According to various international experiences reported in a recent OECD book, a 10 km/h speed limit decrease generally leads to a 2 to 5 km/h decrease in speed, this variability being linked in particular to the associated communication and control actions.
The speed limit in urban areas was set at 60 km/h in 1954 and 50 km/h in 1990. Outside urban areas speed limits were set between 1973 and 1974 at 130 km/h on motorways, 110 km/h on dual carriageways and 90 km/h on single carriageways. The latter value was reduced to 80 km/h on 1 July 2018, except on sections of these roads with at least two lanes assigned to the same traffic direction where it is maintained at 90 km/h. For novice drivers or in the event of rainfall, the limits are lowered to 110 km/h, 100 km/h and 80 km/h respectively. The traffic police authority (prefect, president of the county council or mayor) may lower the speed limit for safety reasons, or increase it to 70 km/h in urban areas. Article L2213-1-1-1 of the General Code of Local Authorities authorises the mayor to set a lower speed limit for all or part of the roads in the urban area open to public traffic than that provided for in the Highway Code, taking into account the need for road safety and traffic, mobility or environmental protection.
Decreasing traffic speeds through safety cameras
France has been implementing automated safety cameras throughout the network since 2003. Traffic speeds hence decreased, which was beneficial for road safety in decreasing serious road traffic accidents. Download here Ifsttar evaluation paper on the programme (by researchers Carnis and Blay).
Speed in road accidents
Speed affects both the driver's ability to adapt to the situations encountered, the occurrence of the accident and the severity of the injury. Despite technical progress, vehicles are not designed to withstand impacts at high speeds (above 55 km/h at impact). Equipment, seatbelts or airbags, and the absorption of energy by deformation are not sufficient to protect the internal organs of the human body. However, most fatal accidents for passenger vehicle occupants occur at residual speeds (after braking) between 40 and 80 km/h.
In 2017, according to police information on the presumed authors of fatal accidents, excessive or inappropriate speed was present in 27% of accidents. This proportion is higher than the average for powered-two wheelers (44%) and lower for utility drivers (18%) and truck drivers (13%). The "excessive or inappropriate speed" factor occurs more often than the average on roads limited to 70 km/h (41%), which probably reflects the difficulty for drivers to perceive the specificity of these sections and to become aware of the associated dangers. It is less present on motorways limited to 130 km/h (16%).
More information :
https://www.itf-oecd.org/sites/default/files/docs/speed-crash-risk.pdf (OECD Speed and crash risk report).