On average, a person is killed in a traffic crash about every 15 minutes in the United States. Although the number of traffic fatalities has come down over recent years, the current numbers average about 100 traffic fatalities a day, according to data from the National Highway Traffic Safety Administration. 

Traffic safety remains a stubbornly persistent challenge all across the country. 

Applying different thinking is often the best way to tackle the most challenging of problems. The safe system paradigm shift, which began in Europe nearly 30 years ago and has gained momentum internationally and within the United States, offers a new perspective. A fundamental premise of safe system thinking is that people will inevitably make mistakes while using the roads. This is true whether they are driving, walking or using some other mode. 

We will make mistakes, and those mistakes may result in traffic crashes. Transportation agencies are rethinking how roads are designed, built and operated, moving toward a system that manages crash forces to reduce the risk of death and serious injury.

Safe system thinking elucidates that eliminating fatalities and serious injuries is a far different objective from reducing crashes. As an engineer, this understanding reoriented my thinking in two important ways. First, I acknowledged the inevitability that traffic crashes will happen because no matter how successful a countermeasure is at reducing human error it will never eradicate it. Second, I realized that to eliminate fatal and serious outcomes, attention needs to shift toward understanding human abilities to absorb the transfer of crash kinetic energy. In other words, my thinking shifted from focusing on crash reduction countermeasures to a crash severity management focus.

Here’s another way of thinking about this shift. Nobody wants to be involved in a car crash. Even a minor “fender-bender” will ruin your day. But unlike vehicles, which can be repaired or replaced if damaged, our human bodies are far more precious, and the value of a human life is immeasurable. That’s why considering crash severity is such a critical factor in traffic safety. Reducing crash severity is the important metric.

Engineers have traditionally focused on crash frequencies and crash severities with the objective of reducing the former across all severities. However, some safety strategies may have different crash reduction influences across different crash severities. In fact, some strategies may increase the frequency of minor crashes even though they may greatly reduce the frequency of severe crashes.

For example, changing from a signalized intersection to a multi-lane roundabout may increase the total frequency of crashes. However, because roundabouts are effective at slowing speeds, the crashes that inevitably occur are rarely severe. Building more roundabouts is a great example of what a change in thinking about safety looks like. It’s a focus on what matters most, preventing the crashes that result in deaths or serious injuries.

Many transportation agencies assign dollar values to different crash types based on severity. These dollar values are used for benefit-cost calculations and performance evaluations in project safety analyses. Even though the value of a human life is beyond measure, for the purpose of benefit-cost calculations a value for fatal, serious injury, minor injury and property damage only crashes must be assigned based on societal and economic impacts. 

For example, when calculating crash costs many agencies use values between $12-16 million for a fatal crash and between $30,000-50,000 for a no-injury property damage-only crash. So, from an economic perspective, it would take more than 400 no-injury crashes to equal the value of a fatal crash. By applying this perspective to a proposed multilane roundabout that could potentially increase total crashes, it would take an increase of more than 400 no-injury crashes to offset the trade-off to prevent only one fatal crash. It’s about focusing on what matters most.

Another core principle of safe system thinking is that safety should be proactive. This means ‘building in’ safety through roadway design and operational practices that eliminate the most severe traffic conflicts and promote vehicular travel speeds that are safe and appropriate for the context and type of roadway facility being designed.

Safety is a consideration in most phases of the transportation project development process. But it is in the earlier stages of concept development and preliminary design where there are greater opportunities to implement features that can meaningfully reduce the likelihood of roadway fatalities and serious injuries. Assessing safety performance in design is about applying various methods to quantitatively examine the differences among project alternatives, or optional design features within alternatives, that may reduce crash frequency and severity.

Applying safe system thinking also means assessing traffic safety through the lenses of exposure, crash likelihood and crash outcome severity. Exposure involves considering crash potential through metrics such as road user numbers (i.e., traffic volumes and pedestrian counts) and the distance or duration of exposure (i.e., pedestrian crossing distances). Considering quantifiable exposure factors helps prioritize where greater attention toward eliminating severe conflicts is appropriate.

Likelihood refers to assessing certain road conditions or factors that have been demonstrated to affect the probability of a crash occurring. These factors are often associated with conditions that add complexity to the decision-making needs of road users. A common example is when a driver is turning left at an intersection and must judge the gaps in oncoming traffic. The complexity is increased when there is more than one lane of oncoming traffic and when vehicle speeds are high. Strategies such as using a “protected-left turn” (green left-turn arrow) signal indication reduce the potential for fatal and serious injury crashes.

If a crash does occur at a conflict point, the severity of the outcome depends on the levels of crash kinetic energy transferred to the human body. Safe system thinking places particular emphasis on avoiding fatal and serious crashes. 

By including crash outcome severity as an assessment factor, traffic conflicts with a low probability of injury if a crash occurs become less of a priority. Assessing traffic conflicts through metrics tied to exposure, likelihood and crash severity is a great example of how applying Safe System thinking in design can help transportation agencies achieve their long-term safety goals.

Although more transportation agencies are embracing safe system principles, a significant obstacle is a limited public understanding of the benefits of certain infrastructure changes. Some safety strategies are perceived as an inconvenience if they involve trade-offs like slower speeds or redirected turning movements and they may even provoke public opposition. Creating a shared common understanding of the traffic safety problem and collaborating with the community to foster public acceptance on safety priorities is key to overcoming this challenge.

Mark Doctor, P.E., PTOE, RSP1, is a senior engineer at Gresham Smith.