.