Until recently, automotive safety efforts have focused on reducing the likelihood of occupant injuries or fatalities during a collision. In the past decade, however, the automotive world has shifted its focus to active safety: systems designed to prevent collisions in the first place.
Electronic Stability Control (ESC) is the first active safety system that assists in directional control. It was developed alongside anti-lock braking systems (ABS), which are an integral component of ESC and were the first active safety systems to assist with speed control. ESC first appeared in production vehicles in 1995 and all vehicles manufactured after September 2011 must be equipped with an ESC system.
Stability control systems work by controlling braking and, in many vehicles, power transmission at individual wheels. The system is designed to prevent the vehicle from turning faster or slower than the driver is steering. In other words, ESC detects if the driver is losing directional control and applies a combination of braking at some wheels and power at others to regain control.
Figure 1. ESC system components.
Every new vehicle with ESC must pass a double lane change maneuver test prescribed by the National Highway Traffic Safety Administration (NHTSA). This test requires each vehicle to undergo a prescribed steering pattern at 80 km/h (50 mph) without going into an uncontrolled spin.
A recent and comprehensive review of ESC effectiveness in the US found that fatalities were 18% lower for vehicles equipped with ESC compared to similar vehicles without ESC. The risk of fatality was 49% lower when considering only single-vehicle collisions (Sivinski, 2011). A similar study in Canada showed that ESC-equipped vehicles were involved in 41% fewer loss-of-control type collisions than similar vehicles without ESC. The reduction increased to 51% on roads covered with ice, snow and slush (Chouinard et al, 2011).
Although the success of ESC in preventing crashes is clear, its effect on reconstructing crashes is more complicated. When investigating a collision where ESC was active, reconstructionists need to consider that the ESC system may have contributed to the vehicle’s behavior at, as well as after, the onset of control loss. This could be relevant in cases where a collision occurs in an oncoming lane, or a collision occurs with objects on the side of the road following a control loss. Vehicles equipped with ESC may also leave tire marks that are different than the yaw marks deposited by non-ESC equipped vehicle. Misinterpreting these marks could lead to incorrect conclusions about vehicle speeds and driver steering behavior if traditional reconstruction methods are applied blindly.
ESC is the first active safety technology to influence the way we investigate collisions, but there are many more to come.
Chouinard A, Lécuyer JF (2011). A study of the effectiveness of Electronic Stability Control in Canada. Accident Analysis and Prevention 43:451-460.
Sivinski, R (2011). Crash Prevention Effectiveness of Light-Vehicle Electronic Stability Control: An Update of the 2007 NHTSA Evaluation (DOT HS 811 486). National Highway Traffic Safety Administration. Washington, DC