(This is assuming black background with white dips leads to pits, and white background with black dips leads to bumps in a bumpmap)

Well -- if you want good pavement, you may want to make a VERY VERY dark bumpmap jpg in photoshop, and give slight subtle dark greys to it to create a bumpy effect like asphault. It all depends on the condition of the road/weather of system aswell.

Example, if its a dry dusty desert road on a hot hot day, I believe its possible to emit a very very short-lived particle cloud in the distance, set to clear BUT with a refractive index (**making sure to set your skybox and other distance objects "visible in refractions"**) to make a heat-wave effect. Turn on raytracing in the renderer if you want to refract

However, if its a temperate well paved road, you may want a very very dark area with a well defined center divider/paint.

If its a sideroad, you may want a very very pale road with a couple of faces set darker to look like asphault patchwork, maybe draw a few exadurated lines in your bumpmap file to mimic cracks.

However, if its a super-futuristic metalic road, you may want a slightly reflective bottom (raytraced reflections if desired) with well defined bumpmaps to look like patched metal.

But most importantly, and most obviously, be careful with your dimensions! Keep things proportionate to whats ontop of it.