The sign is a palimpsest, but the key message can still be deciphered: 'Sécurité sobriété' (safety and sobriety). This anti-drinking campaign began on Parisian public transport in 1955. Within a few years, it spread throughout France and the roadside signs emphasised the dangers of drink-driving. The earlier advert is a design with a blue ground and a large glass; the slogan is at the bottom. (A similar sign with a different slogan can be seen here). The wording inside the glass reads 'Votre voiture est sobre - faites comme elle': roughly, 'Your car is sober - be the same'. The slogan was familiar to drivers in 1966 [PDF], so this sign probably predates that. Research published that year found that the signs were difficult to read, so either their message was lost or they were counterproductive in terms of road safety!
That research seems to have had an effect as the second version is much more emphatic. The slogan is emblazoned in heavy black letters across the top of the sign. It appears to be the 'falling glasses' design here, suggesting that the black cross and square also faintly visible belong to a third advert.
Altogether, not a bad first find!
