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For passengers, the fare mainly depends on who is funding it. When your employer, region or local authority is a partner, the journey often costs one or two euros, or even nothing at all. The driver, meanwhile, receives an allowance for each passenger carried.

Without a partnership, prices remain modest: these are short journeys, so we're talking a few euros at most.

Before signing up, take a look at the list of partner areas and companies on the Karos website: that's what really changes the bill.

The app works throughout France, but it performs best in areas where it has forged partnerships with local authorities: the Île-de-France region, major regional cities, and many suburban and peri-urban areas where public transport options are limited.

This is indeed its primary purpose: to complement trains and buses where they don't run, rather than replace them.

For an occasional long journey, Karos isn't the right tool: compare train, bus and long-distance carpooling instead on Tictactrip.

Karos describes itself as "short carpooling": carpooling designed for the everyday 10 to 40 km journeys. The app analyses your usual routes and automatically suggests drivers or passengers who match your route and schedule.

The more you use it, the more accurate the suggestions become. Bookings are made in two clicks, and the app manages the meeting point, often set at an existing stop or a park-and-ride car park.

Karos also integrates with local transport networks: in some regions, carpooling appears directly as a public transport line.