Firstly, you are right about newly laid asphalt being more slippery than the older stuff. This is to do with the fact that there is a fresh, thin layer of bitumen covering the stone in the material. As this is worn away with trafficking etc, this will expose the natural skid resistance of the stone, and in effect make the road rougher, giving a better grip. For most roads in the city there is a minimum PSV (polished stone value) specified. This is not the same for footpaths though.
Regarding minimum thicknesses, this depends on the size of the aggregate in the layers. For footpaths where a 6mm aggregates material is used, the British standards call for a layer of around 25mm thick. Roads containing 14mm aggregate as a surface course, will be laid around the 40mm mark. The majority of the material being laid on the main roads are probably a proprietary 'thin surfacing' system and will be designed to perform at such depths.