How cold does it have to be outside for the roads to ice up?

The ice will form once the temperature hits zero Celsius or 32 F. however, it may not still freeze on pavement, even if the ambient temperature has reached this for two reasons.

a) There is something called latent heat. it takes some more cold before zero Celsius water turns into zero Celsius ice.

b) the earth below the pavement surface may be warmer than the ambiance, helping water not to freeze.