Wolfsburg, Germany – We are racing down the autobahn at 230 km/h in a Volkswagen Phaeton when a car slides into the lane ahead, prompting our luxurious Volkswagen sedan to gently slow itself down enough to maintain a safe distance between us and the car in front.

Magic? Not at all. This is adaptive cruise control or ACC at work. Using radar or a laser beam to monitor the traffic ahead, ACC adjusts vehicle speed without direct driver input when it senses a vehicle – or perhaps some other object – ahead. It is all automatic and instantaneous. ACC is important because it not only adds a new safety feature to the electronic safety arsenal already on board today’s automobile, it just might help reduce traffic jams, too.

According to computer simulations developed by Craig Davis of the University of Michigan, if 20 per cent of cars were equipped with ACC, traffic jams caused by a slow lead car on a high-speed, single-lane road would be prevented. Lower rates of ACC in vehicles would definitely improve traffic flow, he reports in Physical Review E.

In a nutshell, ACC can have a positive effect on traffic because as a speed control device it is far faster than the one-second reactions of the typical human driver manually pushing on a brake pedal. Slow human reaction times create a kind of accordion effect in traffic as drivers adjust to various flows and patterns that include merging vehicles. The result: traffic jams.

Of course, ACC wasn’t invented and it certainly is not being perfected to keep traffic flowing, though drivers on the 401 at 5 p.m. surely won’t complain. The automakers market ACC as both a safety and a convenience feature for highway drivers. The systems currently on the market are mostly confined to luxury cars.

All ACC systems are designed to sense a vehicle ahead and adjust the throttle, and even brake accordingly. The idea is automatically to keep your vehicle at a safe distance from the one ahead, although if the closing speed is too great, typically an alarm warns you to make an aggressive safety manoeuvre.

While there are differences among the various system for sale today, as a general rule there are two main types: laser and radar. The radar-based systems favoured by the Europeans bounce microwaves off the target vehicle. The laser types preferred by Japanese manufacturers key off the reflection of the vehicle ahead.

Proponents of radar-based systems say they are better because they work in heavy rain, dense fog and even when it is snowing. However, no one should be using any sort of cruise control device in poor weather conditions, anyway.

Regardless of the system, ACC scans the lane ahead, looking for a target which will trigger the computer brain to adjust vehicle speed to maintain a safe driving distance. All are designed to limit the scanned width ahead to about one driving lane, and to avoid looking too far into the lane of oncoming traffic in a turn. Systems that scan too broadly or too far down the road will needlessly activate a braking or throttle response.

For the technically minded, here’s how ACC works: headway sensors normally located in the front grille area of a vehicle collect information that is sent to a digital signal processor. It translates the speed and distance information for a longitudinal controller which orders up the necessary braking or throttle adjustments. When the road is clear, the system will re-accelerate the vehicle back to the set speed.

In practice, the ACC systems in use today work very well – so well they even read motorcycles ahead. On highways with fairly dense traffic they are useful at maintaining the best and safest speed and distance in traffic, while minimizing the number of driver inputs. The systems also shut off below a certain speed — about 30 km/h — thus there is no danger of racing right through a stop sign or the like.

Still, independent groups like the Insurance Institute for Highway Safety say ACC and other so-called “active safety” devices are difficult to test for their effectiveness. So until ACC has been available for a period long enough for a critical amount of data to have been generated in real-world experiences, researchers can only predict the benefits using computer simulations and, of course, track testing in research and development centres.

Indeed, Craig Davis’s results were computer generated, but they still had us dreaming about the end of traffic jams.

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