Adaptive cruise control (ACC)

Ordinary cruise control holds one speed. Adaptive cruise control also looks ahead and adjusts your speed to the car in front. That looking ahead is exactly what most exam questions are about because the system does not see everything.

How does it work?

A radar in the grille, often combined with a camera behind the windscreen, continuously measures the distance to and speed difference with the car ahead. If it slows, ACC lifts off or brakes along. Once the road is clear, it accelerates back to the speed you set.

The following distance is adjustable, usually across three or four settings. In many cars ACC works from around 30 km/h; stop-and-go systems also follow in a jam and come to a full stop by themselves.

What the CBR asks about this

• You set the following distance to what the rijprocedure considers safe, not to whatever pulls away most conveniently. Choosing a short setting because people keep merging in is exactly the negative behavioural adaptation the CBR watches for.
• You keep following both the car and the traffic while ACC does the work. The system takes over the driving, not the looking.
• You maintain an active driving posture: hands on the wheel, foot near the brake, ready to take over immediately.

This system falls under these guidelines from the Rijprocedure B:

Limitations: This is where the exam questions are

Stationary traffic is often not seen

This is the key limitation. The radar is tuned to moving objects so it does not brake for every road sign or crash barrier. A fully stopped queue can therefore be recognised late or not at all. Approaching the tail of a jam at 100 km/h, you have to brake.

Merging cars are picked up late

When someone slots in just ahead of you, the radar takes a moment to register them as the new lead vehicle. During those seconds your following distance is shorter than you think.

Weather and dirt disrupt the measurement

Snow, dried mud or ice on the grille disable the radar. You usually get a warning, but not always immediately.

ACC does not account for the road itself

The system looks at the car in front, not at a sharp bend, an exit or a speed sign. In a bend ACC will simply hold the speed you set.

Three practice questions

Open one for the answer and the explanation.

You are doing 100 km/h on the motorway with ACC and see a completely stationary queue ahead. What do you do?

• Brake yourself; ACC often recognises stationary traffic late or not at all
• Nothing, ACC will brake the car safely to a stop
• Set the following distance to its longest setting
• Just switch off the cruise control and coast

Correct answer: Brake yourself; ACC often recognises stationary traffic late or not at all

Explanation: The radar is optimised for moving objects. A stationary queue falls outside that and is regularly ignored. You stay responsible for braking in time.

What following distance should you set on ACC when cars regularly merge in front of you?

• A distance the rijprocedure considers safe
• The shortest setting, so nobody can fit in
• Whatever the manufacturer set as the default
• It does not matter, the system corrects for it anyway

Correct answer: A distance the rijprocedure considers safe

Explanation: Settings must contribute to the desired driving behaviour. Following closer because the car will brake anyway is negative behavioural adaptation one of the principles in the Rijprocedure B.

What does ACC do when you approach a sharp bend with cruise control engaged?

• Nothing in particular; ACC looks at the car ahead, not at the bend
• It brakes automatically for the bend
• It switches itself off as soon as you steer
• It adopts the speed from the last road sign

Correct answer: Nothing in particular; ACC looks at the car ahead, not at the bend

Explanation: With no vehicle ahead, ACC simply holds the speed you set. Adapting to the road is your job, not the system’s.

Written on the basis of the official CBR Rijprocedure B. These are not real exam questions.