June 4, 2013
Bill Howard

What is adaptive cruise control, and how does it work?

Here at ExtremeTech we see automobiles as much more than just four wheels, an engine, and a few seats. We view automobiles as being the ultimate mobile technology platform and something as worthy of our attention as the latest CPU or smartphone. With that in mind, we’ll be releasing a series of introductory auto tech articles, providing readers with in-depth explanations of today’s important technologies. First up for the series: adaptive cruise control.

Adaptive cruise control basics

Adaptive cruise control (ACC) is an intelligent form of cruise control that slows down and speeds up automatically to keep pace with the car in front of you. The driver sets the maximum speed — just as with cruise control — then a radar sensor watches for traffic ahead, locks on to the car in a lane, and instructs the car to stay 2, 3, or 4 seconds behind the person car ahead of it (the driver sets the follow distance, within reason). ACC is now almost always paired with a pre-crash system that alerts you and often begins braking.

ACC is ideal for stop-and-go traffic and rush hour commuting that swings from 60 mph to a standstill. Adaptive cruise control as of 2013 ranges from $2,500 at the high end to as little as $500. Less costly “partial ACC” only works at speeds of 20 or 25 mph and up, but it’s markedly cheaper.

Expect to pay $2,000-$2,500 for full-range adaptive cruse control, but the price is coming down. The first ACC systems were about $2,800 five years ago.

Adaptive cruise control radar illlustration (Mercedes)

Adaptive cruise control is also called active cruise control, autonomous cruise control, intelligent cruise control, or radar cruise control. This is the case because distance is measured by a small radar unit behind the front grille or under the bumper. Some units employ a laser, while Subaru uses an optical system  based on stereoscopic cameras. Regardless of the technology, ACC works day and night, but its abilities are hampered by heavy rain, fog, or snow.

ACC is a crucial part of the self-driving cars of the near future. On an autonomous driving car, ACC needs to track the car in front but also cars in adjacent lens in case a lane change becomes necessary.

2010 Ford Taurus

Adaptive cruise control is typically paired with forward collision warning that functions even if you don’t have ACC engaged. When ACC is engaged, the car will typically slow under ACC braking at up to half its maximum braking potential. (Beyond that, driver and passenger discomfort with automated braking sets in.) Red lights flash at the driver (as with the Ford Taurus pictured above), the words “Brake!” or “Brake Now!” show on the instrument panel or head-up display, and a loud chime sounds. When ACC isn’t engaged, it’s still tracking traffic in front and intervenes with the warnings if it senses a potential accident.

Using ACC

Mercedes-Benz Distronic PlusTo use adaptive cruise control, you start the same as you would with standard cruise control. The driver turns ACC on, accelerates to the desired speed, then presses the “Set” button. It’s then possible to tweak the “+” and “-” buttons to raise or lower the speed, typically by in 1 or 5 mph increments. Lastly, the driver can set the desired gap behind the next car, most commonly by pressing a button to cycle among short, medium, and long following distances. Some automakers show icons with 1, 2 or 3 distance bars between two vehicle icons. Others, such as Merecedes-Benz, show the following distance in feet, though it’s really in seconds of following gap translated to feet — for example, 200 feet of following distance at 60 mph (88 feet per second) is about 3 seconds.

An indicator in the instrument panel or head-up display shows a car icon and often what looks like converging-at-infinity lines, indicating the roadway. When radar detects a car ahead, a second car icon appears or the lone car icon changes color.

When you’re just starting out in a newly acquired car with ACC, start with the longest following distance. If you set the closest following distance, you’ll get nervous if the following distance seems to get dangerously close and you’re not sure if ACC is actually working. Most likely it is working and the driver may have lightly brushed the brake pedal, didn’t realize it, and now ACC is available but not engaged.

Next page: The technology behind adaptive cruise control…

The technology behind ACC

Adaptive cruise control typically uses radar in a frequency band that doesn’t compete with police radar and doesn’t trigger radar detectors. For full-range ACC, some automakers use two radars — one for close range out to about 100 feet and a second that sees out to about 600 feet, or about 6-7 seconds at highway speeds. Partial ACC is usually a single unit, while some full-range ACC implementations are now able to use a single radar as well.

Early ACC units were a competing mix of laser on some cars and radar on the others. Radar won out because it works better in bad weather and costs came down to be competitive with laser. Even so, some ACC is optical. Subaru uses two cameras flanking the rear view mirror for its EyeSight system. It also provides unassisted automatic braking at low speeds if a pedestrian or stopped car gets in your path.

ACC dual radar MB S-Class 2008

The effectiveness of even radar-based ACC is compromised in bad weather. In heavy rain or snow, it will shut off (you get a warning) or if the sensor in the grille or under the bumper is caked with snow or dirt.

So far, adaptive cruise control doesn’t adjust to changing speed limits. Technology exists to do that: speed limit info is in navigation system map data, and lane departure warning cameras have the ability to read speed limit signs. In theory, you could tell ACC that at highway speeds you want your top speed to be the posted limit plus 5 mph. Then when you hit a construction zone posted for 45 mph, you’ll stay reasonably close to the limit.

Is adaptive cruise control worth it?

Lexus_IS250_ACCAdaptive cruise control makes sense as the price comes down and as you drive more highway miles, especially long trips where your reflexes are dulled from driving five to 10 hours. For that, even partial ACC is helpful, and at $500-$1,000 on a $30,000 car (the typical selling price now), it could be worthwhile. If your commute involves freeways that are clogged every day, then you want full-range ACC for convenience.

The same holds if your long vacation or weekend trips involve leaving and returning big urban areas. Anyone who’s driven Boston to Cape Cod on Friday night knows the pain. Returning to metro New York, Los Angeles, or Chicago, traffic starts slowing down 100 miles away on a Sunday night. Here, adaptive cruise control is a quality of life feature — just hit the “Set Speed” button and don’t worry about cars stopping suddenly or creeping along.

While ACC is a great feature, I advise buyers to get lane departure warning and blind spot detection before you spend money on adaptive cruise control. These two features cost less and do more for you on most trips.

And keep in mind, vecause it’s so complex, you have to order a car with ACC. You can’t go to the parts department and order it added later on.

Who has active cruise control?

More than a dozen automakers in North America are selling cars with adaptive cruise control. Typically it starts on higher-end models and works its way down the line. Full-range ACC or stop-and-go ACC costs around $2,000-$2,500.

Automakers with adaptive cruise control (as of mid-2013)

  • Audi
  • BMW
  • Bentley
  • Buick
  • Cadillac
  • Chevrolet
  • Chrysler
  • Dodge
  • Ford
  • Honda
  • Hyundai
  • Jaguar
  • Land Rover
  • Lexus
  • Lincoln
  • Porsche
  • Toyota
  • Suburu
  • Volvo

Now read: ExtremeTech’s list of the world’s best tech cars