I am currently building several major asphalt roadways on my layout and have found the following technique to provide excellent results:
1) I first draw the roadway onto the layout surface keeping in mind the way actual roads are laid out and how vehicles would negotiate them so my roads don't appear odd to viewers. You don't want to lay out turns that only a go-kart could negotiate. If in doubt, place a large truck or bus in your problem area and figure out how much room such a vehicle would need to access the area.
2) I next glue 1/4 inch wide strips of .040" styrene along the centerlines of each roadway to create a crown in the final road surface. These strips should meet in a "+" at typical roadway intersections.
3) Next, I cut roadway patterns from cardstock to match my roadway dimensions. Several pieces of cardstock can be taped together into a single pattern for complex roadway shapes.
4) Once my cardstock patterns are complete, I trace their outlines onto .040" styrene sheets. I then carefully cut out the styrene road shapes.
5) Prior to gluing the styrene road shapes into position, I use these pieces as patterns to mark and cut my road surface material. I use a product called "Fun Foam" made by a company called "Creatology". This dense 2 mm thick foam product can be found at Michael's in 12" by 18" sheets in various colors (I use black) for under $1 each. The surface of the fun foam has a bit of texture and the occasional bubble (pothole) that fits well with N and HO scales. It can also be roughed up with sandpaper if a bit more texture is desired.
6) Once I have finished using the styrene roadway base pieces as cutting patterns, I glue them down to the layout surface using latex caulking. I run beads of caulking along each roadway edge and atop the central crown strips then press the styrene sheet pieces into place. It may be necessary to tape down the edges of the styrene sheet pieces until the caulking sets, especially at intersection corners. Once the caulking has cured, I spray the underside of the matching Fun Foam pieces with Elmers Multi Purpose Spray Adhesive then press them into place over the appropriate styrene sheet area. I use Woodland Scenics Foam Putty to fill in joints between pieces of Fun Foam.
7) I usually allow everything to set overnight so that painting the road surface doesn't affect the adhesives. I next sand down the WS Foam Putty patches smooth with the adjacent Fun Foam surfaces. This product shrinks a little so these joints may need a second application of Foam Putty.
8) Once I am satisfied with the Fun Foam/Foam Putty surface, I paint the roadways a dark to neutral gray depending on how old I want the road surface to look. I use inexpensive acrylic craft paints. These paints tend to firm up the surface of the Fun Foam when dry and also seem to improve the surface texture a little.
9) Using a sharp hobby knife, I next make numerous light surface cuts perpendicular to the roadway edges and centerlines to simulate surface cracks. I try to use as random a pattern and crack shapes as possible. The Fun Foam acts somewhat like a self healing cutting matt in that the cut will shrink a bit. This works well as even deeper than desired cuts eventually return to believable scale size cracks.
10) I next use a black fine tip Sharpie pen to trace over the cracks to simulate tar applied to seal the cracks by road crews. These last two steps are a little time consuming but really add to the realism of the finished roads.
11) I use white and yellow fine tip paint pens to apply the road striping. Be sure to research roadway striping practices for your layout era and locale. I use a spray can of flat white paint and stencils cut from .020" styrene sheet for "Stop" and "RR Crossing" indications painted on the road surface.
12) The final step is to weather the road surfaces. I start by airbrushing a light gray wash along the edges and centerlines to fade the asphalt color and tone down the starkness of the tar sealed cracks. Once I am satisfied with the "aging" of the asphalt, I airbrush a dark gray wash down the center of each lane to simulate grease and oil spray along the normal driving line. Don't forget the right and left turn driving lines at intersections.
13) Add a few vehicles and the roadways are done (except for sidewalks). I'm still experimenting with how I will create my sidewalks but I will probably cut them from .040" styrene sheet and place them atop two layers of Fun Foam. When positioned against the edge of one of my roadways described above, the sidewalk surface is about 6 to 8 scale inches above the roadway edge. I will probably add curb faces using strips of .020" styrene cemented to the edge of the styrene sidewalks and sanded flush with the top of the sidewalk. Expansion lines will be added using a hobby knife.
One thing I didn't describe is how I handle grade crossings. This is where the .040" styrene roadway base really comes in handy! I first run my centerline crown strips fairly close to the railroad track roadbed material but you don't need to get all that close. I then cut the .040" styrene roadway base to fit right up to the edge of the track ties using the RR track roadbed to support the edge of the .040" styrene piece. When I glue down the styrene, I use a few thick beads of caulking between the RR track roadbed and the end of roadway centerline strip to support the styrene in the "transition area" after the caulking cures. Next, I gently press the styrene road base into the caulking several inches away from the crossing and atop only the edge of the styrene where it sits on the RR track roadbed. This allows the styrene base to assume its own smooth grade transition without creating the dreaded "stunt show jump" as the roadway approaches the grade crossing. Having used the styrene road base pieces as patterns for cutting the Fun Foam, the edge of the Fun Foam road surface stops just above the ends of the RR ties leaving room to add your favorite grade crossing materials.
I have been very pleased with the look of my roadways as they have realistically smooth elevation changes, nice drainage crowns, good surface detail and just enough variation due to the Fun Foam/Foam Putty surface to not look like they were graded using lasers.