My area is rural enough that few secondary roads are paved; most are gravel or downright 'rocky' surface. However, there is a small subdivision about a mile away whose two 'streets' are paved, and I do go there and drive on occasion. As already noted, asphalt pavement *can* be used to help build bone density; it's just a matter of good judgment in slowing building up, never 'overdoing' it, on pavement. I've never had a horse slip, but am always careful in turns, stops, etc. I both walk and trot there; how much/how long depends on the particular horse and its level of maturity and physical readiness and fitness. I wouldn't want to drive on asphalt ALL the time, but it can have its usefullness as a place to SOMETIMES drive.
The road in front of my own house is 'worse', as it had a good deal of fairly large diameter ROCK added a year or two ago, rendering it virtually useless for me to drive on---now I carefully and slowly WALK my horses down to the corner-thankfully, only .2 mile--then I can drive the dirt trails along the highway right-of-way-from where I can access the paved subdivision, along with other less rocky secondary roads and byways.
Margo