FROM the moment the steering wheel vibrated as I moved out to overtake it was clear what the ‘S' stands for in S-Class - safety.

Yes, it also encompasses sumptuous, sleek and stylish but Mercedes-Benz has really pushed the boat out to keep all occupants of the S-Class as safe as possible.

There are monitors that can tell whether you are drowsy and respond to what they perceive to be unintended lane changes. They will also actively stop you steering into the path of another car coming up behind or changing lanes unintentionally - hence the vibrating steering wheel.

Buy the long-wheelbase version I tested and you will also have enough room for your children to invite their mates round for a sleepover as the amount of space available in the rear is palatial.

But back to the safety features which are ground-breaking. Over the years successive generations of the S-Class have been pioneers for remarkable innovations that include anti-lock brakes, airbags, catalytic converters and electronic stability programmes.

The current gizmo pushing the boundaries is called Attention Assist and it pretty much does what it says on the tin. With almost a quarter of serious road accidents caused by drowsiness at the wheel this programme is a major boost for road safety.

Basically the system works by making the car your co-driver so it can keep an eye on how you perform and give advice when necessary. Fire up the S350 CDI and the car's computer begins to put together a profile of your driving behaviour which is then compared to your performance via the data collected from 70 monitors around the car tracking speed, acceleration, erratic steering plus indicator and pedal use.

The journey length, time of day and music being listened to are also assimilated - switch to a rock station from a calm discussion on astrophysics and the computer will know about it - as are the day's driving conditions including wind direction, road surface and the weather.

The key to the system is its ability to detect the crucial moment - which Mercedes refers to as ‘the floating transition' - when the driver slips from alertness to drowsiness.

When that happens the computer wakes you up with a high-pitched warning sound jerking you into consciousness. The digital dashboard then goes blank and in large capital letters the order to take a break flashes on and off until you heed the advice pull over into a service station and fill yourself full of caffeine.

I've got a feeling that Attention Assist was introduced because the S-Class is so smooth to drive. Simply getting behind the wheel of the S350CDI is a pleasure as the driver's seat moves electronically into your favoured position.

The 3.0-litre diesel engine has plenty of power on tap but this is a cruiser rather than a sprinter built for comfort and relaxation. With that in mind little in the way of exterior noise makes it into the cabin and in fact the silence is a little spooky with engine noise largely absent.

The interior is festooned with highly polished wood, premium leather trim and classy aluminium switches with the panels exhibiting the fit and finish you'd expect from a top quality Mercedes-Benz.

The long wheelbase version adds to a space already expanded by the latest S-Class being wider and taller than the previous generation and the boot is enormous.