tominthehat's technique has much to recommend it, particularly for larger scales.
But, also because there are two kinds of bithumenous roads.
One is hot-placed asphalt, where aggregate (engineering term for specifically-sized gravel and sand) is completely coated in bithumen tar, and placed in a single-pass lift. Which are between 1" and 3" thick in US practice.
The other is tar-macadam (also known as tarmac). In that case a gravel surface is set over the compacted subgrade and shaped to the road contours. Over that is sprayed a heavy coat of liquid tar. A finish coat of graded aggregate is poured over that tar, and rolled into the wet tar. tominthehat's technique with talus in the mix replcates tarmac really well.
In US practice, sometimes the two medthods are mixed. For rural roads, the subgrade is often made four lanes wide and the entire surface tarmac-ed. The actual road surface is then paved, two lanes wide in asphalt to render a paved shoulder.
Now, in US practice, asphalt roads get a significant crown, often as much as 6" for a 25' wide lane. Two-lane roads are often built with a "bar ditch" (short for wheelbarrow) where the side of the road way are cut, and the spoil piled to the center of the right-of-way.
In US practice, rural lanes and highways have 12' wide lanes and a 12" wide center space (depending on era and state with a double or single line; amended with dashed lines).
Larger highways will use a 14' lane, and Interstate highways use a 16' lane.
Center stripes are yellow (at least after about 1940-50), and side stripes are white. In the US at least. In France, the center stripe is solid white, and the side line dashed white.
Interestingly, US practice changed on rural highways in the 80s, the center space was increased to 6', the better to help prevent head on collisions. This was later increased in the late 90s to be 12' full feet with 'wake up" texturing (a 2" wide, 1" deep. 6" long every 6" on center impression cut or stamped in the pavement).