Road Test: Mercedes-Benz S Class S500 4dr Auto

When Mercedes throws its entire might into high-tech support for luxury and safety, the results are so dazzling that it's hard to know where to begin.

Try this. I'm telling myself to ignore a speed bump ahead and hit it at about double the sensible speed, puckering myself as it passes beneath the bonnet. Ha! The inevitable brutal shock never materialises. Binocular-vision cameras in the windscreen have seen the bump, causing the fully active suspension to lift each wheel at the exact moment it's due to meet the tyre. Not just softened off a spring or damper, but actively lifted the wheel. The disturbance is what you'd expect from driving over a couple of catseyes. Now, I'm not suggesting you clatter through traffic-calming at car-chase velocity, because a kid might run out from the school gate (although the S-Class also has pretty effective pedestrian recognition and autonomous emergency braking). I'm just saying that the new S's optional Magic Body Control does astounding things for comfort over large bumps and potholes.

Or how about this? We're in sluggish traffic, two lanes of it, down a curving street with traffic lights. I set the adaptive cruise control, touch the throttle and the Mercedes sets off. I remove my hands from the wheel and my feet from the pedals. The car keeps steering, slowing, accelerating and stopping to keep its place behind an Audi. It's building a map of the situation using those stereo cameras as well as three different radar frequencies. This is not a closed-test-track experiment, by the way - it's real traffic on a real rainy weekday afternoon. At one point, a Suzuki Swift dives in front of me. The Benz obligingly makes way, then follows it. In a while, the supermini dodges back into the adjacent lane, and the Mercedes heads off after the Audi. I haven't touched the throttle or brakes yet. I have wobbled the wheel occasionally, but only when the car flashes up a warning to remind me it isn't an autonomous driving system. Oh no, it's a driver-support system, so if I refused to play my part every 10 seconds or so, it would shut itself down.

As with the speed-bump stunt, I'm not advocating you drive without due care. For a start, the self-steering does occasionally lose the plot, and when it does, you're probably bearing down on a kerb or a lorry in the next lane. And in the time it takes to refocus your attention from texting or daydreaming and grab the wheel, you'll probably have had the accident. Which means that absenting yourself entirely from the driving is scary rather than relaxing. But allowing it to support - not replace - you does ease the strain to a surprising extent.

And easing the strain is what the S is all about. This is one of those cars that, over generations, has always defined its market, leaving rivals benchmarking themselves against it and finding spurious points of difference. It knows what it is and plays to its strengths. Relaxed, safe, cosseting, swift.

So while the technologies dominate your thinking about the new S, it's important to remember they're bolted onto what is, at heart, a very fine luxury barge. The self-driving kit and Magic Body Control are a £6,600 hit to the S500 L, but the car isn't sparse without. Even the base S350 diesel has a cabin of lavish plushness, equipped with two 12.3-inch hi-res screens, air suspension, navigation and internet, and even a lighting system consisting entirely of LEDs - headlamps, cabin lamps, everything.

The design of the outside has an elegance that isn't apparent in photos. It avoids the brutally dictatorial aspects that characterised some of its predecessors. Inside, it's not really to my taste - too ornamental and a bit Fifties in its palette of curves and surfaces - but it's done with a conviction that carries the day. And while Audi and BMW simply make their biggest saloon interiors a slightly higher-grade version of their smaller ones, the S-Class stands apart as its own thing.

The longer of the two wheelbases allows space to option a pair of soft and embracing back seats that recline near-horizontally, with electric calf cushions and heating not just for the seat surfaces but even the armrests. Mercedes knows this market so well, it has even provided two different ways to arrange the front passenger seat to maximise stretching room for the potentate behind, depending on whether their culture allows or disapproves of the sight of unshod feet.

The engine and transmission we've seen before: nothing wrong with the silken force of the 455bhp twin-turbo V8. It answers and makes no fuss. Silence is near-uncanny, not just of the engine but passage through the air and over the road, too. The car is lighter (by 100kg in the S350 diesel), thanks to a significant proportion of aluminium in the shell, and it's also stiffer, so you feel noticeably less vibration through the steering column and less body resonance after sharp bumps.

The S500's active suspension has two modes, an automatic one that uses the Magic Body Control and aims always to keep supple. It rolls a little and understeers and heaves if you drive like you've got no passengers, but the ride is unprecedented over big bumps and excellent if not quite unmatched over smaller corrugations. Hit Sport, and the Magic Body Control turns off, but it's still not jarring. In corners, things firm up, the roll disappears and the understeer is cancelled. It's not engaging, but it'd be rapid enough for getting away from a diplomatic incident.

Yet, asked what's his favourite feature, chief engineer Dr Hermann Storp doesn't mention any of the technological panoply. He points at the butter-soft leather cushions in the back seat, with elastic so you can strap them to the headrests and get some deep relaxation. It's a complicated car that makes travel blissfully simple.