Minecraft wiki entry for Building_a_metropolis, associated text: Recommended material: Pretty much anything road-like you can get your hands on.
A freeway is a road designed for high-speed vehicular traffic. No walking or parking is allowed in freeways, so allow only people riding pigs or horses to use it. Entrances and exits should be provided only at intersections that connect to other roads. Opposing directions of travel should be separated by median strips or barriers. Goes nearly straight, never exceeds about 1/8th slope, obliterates or bores through anything necessary excluding centers of cities or enemy territory (then wars are fought over its construction). The road should be about thirty blocks wide and as flat as possible. Bridges and tunnels can be built whenever needed, and no bridge type or tunnel is too expensive or extravagant for something like this. You can also include road tolls the players have to pay in order to use the road.