Generally, "up" or "down" can be used interchangeably, so "he went up the road" = "he went down the road." However, if the road has a slope, going from a higher position to a lower one is definitely "going down the road," and vice versa. Yes, you're right in that "up the road" could also mean "go North along the road" and "down", "go South along the road." But unless there is a clear slope in the road, people will often use "go up" and "go down" the road as in "go along the road."