One morning, at 7.49am, we took the C12 bus from Place Bellecour, and rode all the way to the Hôpital-Feyzin-Vénissieux terminus. The trip across much of the Lyon urban area, in south-east France, is about 20km long and takes an hour, with 30 stops. But for €1.60 ($2) – the price of a single ticket – you get a remarkable slice of life, ranging from the city centre's plush quartiers to the Minguettes tower blocks, passing through leafy middle-class avenues east of the Rhône and the industrial estates of Saint Fons.

Our driver and guide is Anne Sari, 51, who works for TCL, the Lyon transit operator. For the past two years she has snaked her way through the traffic in an 18-metre articulated Iveco bus, which theoretically carries up to 150 passengers ... but often more.

From her observation post Sari sees the invisible divisions all along her route, reflected in the social standing of passengers . A sign above the driver's seat forbids unnecessary talk so we meet Sari at home, for a detailed account of her work. After many years as part of the ground staff at an airport, she was made redundant. An advert for a job as a TCL bus driver caught her eye and, after three months' training, she mastered the art of steering the massive vehicles.

She volunteered for the reputedly difficult C12 route, because the buses are modern enough to carry a radio, enabling her to tune into her favourite channel. The first bus leaves at 4.50am, which means clocking on at 4.30. The last one reaches the depot just after 1am. Drivers work seven-hour shifts, covering the route, back or forth, six or seven times.

Early in the morning Sari picks up cleaners travelling to city offices, chemical-industry workers heading for plants downstream from Lyon, butchers on their way to the abattoirs and logistics operatives employed by warehouses in the eastern suburbs. They are barely awake, yet already exhausted.

A bit before 8am the bus wakes up as the students climb on board. There is a stop outside the Lycée Jacques Brel, on the Minguettes estate. It used to be badly rated but is gradually improving, a yardstick for this neighbourhood struggling to shrug off its bad name – in 1981 it was the scene of one of France's first banlieue riots. Further down the line, at Ludovic-Bonin, the C12 stops to pick up pupils from the Lycée La Xavière, a top-notch private secondary school with a 90% success rate for the baccalauréat exam.

Mid-morning and it is time for the chibanis, north African migrant workers who have opted to stay in France on retiring. On the bus they talk in low voices, mixing French and Arabic. They mostly get off at Place Gabriel Péri, a favourite meeting point. Later in the afternoon Sari takes them home again.

At Vénissieux's Picard stop, the bus unloads women trailing empty bags and children. "There's a Resto du Coeur [NGO handing out meals and food packages] just down the road," Sari explains. Nearby, at Les Marronniers, there used to be a Roma settlement, now demolished. "Everyone thinks badly of them. They don't pay, but they're polite, always saying 'Bonjour'," she adds.

This is not always the case with other passengers, though Sari admits it is partly her fault: "If I look at them when they get on, they greet me. If I ignore them, they do too."

On Place Jean Macé she often sees a street-sweeper boisterously singing old hits. But when he catches the bus to go home he is reserved. "Without his day-glo jacket, it's like he was someone else."

The C12 goes past the Saint Jean de Dieu psychiatric hospital. The stop outside boasts a statue of Sigmund Freud. During the day the bus picks up patients allowed out for the day. Sari recalls being particularly affected by one old lady: "She said she'd forgotten where she should get off, but knew it was a place where they treat Alzheimer's."

Sari has never been assaulted, but the bus has been pelted with stones. At night the kids in the tower blocks try to dazzle drivers with laser beams. A couple of times she has summoned inspectors to pacify drunks or patients from the psychiatric hospital. One evening in June 2009, when Algeria qualified for the World Cup, Les Minguettes went wild and her bus got caught in the crowds. A bunch of youths started kicking the vehicle. One stuck an Algerian flag behind the windscreen wiper. Sari enthusiastically hooted the horn, marking her approval. The tension dropped and she was able to drive on.

Nearing the last stop on Place Bellecour she crosses the Rhône, drives past the affluent buildings and into the vast square in the centre of the old middle-class town, parking the bus outside the post office. Les Minguettes are far away, in another world, to which she will shortly return.

This article originally appeared in Le Monde