Car safety systems: active and passive (ABS, ESP)

Active systems (ABS, ESP, ASR, EBD, AEB) prevent an accident: they watch over grip, the car's stability and the distance to an obstacle. Passive systems (belts, head restraints, airbags, crumple zones) act only during a collision and reduce injuries. None replaces the driver. With ABS you brake hard, without pumping.

How do active systems differ from passive ones?

Active systems work before an accident happens. They watch over grip, the car's stability and the distance to an obstacle, and when needed they brake one wheel or the whole car by themselves. ABS, ESP, ASR, EBD and AEB belong to this group.

Passive systems only come into play at the moment of a collision and limit its consequences. Seat belts with pretensioners, airbags, head restraints and crumple zones will not prevent a collision, but they reduce injuries. In the theory test both groups appear in the specialist questions, meaning the ones with answers A, B, C.

A simple test: if a system works while you are driving and affects the car's line or speed, it is active. If it wakes up in a fraction of a second after an impact, it is passive.

Table of abbreviations: which system does what

Trade names vary (ESC, DSC, VSC for stability control; TCS for traction control), but the principle is the same. In the test questions you will meet the abbreviations ABS, ASR and ESP.

How ABS works and how to brake in a car with ABS?

Sensors on the wheels measure the rotational speed of each one. When a wheel starts to lock, the controller reduces the pressure in its brake for a fraction of a second, then raises it again. The cycle repeats many times a second. The wheel rolls instead of sliding, so it keeps transmitting lateral forces and the car responds to the steering.

Without ABS, braking too hard locks the wheels and ends in a loss of steering control. That is the correct answer to question 3857. With ABS you brake differently from what was once taught:

1. Press the brake pedal quickly and hard, all the way down. That is the most effective method according to question 3858 from the catalogue.
2. Do not pump. Pulsing and the pedal pushing back are the normal operation of the system; keep the pressure on (question 3859).
3. Steer where you want to go. ABS helps you keep steering control even when the car has gone into a sideways skid while braking (question 3860).
4. Do not start with the clutch. Pressing the clutch first and only then the brake is the wrong answer in question 3858.

The purpose of ABS in question 4495 reads: to prevent loss of stability and to keep the vehicle steerable while braking. Answers about protection from skidding "regardless of speed and conditions" or about halving the braking distance are wrong in the catalogue.

ESP and ASR: what do they do on a bend and when moving off?

ESP compares the steering angle with the car's actual movement. When the front will not turn in (understeer) or the rear overtakes the front (oversteer), the controller brakes an individual wheel and briefly reduces engine torque. The car returns to its line. In the question catalogue the purpose of ESP is to stabilise the vehicle's movement when driving round a bend (question 2026).

ASR works when moving off and accelerating. When a driven wheel starts to spin on a wet surface, snow or sand, the system brakes it or limits engine power. The answer from question 2025: counteracting wheelspin when moving off. ESP uses the same sensors and brakes as ABS, which is why a car with ESP always has ABS and usually ASR as well.

EBD: what is it and why do you not feel it?

EBD stands for electronic brakeforce distribution. Under braking the car's weight shifts to the front axle and the rear becomes lighter. If the rear wheels received the same braking force as the front ones, they would lock first and the car could spin.

EBD meters the brake pressure between the axles depending on the load: one way with an empty boot, another with four passengers and suitcases. It does this on every braking, not only in an emergency, and without any signal to the driver. There is no separate warning light, because EBD is a function of the ABS controller and reports a fault together with it.

AEB: when does the car brake by itself?

AEB is an advanced emergency braking system. Regulation (EU) 2019/2144 defines it as a system that can automatically detect a potential collision and activate the vehicle's braking system to reduce speed in order to avoid the collision or limit its consequences. A radar or camera watches the road ahead, and the controller calculates the distance and the rate of approach to the obstacle.

The sequence usually looks like this: first an audible warning and one on the dashboard, then the brakes are primed, and finally the car brakes by itself if the driver does not react. Newer versions also detect pedestrians and cyclists. Euro NCAP rates such systems in a separate Safety Assist category, so in the test results for a given model you will find a separate score for the assistance systems, not just the crash result.

Passive systems: seat belts, head restraints, airbags

Seat belts are the most important passive element. The obligation to wear them applies to the driver and all passengers, on all roads (questions 6388 and 6393). A belt only works properly when it is adjusted, fits close to the body and is not twisted. A twisted belt will not work correctly (question 4486), and an unfastened one means greater injuries to the driver in an accident (question 6432).

A head restraint protects the cervical vertebrae (question 4484). Set it so that its top is level with the top of your head (question 6336), close to the back of your head. In the practical test, adjusting the head restraint is one of the elements of preparing to drive, described in our article on vehicle checks in the driving test.

Airbags reduce bodily injuries in a collision (question 6340). They do not give full protection and do not replace seat belts: an airbag is designed to work together with a fastened belt. A child in a rear-facing seat on the front seat may travel only with the passenger airbag switched off. When the airbag is active, a child in the front may travel only in a forward-facing seat (question 11028).

What is mandatory in new cars?

From 7 July 2024 every car and van newly registered in the Union must have, among other things, ISA, AEB, ELKS, drowsiness warning, reversing detection, the emergency stop signal, an event data recorder and TPMS. On 7 July 2026 further requirements were added, including AEB that responds to pedestrians and cyclists.

The dates, the full list and who the rules apply to are covered in our separate article on mandatory safety systems in new cars (GSR2). Nobody requires a used car to be retrofitted.

Do the systems replace the driver?

No. ABS will not cut the braking distance on ice to zero, ESP will not beat physics on a bend taken too fast, and AEB may not react in time to an obstacle that suddenly pulls out from the side. Each of these systems works within the limits of tyre grip. Tyre pressure that is too low reduces grip (question 6626), so take a TPMS warning seriously.

That is why in the question catalogue the wrong answers read "protects regardless of speed and conditions" or "provides full protection". The correct ones speak of reducing risk, stabilising movement and helping to maintain steering control. That difference in wording often decides which answer is correct.

How to check what your car has?

1. The owner's manual: the safety chapter lists the systems and their warning lights.
2. The dashboard after turning the key: the ABS, ESP and airbag lights come on for a moment and go out. A light that stays on signals a fault.
3. The assistance menu in the on-board computer: that is where you switch, for example, lane departure warning on or off.
4. A button labelled ESP OFF or with an icon of a car with skid marks: it lets you switch off stability control, for example when digging the car out of snow. In most cars the system comes back when the engine is restarted.
5. The registration document will not help: it has no field for safety systems.

Before you get into an unfamiliar car, hired or borrowed from family, spend a minute on the warning lights and the assistance menu. You can practise questions about ABS, ESP, seat belts and head restraints in the category B mock test, in the specialist pool.