Paris, millions live here and the metropolitan traffic is chaotic. Every day a never-ending line of vehicles, nose to tail, crawls towards the city centre. The road network is on the verge of collapse. Statistics put the average speed at seven kilometres per hour. And here we are, stuck in the middle of the morning hold-up.

The people alongside look at the odd paintwork on the Zafira, not quite sure what they are to make of the words fuel cell and hydrogen. And so they don’t realize that what they are seeing is, without any shadow of doubt, the cleanest car in France. Harmful emissions? Zero. All that comes out of the exhaust pipe is water, pure water. Noise? Practically none, just a gentle whoosh when the compressor forces air into the cells and the power generated operates the electro-engine.

Fortunately we are moving out of the city, eastwards, heading for Reims and Metz to cross the border into Germany. The traffic thins. We have 540 kilometres ahead of us. The fuel cell Zafira is purring on towards the half-way mark. By this evening it will have clocked up 5,000 kilometres. And the engineers, who are surprised by the high fuel consumption, will have made lots of notes in their log book. Every 150 kilometres at the latest we have to stop for fuel, which a Linde truck provides for us: about 75 litres of liquid hydrogen, weighing less than five kilos, cooled to minus 253 degrees Celsius, is one tank filling.

In Paris we met Jon Bereisa. Jon is the strategy director for GM in Detroit, he develops concept models for fuel cell cars we might be driving in eight or twelve years’ time or whenever. Asked whether he would buy this kind of car, Jon replies: "Of course, but only if it’s a genuine fuel cell vehicle, not a conventional car with a fuel cell."

Because to Jon there’s a big difference. If it’s used cleverly, an electric power unit means that vehicles can be designed in completely new ways. Forget the three boxes design; engine, passenger cell, boot. Obviously there will be transitional versions to start with, because they would not be produced in great numbers and a new architecture would cost incredible amounts of money.

Basically the situation is the same as 120 years ago when the Otto and diesel engine were invented. First of all these power units were built into coaches and carriages. Proper cars, as we now know them, came later. The same will happen with the fuel cell.

The Hy-wire, which GM presented about two years ago, is one such first idea of the kind of architecture that might be used with the fuel cell. The flat chassis, which looks like a big skateboard, contains all that is needed to propel the car. According to intended use or customer’s wishes, a wide variety of different bodies can be mounted. At next year’s Detroit Motor Show GM intends to present the Hy-wire II, designed to bring the future a step closer.

But for now we are heading for Darmstadt, where the Zafira will be laid up for a day to allow time to check all the systems and have the car fit and ready for the second stage of the marathon.