How we test cars Advanced braking systems

The future of car safety lies in preventing crashes, rather than simply reducing their effects. That's why Which? Car has carried out the first comparative test of advanced braking systems that use cameras, radars, and automatic braking to stop a collision before it happens.

Volvo City Safe

Volvo's City Safe came out top in our advanced braking systems test

All the systems we tested were judged as worth having, but they are currently available only as pricey optional extras. We hope that, like electronic stability control, they will become standard in cars over the next few years.

Advanced braking explained

The cars we tested were the Volvo V60, new Mercedes CLS, Audi A7, Volkswagen Passat Estate, BMW 530d and Infiniti M37S.

They all come with advanced safety equipment including radar sensors, cameras and lasers that recognise the vehicle in front and alert the driver if a collision is imminent. If the driver does not react and the collision cannot be avoided (by swerving), the system automatically brakes to reduce the speed of the impact or to prevent it altogether.

Out test looked at how well the advanced braking systems reduced impact speed, as well as how effectively the driver is alerted to an imminent collision. We also looked at the reliability of the systems and for the chance of false alarms or unnecessary emergency braking.

Volvo City Safe wins

Advanced braking systems compared
Advanced Braking SystemCar testedRating
Volvo City SafeV60 D5excellent
Mercedes Pre-SafeCLS 350 CGIexcellent
Audi Pre-SenseA7 3.0 TFSIgood
VW Front AssistPassat Estate 2.0 TFSIgood
BMW Adaptive Brake Assistant530d Automaticsatisfactory
Infiniti Intelligent Brake AssistM37S Premiumsatisfactory
The best overall system we tried was Volvo's City Safe, which gained a five-star rating. It showed a high potential to prevent crashes into stationary objects, performing well at high speeds with early warning alarms. At speeds of up to 25mph, it completely prevented the V60 from crashing.
Mercedes Pre-Safe

The Mercedes Pre-Safe system came a close second but didn't completely prevent a collision

The Mercedes Pre-Safe and Audi's Pre-Sense outperformed the Volvo in terms of cutting speed and lessening the severity of potential injuries, but couldn't completely prevent a collision or come to a stop in front of stationary objects. The Mercedes system had the edge of the two, just scoring five stars, while the Audi earned a high four-star rating. However, both manufacturers have since announced upgraded systems since our test.

The four-star VW Front Assist system in the Passat estate took fourth place. It did stop in front of stationary obstacles, but only at very low speeds. At high speeds, it was less reliable than we'd like - setting off the driver warnings too late or not at all.

BMW's Adaptive Brake Assistant lagged behind the four top scorers, getting three stars. It warns the driver very early, but not very forcefully. If the driver does not react, it brakes only partially, reducing speed less effectively than the rival systems. With a stationary obstacle, it did not brake at all.

The three-star Infiniti Intelligent Brake Assist was the poorest performer. It was ineffective in terms of warning the driver and reducing speed. One plus is the innovative accelerator pedal, which pushes back against the driver's foot if the distance to the car in front is inadequate - but this lacks fine-tuning and is likely to feel strange for most drivers (at present).

The results

While the overall results vary greatly, all the advanced braking systems we tested do offer enhanced safety. Even if they lower impact speed only minimally, any reduction at all can save lives.

We urge car manufacturers to introduce systems like these more widely across their ranges, and reduce their cost, as soon as possible.

View our reviews of the tested cars:

How we tested the systems

Our scores were made up of each system's performance in five areas:

  • Approaching a slow-moving object
  • Approaching a steadily-decelerating object
  • Approaching an object which comes to a halt
  • Approaching a stationary object
  • The system's alerts - speed an effectiveness

Each made up 20% of the total score. Here's how we tested each, and how they compared:

Approaching a slow-moving object

This test was carried out at two speeds: once with the test car travelling at 50kph and the other travelling at 20kph, and another time with the speeds at 100kph and 60kph respectively.

1. VW Passat   4 stars

2. Volvo V60   4 stars

3. Mercedes CLS   4 stars

4. Audi A7   3 stars

5. BMW 5-Series   3 stars

6. Infiniti M37S   2 stars

Approaching a steadily-decelerating object

In this test scenario, both the test car and the target car started at 60kph and 40 metres apart. The target car then started decelerating steadily.

1. Audi A7   5 stars

2. Mercedes CLS   5 stars

3. Volvo V60   4 stars

4. VW Passat   4 stars

5. BMW 5-Series   4 stars

6. Infiniti M37S   4 stars

Approaching an object which comes to a halt

In this scenario, the vehicle in front suddenly brakes to a standstill at quite a distance (120m). This is test represents what happens when you approach the tail end of a traffic jam.

1. Mercedes CLS   5 stars

2. Volvo V60   4 stars

3. VW Passat   4 stars

4. BMW 5-Series   4 stars

5. Infiniti M37S   4 stars

6. Audi A7   3 stars

Approaching a stationary object

Automatically stopping a car before it hits a stationary object is one of the most challenging tasks for carmakers. We tested each car's performance at 20kph, 30kph, 40kph and 70kph.

1. Volvo V60   5 stars

2. Infiniti M37S   4 stars

3. Mercedes CLS   3 stars

4. VW Passat   2 stars

5. BMW 5-Series   2 stars

6. Audi A7   1 star

System alerts

The assessment of each system's alerts includes the activation time and whether or not the alarm is noticeable and effective. The 'cascade' of alerts include sounds and flashing icons and partial braking to grab the driver's attention.

1. Audi A7   5 stars

2. VW Passat   5 stars

=3. Volvo V60   4 stars

=3. Mercedes CLS   4 stars

5. BMW 5-Series   4 stars

6. Infiniti M37S   1 star

Other factors

Bonus points were also awarded if the system added extra brake pressure if not enough was applied to prevent a collision, and for a good early distance warning.

Points were taken from the overall score if the system gave false alerts in cornering or overtaking manoeuvres.

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