"Keep your feet flat on the ground, keep your feet flat on the ground."
This is the constant refrain at Montparnasse station's main interchange inside the Paris metro.
And there is a good reason for it - failing to comply could land you flat on your face.
The message is directed at people using a new high-speed travelator, an invention that some say could revolutionise the way we get around big cities.
The trottoir roulant rapide (fast rolling pavement) or TRR is on trial until October, when the metro's safety committee will decide whether it has been a success - and whether to roll it out elsewhere.
The prototype carries passengers the length of Montparnasse station at 9 km/h - three times as fast as normal travelators, and about the average speed of a Paris bus.
TROTTOIR ROULANT RAPIDE
Length: 180 metres
Top speed: 11 km/h (7 mph)
Capacity: 110,000 people per day
4m people carried so far
Nickname: TGV - the trottoir, not the train, a grande vitesse
In development for 10 years
It is easy to spot old hands who use the trottoir daily and stride boldly along its length.
But new users also appear every day, and a small proportion promptly fall and hurt themselves.
In some cases the Paris metro has had to pay compensation.
"People have to learn how to use it and that takes time," Anselme Cote, project manager for the Paris metro, told BBC News Online.
He added that escalators had presented travellers with a similar challenge when they were first introduced.
People stepping directly on to the TRR would be sure to lose their balance, so they first have to be accelerated - and then decelerated again at the other end.
The TRR could be ideal for transfers between stations
"The problem lies in the transitions; one has to glide from one phase to the next; we ask people not to move, but they are not used to it," says Mr Cote.
"One must keep one's feet flat between the two phases, but people walk. There's a technique to it. But people get used to it very quickly."
As they get off the belt, in a more or less orderly manner, new users seem either relieved or elated.
But some remain sceptical.
"It's crap," 25-year-old Nouria El-Gouy, an auxiliary nurse who was using it for the first time, told BBC News Online.
The real problem nowadays is how to move crowds - they can travel fast over long distances but not over short distances
Anselme Cote, inventor
"I'm really not convinced. I'm with my mum and she was really scared. That's bad."
Another first-time user was also unimpressed, but for the opposite reason.
"It's the first time I've used it and it's not as fast as I thought," says Nicolas Dejenn, a 28-year-old student from Nantes, loaded with luggage.
"It was less scary than I expected."
Some regular users say it is a great timesaver, but that they would not dare use it with a rolling suitcase or a pushchair.
Experts from all over the world have flocked to see the trottoir
People who use walking sticks are also advised to steer clear.
Mr Cote says one obvious application would be to speed up transfer times between train stations - for example the Gare de Lyon and Chatelet in Paris, or Euston and King's Cross train stations in London.
Two TRRs in sequence could be used over a distance of one kilometre - on the Champs-Elysees or at Roissy-Charles de Gaulle airport for example, Mr Cote says.
"The real problem nowadays is how to move crowds; they can travel fast over long distances with the TGV (high-speed train) or airplanes, but not over short distances (under 1km)," he says.
You can travel from Le Mans to Paris in 50 minutes, he points out, but crossing Montparnasse Station may take you 20 minutes.
This explains the enormous international interest the TRR has aroused.
Experts from all over the world have gone to Paris to see the magic trottoir in action.
They include airport operators in Toronto or Hong Kong, Transport for London - which runs the London Underground - and the organisers of the forthcoming Olympic Games in Beijing.
Peter White, professor of public transport systems at the University of Westminster in London, says the trottoir could have a big future if it is not too costly.
He says there is a need for an "intermediate means of transport" in big cities - but he points out that the TRR needs a site that is flat and straight.
For example, the ancient London Underground, with its narrow and crooked tunnels, might not be appropriate.
For now the inventors need to keep their feet on the ground - but they, and the trottoir, could go far.