Auto safety and human adaptation

Issues in Science and Technology, Winter 2000/2001 by Smiley, Alison

Adaptation is intrinsically human. It is one of our most valuable characteristics and the reason why a human presence is desirable in monitoring even the most highly automated systems: to adapt to and therefore deal with the unexpected. Adaptation is a manifestation of intelligent behavior.

However, engineers who develop new devices to assist drivers frequently assume that drivers will not change their behavior. For example, when anti-lock brakes were introduced, predictions about their impact on safety were based on the assumption that only stopping distance and directional control during braking would change; speed and headways would not be affected. But that has proved incorrect.

Why do engineers make such assumptions? According to Ezra Hauer, a civil engineering professor at the University of Toronto, engineers are trained to deal with the characteristics of inanimate matter such as loads, flows, stress, strain, and so forth. Once the physics of the situation and the properties of the materials are understood, engineers can predict fairly well what will happen and make the corresponding design choices. But drivers adapt, and speed and headway choices and reaction times cannot be considered to be invariant quantities that remain the same once the roadway or the vehicle has changed. That adaptation will occur is predictable. We should be more surprised by its absence.

Unfulfilled predictions

A prime example of unfulfilled predictions because of adaptation is anti-lock braking. In early proof-of-concept studies, test drivers drove at a designated speed and then braked. Not surprisingly, braking distances were found to decrease on wet surfaces. Moreover, directional control was maintained during braking on wet or dry surfaces. Based on such studies, optimistic predictions were made. For example, one German engineer concluded that the universal adoption of anti-lock brakes in Germany would result in a 10 to 15 percent reduction in accidents involving heavy damages and/or injuries.

Later studies considered the possibility of adaptation. A test track study showed that when drivers could choose their speed, they traveled slightly faster after practicing with anti-lock brakes on wet surfaces, with the result that emergency stopping distance was no different than with standard brakes. Other researchers observed 213 taxi drivers en route to an airport and likely to be pressed for time. Drivers whose vehicles were equipped with anti-lock brakes were found to allow significantly shorter headways to the vehicles in front of them.

How was safety affected? In an extensive study, the Highway Loss Data Institute compared claim frequency and size of 1991 models without anti-lock brakes to those of 1992 models with the system. No significant differences were found in either claim frequency (8 per 100 vehicles) or size (an average of $2,215 per 1991 model claim versus $2,293 per 1992 claim). Researchers then examined a subsample from the northern states in the winter and still found no significant differences. Based on the performance studies and on this crash rate study, it appears that drivers with anti-lock brakes adapted by trading off safety for mobility to the extent that there was no safety benefit-a far cry from the predicted 10 to 15 percent improvement.