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Center for Transportation Studies

Maintaining Safe Headways while Driving

Presentation by Dr. David Shinar, Ben Gurion University of the Negev, Israel

September 28, 2004

For drivers, maintaining a safe headway between their vehicle and the next vehicle ahead is a critical cognitive task. For human factors researchers, understanding how drivers keep their distance--and why they fail to do so--is an ongoing challenge with obvious implications for improving roadway safety.

Dr. David Shinar has been studying how drivers maintain vehicle headway for more than ten years. Shinar oversees a human factors research program at Israel's Ben Gurion University of the Negev with more than 40 staff members. At a seminar on September 28, 2004, he outlined several experiments recently carried out on how drivers maintain safe headways, and how headway skills could potentially be improved.

Crashes due to insufficient vehicle headway, Shinar stated, account for a significant portion of all crashes: over 29 percent in the United States. In Israel, where statistics are available for injury crashes only, this type of crash accounts for roughly 13 percent of injury crashes.

Shinar outlined diverse strategies used by transportation agencies around the world to encourage drivers to maintain safe headways. These include various types of signage, some simply advising drivers to keep a "safe distance" and others using more sophisticated means such as pavement markings to help drivers determine what a safe distance is. Although these pavement marking schemes may be moderately helpful, Shinar noted that the fixed distance between markings does not account for the shorter time gap between vehicles as speeds increase.

In Israel, a novel strategy developed with input from high-school students involves bumper stickers printed with numbers "5 7 9" in increasing sizes. The idea is that a number should be barely legible at a corresponding speeed; for example, a driver traveling at 50 kilometers per hour should just be able to read the numeral "5" on the bumper of the car in front of her.

While such methods may offer some safety benefits, they leave open the central questions of how drivers choose and maintain a headway distance, and how their performance of this task can be improved.

Shinar outlined a small-scale field experiment in which drivers were instructed to follow a control vehicle and maintain "comfortable" and "minimum safe" headway distances at various times, while the distances were tracked by a laser rangefinder. He found that drivers are in fact very good at maintaining a constant time headway, but that the minimum safe headway they maintained was often too short for safety - an average of 0.7 seconds, versus the one or two seconds recommended in Israeli laws and safety standards. Many drivers consistently maintained a headway of less than half a second, a situation the researchers riding with them found alarming.

In a second study, researchers used pavement-embedded loop detectors to observe the behavior of drivers in multiple lanes on a Tel Aviv freeway. Observations were restricted to pairs of vehicles separated by less than 2.5 seconds, which were deemed to be in "car-following mode." Observations in this study confirmed the finding of the control-vehicle research that time headways were too small for optimum safety.

A third study focused on the question of whether drivers can learn to improve their headways while driving by using a "headway-o-meter" consisting of a laser rangefinder attached to a warning display. The researchers were especially concerned about the effects of such a device if it was not completely reliable, so they designed the device to operate at varying levels of reliability from 60 to 95 percent, set without the knowledge of the driver.

Participants were instructed to drive a fixed 70 km route on the freeway, reaching their destination in a minimum time without exceeding the 100 k.p.h. speed limit, maintaining a headway of greater than one second at all times, and passing other vehicles only when legal. A buzzer sounded if the distance to the vehicle in front dropped below 1.2 seconds (subject to the reliability setting of the headway meter). Drivers were then instructed to continue for a further 20 km without feedback, while the rangefinder measured their headway.

Results showed that feedback increased headway distances, and that there was no discernable difference between males and females. In a subsequent test six months later, the drivers reproduced the same headway profiles established after exposure to the headway feedback, demonstrating that good distance keeping could be learned and retained, even when using a rangefinder with imperfect feedback performance.

Finally, in a fourth headway experiment, 43 drivers using company cars were first monitored for three weeks without feedback, then given feedback on their headways for a second three weeks. Feedback was again found to reduce the incidence of short headway distances, and increase the incidence of longer distances.

In terms of future work in this area, Shinar highlighted several potentially fertile areas of exploration, including interactive inter-vehicle communication to signal short headways, or the use of adaptive infrastructure-based techniques such as projecting lights on the wall of a tunnel to help regulate distances between vehicles. Because Adaptive Cruise Control (ACC) technology assumes the existence of a minimum safe headway, Shinar said, it is another area that is ripe for experimentation.