Villages which spawn in non-flat terrain occasionally have collapsed portions of roads where they overhang ravines, caverns, caves, etc. Assuming the villagers are residents, it would be more appropriate if the roads were safe. This can be acheived by using a standardized non-falling block (such as dirt), or a biome specific non-falling block (stone in mountain, mylecium in mushroom, dirt in taiga, forest, and so on) underneath the gravel used for roads.