From a strictly personal perspective, the steel gratings that I have had the "pleasure" of driving on differed greatly depending on age. The new ones had traction galore, those that had been in service for some time and driven on regularly were very slick. It has much to do with how many tires have driven over them and smoothed off the edges of the steel. It also varies greatly depending on the relative hardness of the tire compound, a soft tire on a new grate will have very good traction, to the point that it will try to shred a tire, older grates with hard compound tires will slide at an alarming rate.
YMMV
George