I've used the same basic method for making my HO scale roads for some time now. I use the 2mm thick foamie sheets available from the local craft store. These are typically used by kids to cut out shapes and fill up the landfills in your area. The sheets are cheap, I think less than $1. The sheets give a nice base to the roads and have just a little texture on their own to add to the overall effect.
In my brewery area, I needed a large paved area for parking plus a road that went across the tracks to the freight house. I also wanted a short section of road on the other side of the access aisle.
First, you cut your foam sheets to fit the area. Then glue it down with white glue.
I like to take plaster and blend the road edge into the base of the layout. I also use it to fill any seams between sheets.
I also had to cut pieces to fit between the tracks for the crossings and then weighed them down until the glue dried. (This step was done before the plastering.)
Next, I use a Ralph Lauren specialty paint from Home Depot. It's got sand in it, and give a really nice texture because the black sand flecks show through the gray base. Paint all your foamie sheets.
At this point, I also needed to make a retaining wall to go with my road. I used black foam core as the base, and glued a texture printed out from my computer. Here I'm test fitting the pieces.
I fixed them in place using caulk.
But what to do about those vertical seams? Vines to the rescue:
The last step is to add cracks and weathering to your roads. You can also make patches for your blacktop. I use a micron pen to draw cracks on the surface. Just draw them in sort of a lightning bolt pattern with cracks radiating off of other cracks. All you have to do is look at a real road for plenty of ideas. The end result looks pretty good. I still need to add a white stripe down the middle, and then I'll be done.
I'll also add some black chalk weathering to both the road and the parking lot for the final details, but won't bother doing that until I'm done doing all the landscaping in the area.