If the driver takes no action by braking, or steering to avoid an obstacle, the car applies the brakes automatically.
In theory that sounds simple enough, but it can be a pain in the neck - literally.
I had the chance to test the system at Ford's Australia design centre on the outskirts of Melbourne.
I'm sitting behind the steering wheel of a Focus at the end of a carpark near the design studio with an engineer as my front-seat passenger.
At the end of the carpark area is a large Ford sign - about the size of the rear of a car.
"Start driving towards the sign until you hit 15km/h and then maintain that speed," the engineer instructs me.
Easy enough. Hit 15km/h and the car ploughs towards the sign - and some Australian motoring journalists standing nearby.
Just two metres away from the sign - and certain doom - I apply the brakes, over-riding the system.
The engineer - who now would rather be designing the inside of a wheel nut than sitting next to this nut - tells me, in a monotone voice, to do it again.
"Sorry," I say as I reverse back to the start of the carpark area, "I'm just a control freak."
So I start again, hitting 15km/h quick smart, and as I near the sign I turn my head and look at the engineer to prevent me braking again.
And half a metre away from the sign the Active City Safe system kicks in hard - and the brakes are applied with such force that I think, "Hmm, must make an appointment to see my chiropractor Simon Kelly soon."
But a chiropractor visit is cheaper financially and emotionally than a nose-to-tail collision that the system has prevented.
You don't need to worry about making that call to your insurance company or the hassle of a visit to the panel beaters - and dealing with the other car and driver involved.
Instead I'm just nursing a stiff neck because I had turned to look at the engineer.
Of course, I should have suggested to him that as well as applying the brakes automatically, the Focus should also have activated the horn and maybe let out a few expletives to the "car" in front.
"Active City Safe is another smart technology from Ford that drivers can use every day," says Ford New Zealand's communications manager, Tom Clancy. "It's especially convenient but also adds an extra element of safety, especially for drivers who often end up in heavy traffic while commuting.
"It only takes a split second of inattention to end up bumping the person in front of you, either in rush-hour traffic or even at roundabouts. Active City Safe can help avoid those."
While self-driving cars may seem like a dream to some people (my nervous driver mum included), the Active Stop experiment in Melbourne proved to control-freak me that I need to have my hands on the wheel and feet on the pedals at all times.
Sure, Google's experiments with self-driving cars have logged 480,000km during trials around Nevada, USA, but with the potential for law suits, having your car in control is a long way off.
But Ford and other automakers are participating in a pilot programme that enables cars to maintain safe driving instead of self-driving. Sponsored by the US Department of Transportation, the system tests vehicle-to-vehicle communication technology.
Cars would alert each other to traffic jams and accidents while also talking to stoplights and other transport infrastructure.
Now that sort of automation I can feel comfortable with - and here's my suggestion for that experiment: vehicle-to-vehicle communication where you can tell the car ahead it should have indicated when it moved across the lane in front of you.
Oh that's what the horn is for!
