4.4 Reassessing Crash Causation Factors and Crash Types

The Safe System approach provides a different way to look at crash causation, and at the key crash types that contribute to fatal and serious injury.

Crash Causation

The traditional understanding of crash causation was based on the belief driver or other road user error was the primary cause of most crashes. This led to a focus on addressing human behaviour as the main solution. As discussed in Chapter 8. Designing For Road User Characteristics and Compliance , it is important to recognize that human behaviour is influenced by various factors, and simply changing this behaviour may not be sufficient to prevent road crashes. A more holistic approach that considers the interaction between human, environment, and vehicle factors is needed to address the underlying causes of crashes.

The following example and Table 4.1 illustrate this complex interaction.

Example: A 20-year-old man is driving to an important appointment. Rain makes the road slippery, and his tires are not in good condition. While travelling, he enters a curve with a radius of curvature that is less than the minimum standard, loses control and runs off the road into a tree at the roadside.

This example illustrates the following principles:

• Each crash is the result of a succession of events occurring under specific circumstances.
• Each event can be related to one of the components of the Safe System.
• An unfavourable concurrence of events is the explanatory cause of the system.
• Each separate event is largely determined by the preceding events and their circumstances.

Thus, the human-environment-vehicle system provides a conceptual framework for analysing the road crash to identify factors on which to intervene. The Safe System approach is based on the principle that humans make mistakes, but these mistakes should not result in fatality or serious injury. Therefore, human, environmental, and vehicle factors must be addressed with road user limitations in mind, aiming to prevent errors as much as possible (‘self-explaining system’) and mitigate their consequences when they occur (‘forgiving system’).

The human-environment-vehicle system may be represented by the Haddon matrix (Table 4.2), which combines the three components of the system and the three phases of a road crash (before, during, after).

Kimber (2003) suggests past post-crash assessments of crash contribution by researchers resulted in too great a focus on driver behaviour at the time the data was collected. Interventions with potentially greater effect were easily overlooked. Driver behaviour is a wide category, and it was easy to populate it by default when the evidence was incomplete, or a better explanation was not available. Due to this perceived driver failure predominance, the main priority for many years was to concentrate on measures to change driver behaviour (rather than focusing on reengineering other parts of the road, vehicle or driver system) to eliminate the failures.

This mindset is changing, but the misguided focus on the significance of driver error still remains predominant in too much of the thinking across international communities.

Human error is to be expected. It is unhelpful to regard all human error as being capable of somehow being eliminated and the consequences of it therefore being avoided. When the circumstances of road and vehicle allow, routine driver errors translate into collisions, sometimes with injury or death resulting. A focus on the infrastructure and vehicle safety levels that interact with routine driver error is a much more useful means of identifying actions to reduce serious casualty outcomes.

Elvik & Vaa (2004) indicate that, even if all road users complied with all road rules, fatalities would only fall by around 60% and injuries by 40%. Specifically, they note that around 37% of fatalities and 63% of serious injuries do not involve non-compliance with road rules. This indicates that routine human error leading to crashes, rather than deliberate or unintentional breaking of road rules, is a feature of human existence and road use.

While achieving compliance with road rules by road users remains critically important, this approach alone will not achieve the desired road safety gains in any country.

As practitioners from LMICs recognise, there is often a lower level of compliance with road rules and a lesser respect for the rule of law in most LMICs than for many HICs. This deliberate reluctance to act in accordance with the law would affect fatality and serious injury rates in these countries and this is one key difference in comparison to serious crash experience in many HICs.

While a substantial potential benefit is available in the medium term through changing this illegal road user behaviour in LMICs, a further focus upon improvements in infrastructure and vehicle safety over in the medium to longer term will be essential in providing a forgiving system (a Safe System) for crashes arising from underlying (and not illegal) human error, as is the current and strengthening focus in many HICs.

Therefore, research findings from HICs about the role of safer roads and vehicles are very relevant to LMICs.

Stigson (2011) conducted further analysis on the different weaknesses in the traffic system’s traditional components and determined that they play a greater or lesser role in influencing the outcome of crashes. The analysis confirms the potential for road infrastructure to more substantially impact upon fatal crash outcomes for car occupants than other factors in HICs.

While system weaknesses should usefully be analysed for two-wheeler, cyclists and pedestrians in HICs and also in LMICs, vehicle safety improvement (as users shift over the next decades from two wheelers to vehicles) and improved behavioural compliance will offer great opportunities for reducing crashes in most LMICs compared to most HICs. However, the potential contribution of safer infrastructure to substantially reduce fatalities should be reinforced with all road authorities.

Key Crash Types

The focus on reducing fatal and serious injuries through the Safe System approach (as opposed to casualty crash reduction) has had a profound impact on understanding key crash types.

The shift in focus from crash numbers to casualties has a subtle but important impact on crash assessment and the strategies to address risk. Different types of crashes will produce different crash outcomes, including greater or lesser numbers of injuries per crash. For example, analysis from New Zealand on crashes and casualties demonstrates this point. Table 4.3 (NZTA, 2011) shows the proportions of the three most common types of crashes and casualties that result in fatalities and serious injuries for rural roads (excluding motorways).

Table 4.3 shows that 92% of fatal and serious injuries on rural roads in New Zealand are attributed to the three key crash types. The table also shows that a higher proportion (1.3 to 1) of severe casualties (fatal and serious injuries) occur through head-on crashes (25%) than is reflected in the proportion of severe head-on crashes (19%). This indicates that the head-on crash type is of greater significance for overall fatalities than the severe crash numbers would indicate.

Similarly, analysis of crashes by fatal and serious injury (compared to all injuries) is likely to provide a different picture of the risk across the network.

The relative incidence of various fatal and serious crash types will differ from most HICs to most LMICs due to the differences in traffic environments. The types of vehicles and their relative share of the overall traffic volume are two examples of likely difference. It is essential for the road safety agencies and road authorities to know what the major crash types are in their country and where they occur (also see Chapter 7. Roles, Responsibilities, Policy Development and Programmes and Chapter 10. Assessing Potential Risks and Identifying Issues ). Agencies should be able to identify road sections with a higher risk of fatalities and serious injuries on their network.

The predominant road crash types that result in fatalities and serious injuries in high-income countries are typically:

• Crashes involving vehicle occupants only:
  • Head-on (between two vehicles) crashes (most often on rural roads).
  • Run-off-road crashes (most often on rural roads).
  • Intersection crashes (particularly side impact) (most often on urban roads).
• Crashes involving vehicles and vulnerable road users (involving pedestrians, motorcyclists and cyclists).

For many low- and middle-income countries, key crash types include:

• Pedestrian/vehicle crashes (most often on urban roads and within sections of linear urban development).
• Motorcyclist (single vehicle or with another vehicle) involved crashes (most often on rural roads).
• Light passenger van (single vehicle or with another vehicle) involved crashes (most often on rural roads).
• Truck and bus involved crashes with another vehicle, particularly head-on and rear-end crashes with motorcycles (most often on rural roads).
• Cyclist/vehicle crashes (to varying degrees in low- and middle-income countries) (most often on urban roads).

The case study from India discusses the implementation of a speed regulation system.