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Traffic jams

Adapting to road conditions

Jul 1st 2004
From The Economist print edition

The spread of adaptive cruise control may bring an unexpected bonus

Adapt or die—of boredom

SITTING in stationary traffic is, at best, a Zen experience. Drivers mired in a jam learn to cede control to the powers that be, becoming at one with the universe as they breathe in the mind-numbing fumes all around. At worst, it is an ongoing battle for sanity. But now, according to several groups of researchers in America and Germany, there is something that drivers can do to take back control over the roads. Get adaptive cruise control. And, of course, use it.

Adaptive cruise control (ACC), as its name suggests, is a modified version of traditional cruise control. It employs radar to monitor the road ahead of a vehicle, automatically adjusting that vehicle's speed to maintain a safe distance from the one in front. This is safer than manual driving because it reduces the system's reaction time from nearly a second (human) to practically instantaneous (machine), thus helping to forestall shunts. But ACC may have a useful side-effect, arising from the fact that another effect of slow human reaction times is to produce traffic jams on apparently open roads.

Such jams start when a car slows suddenly to allow, for example, another vehicle to enter the traffic stream. Slow reaction times mean that instead of responding smoothly, the drivers behind such a vehicle often end up slamming on the brakes. That slamming propagates backwards, and before long the traffic is at a standstill. So it makes sense that ACC would reduce not only collisions, but also congestion. What is unexpected is how few vehicles need to have it operating for all to benefit. As Craig Davis of the University of Michigan reports in Physical Review E, only 20% of cars need to employ ACC in order to prevent completely those jams that are caused by a slow lead car on a high-speed, single-lane road. According to Dr Davis's computer model, even a rate of use of ACC as low as 13% can improve the flow of traffic significantly.

Drivers wanted
Mar 11th 2004
Beyond cruise control
Jun 21st 2001
Future car technologies
Nov 26th 1998

The motor industry

Howstuffworks.com explains the basics of cruise control and links to online resources on the subject. The report by Craig Davis and colleagues is published in Physical Review E. Martin Treiber's homepage outlines his research on traffic jams.

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ACC is not a panacea. Dr Davis got less promising results for more complex road conditions, particularly those near junctions. And a similar model built by Boris Kerner, a researcher for DaimlerChrysler, in Stuttgart, Germany, indicates that in certain bottleneck conditions, ACC may even cause extra congestion. These disappointing results can, however, be ameliorated by shortening the “headway” in ACC-equipped vehicles, according to Martin Treiber of the Dresden University of Technology, also in Germany.

Headway is the gap, measured in seconds, that a driver puts between himself and the car ahead. Since ACC reacts more quickly than a human, people who have it fitted can afford to allow less headway.

In his simulation, Dr Davis instructed both his ACC-equipped and his manually driven cars to leave about 1.1 seconds of headway (equivalent to 28 metres at about 100km an hour). Dr Kerner allowed for as much as 1.8 seconds, the distance at which drivers in Germany are taught to follow. But Dr Treiber says 0.9 seconds is safe and realistic. Using this headway in their simulations, he and his colleague Dirk Helbing saw big improvements in traffic flow even in the most dire road conditions. The question remains, of course, of whether real drivers will trust the technology sufficiently to allow it to follow the car in front so closely.

ACC has been an option in a number of luxury vehicles for several years. Now it is becoming available in mid-range cars. But its presence on the roads is still limited, and many people who have it do not use it. If drivers could be convinced to fit ACC and use it, it would not only improve their own driving experience, but also that of their fellow motorists.

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