I've been using HO cork switch pads. It the right width for a two lane road in O scale. I paint it with Woodland Scenic's asphalt paint. It looks similar to some of the various tints of different asphalt down here in Texas. One can tell when driving thru Texas that you have entered a new county because the color and feel of the road changes.
Since our layout is modular I have never setup my city exactly the same way twice. To this end I have to be flexible so many times I use black poster board and cut out the shape of the road I need. I have lots of pieces stashed in a box along with more cork.