Adaptive Cruise Control - 2011 Infiniti M56 Long-Term Road Test

2011 Infiniti M56 Long-Term Road Test

2011 Infiniti M56: Adaptive Cruise Control

September 21, 2011


When our 2011 Infiniti M56 passed through 15,000 miles a couple of weeks ago, observant readers studying the celebratory photograph noticed the car was in adaptive cruise control (ACC) mode. How could they tell? The indicated speed was a few mph lower than the cruise control "set speed", a situation that can only occur on flat ground when one is following another motorist who is travelling slower than the ACC system's target velocity.

Futhermore, the presence of only one (out of a possible three) illuminated bars on the dash was proof that I had set the system to the "closest" following distance. A mild scolding ensued.

The above photograph was taken at that closest following distance, with the image compressed slightly because the camera is zoomed in a little.

This is the unzoomed shot...


Remember, this is one "bar", the closest ACC following distance setting. One bar is, as near as I can tell, roughly equivalent to the amount of space sufficient to invite one car to move in between me and the van ahead without thinking for one second that I'd been cut off.

Two bars is enough for two cars to move into the space ahead without any of the involved parties taking offense or trading rude hand gestures. Three bars allows for three lane changers. That's not exactly the defined intent, mind you, but that's how it works out in our M56.

But in this common no-need-to-panic situation the M's ACC system panics anyway. It responds with an immediate chop of the throttle and dab of brakes in a situation where you or I would simply breathe imperceptibly off the gas and allow the gap to restore itself gradually -- if we let it restore at all.

It wasn't supposed to be this way. ACC systems were originally developed over two decades ago as part of the Intelligent Vehicle Highway System (IVHS) initiative, an engineering exercise that sought to explore and define intelligent systems such as this that could pack cars closer together than drivers would otherwise do on their own, and do it safely. Computers monitoring the space ahead while they controlled an electronic throttle and computer-regulated brakes were supposed to allow more traffic to occupy the same freeway at a productive speed, improving traffic flow while avoiding billions in roadbuilding expense.

The technical elements are here today but it seems ACC will never be allowed to carry out this mission in a legal climate that leads to following distances for such systems that are GREATER than what drivers would otherwise choose for themselves.

Ironically, big following distances and the specific way in which ACC reacts to interlopers can cause problems in the very sorts of moderate freeway traffic situations in which ACC was supposed thrive. Opportunistic drivers understandably move into inviting open spaces, which in turn triggers the above-described exaggerted ACC response that attempts to restore the gap ASAP with an urgency that is frankly startling to the car following directly behind.

The miffed pilot of said car will most likely zoom around and into the freshly restored gap, possibly accompanied by a helpful hand gesture proclaiming your Number One status, therby continuing the cycle and marching you steadily backward until you turn the damn thing off.

ACC systems work best in two places: 1) on the open road, where you don't need it anyway unless you're not paying attention when you come up behind another car or; 2) when travelling in LA-style car pool lanes as shown in the above photo, where infrequent access points are spaced a couple miles apart.

As it stands now, adaptive cruise is an expensive option that I don't need or recommend.

Dan Edmunds, Director of Vehicle Testing

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