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Forward collision warning (FCW) and autonomous emergency braking (AEB): why they are essential?

Forward collision warning (FCW) is a vehicle technology that warns the driver if it detects there’s a risk of a crash. Autonomous emergency braking (AEB) takes this one step further by also braking if the driver doesn’t respond to the forward collision warning. They are the best technological solution to reducing crashes since electronic stability control.

The types of crashes that FCW and AEB prevent are general inattention (distraction) crashes; they won’t prevent you from sliding off the road if you’ve taken a corner too fast. It’s easy to become distracted as there are a lot of things vying for our attention such as our personal feelings, the amount of clutter on the roads, talking to a passenger, etc. FCW and AEB act as an extra pair of eyes to watch for danger, and a backup plan if you still fail to respond.

The process for evaluating danger on the road

As we gain more experience behind the wheel we come across situations we learn from. The vast majority of these are benign and fall into four categories:

  1. being told by someone else
  2. deducing an outcome for yourself
  3. watching another person learn from a mistake (e.g. nearly have an accident)
  4. learning from your own mistakes (e.g. nearly having an accident)

So, it’s obviously more risky for new drivers who don’t have experience, and that’s one of the reasons why they tend to crash more.

When we see a potential danger unfolding, our brains have to make some judgments such as:

  • Have you seen or experienced a similar situation in the past, and what were the actions and results
  • Can you recognise an obvious danger using common sense (e.g. you may not have seen an escaped zoo elephant in the road, but it’s obvious that it could present a problem in the very near future)
  • What is your situation (e.g. how fast are you travelling, what type of road surface, etc)
  • What is the appropriate action to take (brake, swerve, brake and swerve, nothing, accelerate, etc)

Reaction time

While our brains are evaluating these inputs we continue to travel towards the hazard. The amount of time you take from first seeing the hazard to taking action is the reaction time. Typically this is between 1-2 seconds depending on your age and experience. Younger people might have faster general reactions but not the experience to make the decision quickly enough, while older people have slower reactions but more experience.

At 70mph we’re covering 31.3 metres per second. Therefore if there’s an incident ahead on the motorway, you’ll cover at least 31.3 metres and probably more like 60 metres before your foot even hits the brake. From 70mph it could take upwards of 90 metres to stop, meaning your total stopping distance will be around 150m. If AEB can shave a second off your reaction time, you’ll have 31.3m extra to play with and that could mean the difference between hitting the obstacle at around 35mph, or stopping completely.

How to FCW and AEB work?

Radar, cameras or a mixture of both are used to look ahead and determine what is happening. Some systems will look ahead 200m and can identify a potential issue well in advance of the average driver.

Both systems are becoming common on cars because they are needed to score a maximum EuroNCAP crash test score, and also have been introduced on trucks and buses.

What are FCW and AEB’s limitations?

  • AEB doesn’t necessarily react much quicker than you could as it’ll warn you first and will only brake as a last resort
  • The technology doesn’t know the condition of your tyres and suspension, and therefore your overall braking ability
  • The technology doesn’t know how much grip the road surface has; if it’s wet then your stopping distance will be longer
  • City environments with lots of other vehicles and pedestrian traffic can cause false triggering occasionally
  • Camera-based technology won’t work well in heavy fog and rain, or if your windscreen is dirty
  • Neither technology can warn you if you’re approaching a corner too fast.

Here’s how autonomous emergency braking works:

Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.

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