The Singapore Law Gazette

Contributory Factors to Traffic Collisions

Identifying the Human, Vehicular and Environmental Factors in Road Traffic Accidents

Introduction

One accident is one too many. In 2019, Singapore had 7,666 road traffic accidents causing injuries to 9,773 persons and 116 fatal accidents resulting in 117 fatalities.¹Yusof, Z. (10 February 2020). Fatal accidents, road deaths at record low in 2019, but more accidents involve elderly and motorcyclists. https://www.straitstimes.com/singapore/transport/overall-improvement-in-road-traffic-situation-for-2019-but-more-accidents Injuries not only cause physical and emotional harm to individuals and their families but also exact a heavy socio-economic toll on the nation. Traffic collisions that result in serious injuries, fatalities and significant property and vehicle damage need to be investigated to determine the circumstances leading to the collision, and the liabilities of the parties involved. Liability²Guidelines for Assessing Liability of Parties in Traffic Accidents. (2017, December 01). https://singaporelegaladvice.com/law-articles/guidelines-for-assessing-liability^,3Sandhu, V. (18 October 2018). What happens if you cause a fatal traffic accident in Singapore? Retrieved from https://learn.asialawnetwork.com/2018/09/06/happens-cause-fatal-traffic-accident-singapore/ connotes responsibility, fault, human error, negligence or recklessness, blame, compensation, and punishment. Criminal charges and civil or insurance claims may be brought against motorists who cause death, serious injuries, or extensive vehicle damage. Circumstances where culpability would be increased include attitudinal and behavioural factors such speeding, aggressive driving, sleep-deprived driving, driving under the influence of alcohol or drugs, distracted driving while using a mobile phone, violation of traffic rules, or poor control of the vehicle.⁴(2018) SGHC 18

A traffic collision is a system failure or breakdown triggered by a chain of events, and seldom results from a single cause. It is an exceptional or out-of-the-ordinary event for the typical road user. If the causal chain is interrupted by appropriate and timely evasive actions, the incident can be averted.

Traffic Collision Investigations and Reconstructions

The objectives of a traffic accident reconstruction (‘TAR’)⁵Traffic Accident Reconstruction – A primer for lawyers, Law Gazette, Jan 2017^{, 6}Forensic Science, Briefs for the Legal Practitioner, Chapter 13: Traffic Accident Reconstruction, The Forensic Experts Group, 2017. are to answer questions on who was involved, what happened, the type of accident and events and conditions leading to it, where and when the collision occurred, how and why it occurred, and whether the collision was avoidable. Findings from traffic investigations are used not only for court and litigation purposes but also for improvements to the operational safety of transport infrastructure, vehicle design and safety features, public and driver education, and for drawing up countermeasures to prevent recurrences.

Factors Contributing to Traffic Collisions

Driving involves operating a vehicle within a dynamic roadway environment in close interaction with other road users. Traffic collisions are typically caused by a combination of factors related to three essential elements of the road traffic system: the road user, the vehicle, and the road system. We will refer to these contributing factors as the human, vehicular, and environmental factors, respectively. Refer to Figure 1. Among them, human factors are cited as probable causes in 80 to over 90 per cent of collisions.

Figure 1: Venn diagram showing the contributory factors of traffic collisions

The road user’s behavioural characteristics as a pedestrian, driver, motorcyclist, cyclist, perceiver, decision maker and responder in a traffic situation are critical to road safety. His performance and role in the collision are contingent on his abilities, limitations, exercise of due care and attention, and his interactions with other road users, vehicles, and the road environment.

The environment refers to the internal environment within the vehicle, and the external roadway environment. It could be the constant ringing of a mobile device or the presence of a fallen tree across the roadway just beyond a blind bend.

The vehicle and its characteristics include vehicle-related deficiencies and some input-output relationships that are poorly understood by the motorist. For instance, few motorists have experienced or are prepared for the effects of hard braking on a muddy road surface.

In this article, we will first discuss the characteristics and behaviour of the motorist and pedestrian, their abilities and limitations in a traffic situation, followed by the vehicular and environmental factors.

Human Factors: Motorist Characteristics and Behaviour

Motorist characteristics that can result in collisions include poor vision and acuity, poor driving strategy, motorist expectancy not matching traffic circumstances, poor evasive manoeuvres, influence of alcohol and drugs, medical impairment, inattention and distractions, disregard and violation of traffic regulations and controls, and risky behaviour.

Inappropriate and Risk-taking Behaviour

Risk-taking behaviour includes aggressive driving, thrill seeking, not adhering to traffic regulations, illegal road manoeuvres, not wearing seatbelt or helmet and alcohol consumption. 

Common violations include unsafe and inappropriate speeds, failure to yield right-of-way, following too closely, improper turning, lane changing or overtaking. In congested slow-moving traffic, motorists may become frustrated, tired, impatient, inattentive, and distracted. Sudden stops in stop-and-go bumper-to-bumper traffic may result in rear-end collisions. Sudden lane changes in congested traffic to get onto a faster lane may also create traffic conflicts.

Human Abilities and Limitations

Human factors is an established science that encompasses the range and limits of human performance such as visual acuity, motor skills, perceptual abilities of hazards, working memory, risk assessment and reaction times. The key to avoiding a collision may be dependent on, and limited by human ability and time required to perform tasks such as reading traffic signs, checking mirrors, scanning the road, perceiving and identifying hazards, turning the steering wheel or moving the foot to the brake pedal and depressing it.

There is a wide variation among individuals, as well as day-to-day variation for an individual’s ability. An individual’s ability to perceive and recognise a developing conflict or hazard, and make appropriate decisions is crucial for anticipating and avoiding an accident. For example, traffic collisions may arise from discretionary right turn situations at junctions where the driver making the turn incorrectly judges the gap in the traffic stream and the estimated arrival time of the approaching vehicles.

Motorists are not able to respond instantly to a situation; they need time to comprehend an event, identify its nature and decide whether evasive actions are required to intervene or mitigate the situation. This delay or lag in response is referred to as the Perception-Reaction-Time (‘PRT’). Simple straight-forward and familiar situations such as braking on approach to a traffic junction during the red-light phase have a low PRT. On the other hand, a longer PRT is required for a more complex reaction where a driver is confronted with the unexpected and sudden intrusion of another vehicle from the opposite direction.

Vision, Acuity and Field of View

Of the five senses, sight or vision is the most important sense for the driver and road user. Driving is a predominantly visual task. While driving, the motorist conducts a visual search and is actively scanning the visual field for targets, events and potential conflict zones, not knowing when and where they may appear. Visual search is required to coordinate driving with the surrounding traffic and to detect hazards in a timely manner.

Figure 2: A driver’s view

Although a reasonably alert driver spends most of his time looking ahead, a large part of information comes from his peripheral vision. A driver must therefore divide his attention between what is ahead with what is at the side. Clear (foveal) vision is limited to only about two to four degrees beyond straight ahead of where one is looking, and acuity decreases significantly beyond this central sector of the visual field. Refer to Figure 2. The useful field of view while driving is 20 to 30 degrees to either side.

Drivers need to address the competing demands of multiple perceptual and cognitive tasks, and judiciously allocate attention and their limited visual resources and mental capacity. For instance, the average duration of a single glance to the mirrors is reported to range from 0.5 to 1.1 second; eyes would be off the road during this time interval. Today’s high vehicle speeds create a highly dynamic road environment requiring fast and extensive processing of visuo-spatial information in short time intervals on the part of road users. Speed compresses time and distance scales, reducing the buffer for avoidance and correction of any initial error or misjudgement. Split seconds can make a difference between collision avoidance, near-crashes and crashes. Insufficient or ambiguous visual information due to poor night-time visibility and age-related eye deterioration may also contribute to accidents.  

Expectancy

Expectancy refers to a road user’s readiness to respond to conditions, situations, events and information. Drivers expect normal traffic flow and compliance with traffic regulations by other road users. Perception is strongly influenced by expectations. Various factors may significantly interfere with the speed and effectiveness of information processing in a driver’s response, which translates to success or failure in coping with the sudden, unexpected situation. When confronted with an unforeseen real-life emergency or violation of expectancy, a road user may panic and react instinctively, make an irrational decision, flinch or do nothing during the critical moments before a collision.

Driver expectancy may be a dominant factor that leads to a collision, especially in cases where a driver makes an initial assumption, interpretation or decision which subsequently turns out to be erroneous. Safe coordination on the road requires clear communication and understanding among road users. Adequate, timely visual cues or signals must be provided to other road users. Correspondingly, drivers need to perceive and correctly interpret the intent of other road users. Lack of information and conflicting signals can cause hesitation, indecision, and delayed or even inappropriate reaction.

Identification errors commonly arise from right-of-way violations and misleading cues where the first event resulted in an erroneous identification by the driver. In the subsequent event, the PRT starts when the driver becomes aware that the other road user is not behaving as expected and a collision is imminent. In the case study below, when the car was upstream at the traffic junction, the car driver who was travelling in the left lane witnessed the cyclist changing from the left to the middle lane, as if crossing to the divider. Shortly after the car driver cleared the junction, the cyclist, without any hand signals or turning back to look, suddenly and unexpectedly changed back to the left lane, just in front of the car. The critical question in such an event is how long it takes for the driver to overcome the misleading expectation resulting from the first perception that the cyclist had just changed lanes and would not enter her pathway.

Since expectancy is closely associated with perception and response, two persons who are performing the same task – an examiner with hindsight knowledge of its occurrence, and the driver without prior cues – will not have the same perception. For example, a person analysing a video footage can replay it multiple times, stop and pause to look at the images as long as desired.⁸Do you see what I see – Pitfalls of video analysis in traffic collision cases, Law Gazette, Oct 2018 This person has prior knowledge of the collision and knows exactly where to look and what to look for. He is also able to capture substantially more details of the scene with each replay. Repeated viewings of the video footage create a very different set of “viewing conditions” compared with a driver who was caught in an unexpected traffic situation and forced to make a split-second decision. The driver may see an object in his peripheral vision, but he will not respond to it unless it is conspicuous, and he identifies it as hazardous. Hence, caution must be exercised when using findings obtained from video analysis, as well as those from simulation experiments where the human subjects were given information and instructions and received prior knowledge of the occurrence of an event.

It is important to understand that drivers focus their attention on relevant objects and locations where they need to find information for their task at hand; the remaining visual information is filtered away and is not consciously perceived or remembered.

Influence of Alcohol and Drug Consumption

Consumption of alcohol, prescribed medications with sedative effects, and illicit psychoactive drugs causes drowsiness, and reduces concentration and alertness levels. Perception and response are delayed, and coordination, reflexes and motor skills impaired, thus increasing the risk of road accidents. Alcohol-impaired drivers tend to drive faster and are more likely to be distracted than others as well as engage in risky driving behaviour.

Fatigue, Sleep Deprivation and Impairment

According to a 2014 worldwide study, Singapore is the third most sleep-deprived city among 43 cities. Studies have shown the association between fatigued driving and high incidence of traffic collisions.⁹Connor, J., Whitlock, G., Norton, R., & Jackson, R. (2001). The role of driver sleepiness in car crashes: a systematic review of epidemiological studies. Accident Analysis & Prevention, 33(1), 31-41. According to the National Safety Council,¹⁰(2020) Drivers are Falling Asleep Behind the Wheel. National Safety Council. https://www.nsc.org/road-safety/safety-topics/fatigued-driving driving while drowsy is similar to driving under the influence of alcohol, and a fatigued driver would be three times more likely to be involved in a car crash. Driving after going for more than 20 hours without sleep is equivalent to driving with a blood-alcohol concentration of 80 mg% (0.08%), the Singapore legal limit. Drowsy drivers often struggle to keep their eyes open and heads up, may miss road signs or turns, and have difficulty maintaining speed and keeping to their lanes. Sleep deprivation, drowsiness, fatigue and alcohol impairment adversely affects attention, alertness, hazard perception, judgement and reaction times. Research has established a correlation between accident frequency and the time of the day. A higher proportion of sleep-related accidents occur mostly in the early morning (2 to 5 a.m.) and early afternoon (1 to 4 p.m.).¹¹Zhang, Q., Wu, C., & Zhang, H. (2020). Driving fatigue prediction model considering schedule and circadian rhythm. Journal of advanced transportation, 2020.

Human Factors: Pedestrian Characteristics and Behaviour

Pedestrian characteristics are important because of the high number of collisions involving this group of vulnerable road users whenever their paths cross or overlap with those of moving vehicles. They may be overconfident of their own abilities and those of the driver’s, and unaware of the motorist’s and vehicular limitations, as well as the importance of being visible. They typically misjudge the vehicle’s speed and distance, and overestimate their visibility to motorists, especially in night scenes. They fail to realise their reduced conspicuity at night as they are not self-illuminated and generally do not wear retroreflective clothing. Pedestrians may be less attentive to traffic during poor weather conditions, increasing the risk of traffic conflicts. The sudden presence of pedestrians in unexpected places and their relatively small size and lack of conspicuity also reduce driver expectancy, detection and recognition.

Other pedestrian-related factors include crossing the road without precautions (distraction by electronic gadgets, impairment by alcohol or drugs, jay-walking between moving vehicles, darting across the road, and not using designated pedestrian crossings), vision or hearing impairment and slow walking.

A pedestrian’s age can be a contributing factor in vehicle-pedestrian crashes. Due to decreased psycho-physical abilities, young and elderly pedestrians are at greater risk. Elderly pedestrians are vulnerable due to impaired hearing, vision and mobility. Collisions with young children often occur in their neighbourhoods where they have a false sense of security in familiar areas that they frequent. Young children, because of their small size, may not be easily observable by the driver.

Case study: Reversing vehicle accident¹²(2011, June 15) ‘Boy, 2, dies after being hit by mum’s car’, AsiaOne

Having discussed the characteristics and behaviour of road users, we will further discuss how their interactions with vehicular and environmental factors result in traffic collisions.

Vehicular Factors

Vehicular factors are all vehicle-related characteristics, deficiencies and defects, as well as associated motorist’s interactions and response that result in an accident or increase the severity of vehicle impact.

Key Components of a Vehicle

The key components for the safe operation of a vehicle are its brakes, steering system and the less-understood suspension system. Vehicle characteristics include vehicle type (e.g. two-wheeler or four-wheeler), vehicle dimensions, weight, tyre tread depth and wear, wheel base and wheel track, turning radius, anti-lock braking systems, and headlamp intensity and brightness.

Design or Manufacturing Defects and Vehicle Recalls

When a vehicle manufacturer determines that a vehicle or vehicle part creates a safety risk or fails to meet minimum safety standards, a recall is issued for the manufacturer to rectify the defect. The biggest vehicle recall in history involved airbag manufacturer Takata,¹³Consumer Reports (2019, March 29) Takata Airbag Recall: Everything You Need to Know. What this recall means to you and what actions you should take. https://www.consumerreports.org/car-recalls-defects/takata-airbag-recall-everything-you-need-to-know and affected more than 40 million vehicles worldwide.

Mechanical Defects in Vehicular Parts Due to Wear and Tear

Mechanical problems that may lead to traffic collision include defective brakes, steering failure, faulty headlights or signal lights, oversized or unevenly distributed loads, and tyre failure. Common forms of tyre failure are air pressure loss, blow-out and tread separation. Contrary to common belief, a dysfunctional tyre does not inevitably cause loss of vehicle control as the majority of drivers are able to respond safely to a tyre failure.

Loss of Vehicle Control

Wet roads and dangerous road contaminants such as sand and oil pose a potential risk to motorists because they can lead to loss of vehicular control if extreme manoeuvres such as hard braking and sudden swerving are attempted. More importantly, the motorist may not have prior experience with driving on a contaminated road surface and may not understand its effects on the control of his vehicle. Braking gently is counter-intuitive to him when he encounters a slippery road surface; little does he know that hard braking would cause his vehicle to skid.

Vehicle Blind Zones

Blind zone accidents can cause severe injuries to small-sized, less conspicuous vulnerable road users such as pedestrians, cyclists, motorcyclists and personal mobility device (PMD) users. A blind zone or spot is an area of the road outside the driver’s field of vision that the driver cannot see through the windscreen and windows or in the side and rear-view mirrors. Blind zones occur in front, at the rear and sides of a vehicle. They depend on the heights of the driver as well as person or object to be seen by the driver. Their size and location vary according to the type, geometrical shape and structure of the vehicle. A driver with lower ocular (eye) height would have a larger blind zone around the vehicle. Blind zones are enlarged by visual obscuration (obstruction) due to the side-view (wing) mirrors, pillars and head-rests of the vehicle. Front blind zones are larger when the driver’s cabin is higher, the bonnet is longer, and when the A-pillars are thicker. Vehicles with no rear windows or with rear views visually obstructed by cargo as in large high trucks, vans and construction vehicles are particularly dangerous when reversing (backing). Fatal accidents when vehicles with limited rear visibility back into people affect especially small children and the elderly. 

Environmental Factors

Environmental factors refer to elements present in the interior of the vehicle as well as the roadway and its environs. A driver’s response may be affected by distractions present in a vehicle such as navigational aids, internal controls for radio, air-conditioning, window and mirror adjustments, as well as conversation or arguments with passengers and young children. Research has also reported that loud music can have negative effects similar to those of noise on a driver’s reaction time and performance. Distracted driving while using the mobile phone is also not uncommon. The roadway environment comprises features of the roadway and the ambient environment such as weather and darkness.

Roadway Characteristics and Structures

Roadway characteristics include traffic density, road dimensions, geometry and alignment (slopes, straight or curved road), and road surface condition (newly surfaced or old, potholes, fallen objects). Roadside structures which may affect the severity of impact and injury include street lights and traffic light posts, guardrails, traffic signs and control devices, trees, buildings or walls, kerbs, fences, railings or barriers, earth embankment, drains, fire hydrants, and other fixed objects. Construction works on roadways and temporary alteration of the original road geometry may increase collision risk when they involve road diversions and bends, relocation of traffic lights, as well as cordons, barriers and equipment which may affect visibility.

Visibility and Conspicuity

Limited visibility is commonly cited as a factor that affects a driver’s performance. Poor visibility can result from poor illumination for night scenes or disability glare caused by the headlights of other vehicles. Closely associated with visibility is background contrast and conspicuity of an object, vehicle or pedestrian. For example, during the night, against a dark background, a pedestrian dressed in light-coloured clothing is more easily visible than one who is dressed in dark-coloured clothing.

Other Environmental Factors

Other commonly cited environmental factors that may contribute to a traffic collision are:

1. visual clutter, obstructed or limited view (line of sight),
2. improper or non-working traffic controls,
3. presence of potholes, deep ruts or bumps, road under construction,
4. presence of contaminants (oil, sand, gravel) and debris on the roadway,
5. wet and slippery surface, water pooling,
6. improperly parked vehicle on road shoulder, and
7. unexpected intrusion of an animal.

Black Spots

Advances in traffic engineering and road design have improved road geometry and the safety of the driving environment. One component of road safety improvement is the identification and analysis of accident-prone locations – black spots which are over-represented in accident frequency and severity. Following several fatal pedestrian and vehicular T-bone (broadside) accidents at signalised junctions allowing discretionary right turns, the Land Transport Authority decided to phase out discretionary right turns and implement red-amber-green arrows signaling at most traffic junctions by 2023, meaning motorists can only turn right on a green arrow.¹⁶Co, C. & Mahmud, A. (2019, July 8). Scrap discretionary right turns at all junctions soon, MPs urge as Road Traffic Act amendments passed. CNA. https://www.channelnewsasia.com/news/singapore/remove-discretionary-right-turns-road-traffic-act-11702060.

Conclusions

Road traffic systems involve dynamic situations that change rapidly and unexpectedly. Factors contributing to traffic accidents can be classified in three broad categories: human factors, vehicle factors and environmental factors. In a traffic conflict, we are evaluating the road user’s performance and role in the collision by examining his abilities and limitations, his interactions with other road users, vehicles and the environment. Until the day self-driving vehicles become a norm on our roads, human vision, perception, judgement and performance will remain key aspects of driving.

A systematic, holistic and balanced approach must be adopted to reliably identify the causal or contributing factors. Thus, it is essential to consider in totality the various circumstances and events leading to the collision.