CTT advise h+lm+ts, but they're not compulsory. The unfortunates doing TT's in Jockland, though, do have to wear them. IIRC anything under the auspices of ASO, the UCI and/or a national federation will have mandatory h+lm+ts too.
Mass start road racing requires a helmet though.
Most cyclosportives (in the UK and elsewhere) now seem to require you to wear a helmet.