In France, in winter, when do the authorities put salt on the roads?

According to the department in charge of taking care of french national roads near Paris, it is decided at a local scale according to weather forecast and conditions of road surface.

There are 20 centers of intervention and maintenance in the Ile de France for national highways. In each center, the manager in charge of intervention decides whether deicing agent (dry salts or brine) is to to be used or whether the snow is to be mechanically removed.

The french wheather agency météo france emits a map of France twice a day (6am 16pm) featuring regional alerts concerning cold (grand froid) or snow/ black ice (neige verglas).

The ideal situation is to put the deicing agent, when potentially effective, right before the rain/snow fall and right before traffic peaks. Otherwise, the snow is not removed until the snow stops falling. The trucks removing the snow also dispatch salts or sand. Nothing is done if the black ice becomes too thick or if the temperature is too low for salt to be effective.

In the Paris region, there is a plan defined by the "préfecture" (regional services representing the national authority, especially for police) in case of snow/black ice conditions. 1st level is always activated during winter, second level triggers additionnal surveillance to mitigate local jamming. As the third level is activated, truck heavier than 7.5t are stored on 43 parking lots. Following the crisis that occured in februar 2018, the closure of the RN118 highway may be decided and enforced using passages through the median strip. Please keep in mind that french drivers near Paris or in Southern France are not accustomized to snow or black ice. Snow tyres are not mandatory in winter. A few snowflakes may trigger traffic jams and the Paris region could be completely paralysed.

The website for the french professionnals in charge of "viabilité hivernale" (winter maintenance) reports the standards regarding tools, materials, weather and conditions reporting and provide links to technical documents as well as an email adress to contact the network.

• This document reports the use of weather stations near roads and sensors recording the temperature or salinity of road surface feeding a decision-making software SAD-VH helping local center CEI taking optimal decisions.
• This document advises not to use corrosive deicing agent on old steel bridges or brigdes cleary featuirng corrosion issues.

The most actual french guidelines for road managers are available here and here, though it is not free.

Counties "Département" and cities "communes" are responsible for local roads.

Here is the DOVH / POVH, that is the organization plan for the Doubs county ("département"), in charge of 3700km of roads in the Jura mountain. The aim is that a car equiped with snow tires can safely be driven at any time, except for exceptionnal situations or for specific roads. The plan defines roads where the deicing treatment is reinforced (860km of roads). 17 circuits are patrolled early in the morning, reporting road conditions to decision makers. Then, if needed, 53+81 circuits are deicied by civil servants or private contractors.

Here is the DOVH / POVH, that is the organization plan for a small city named Bourogne, 1900habs, in the Jura mountains. 4 agents are to be ready in case of alerts. One must wake up at 3am and 5am to check the weather and trigger the arlert. Some streets are deiced before others. The document states that the inhabitants and shopkeepers and are responsible for deicing sidewalks in front of their houses and shops. It's a very common rule in France and yellow plastic boxes containing salts are placed here and there for the winter by the cities frequently concerned by snow or black ice.

In Paris, the authorities observe the weather and scatter salt just in time (hopefully).

Source: I lived there for about two decades.