why are frost levels deeper on roads and highways?

Answer 1

Roads and highways give up their heat more readily than earth (soil) especially when there is radiation cooling on a clear cold night. That allows the pavement to fall below the dew point of the moisture in the air permitting condensation and frost. During a winter in Louisiana I recall driving down a road that seemed perfectly fine then doing a 180 on an overpass with a thin roadbed that cooled enough (above and below) to develop slippery frost. It was totally unexpected.