Now and then I see a professional study of a transit line — often light rail or streetcar — that suggests that the two directions of service should be a little bit apart from each other, say on different streets, so that they "cover more area."

This is the clearest and simplest example I've seen of the conflict between symbolic transit and actual transit. If you are creating transit for symbolic purposes — say, to give the appearance of permanent mobility so as to stimulate development, then it's certainly true that separating the two directions, so that rails and stops appear on two streets instead of one, will spread that appearance over a larger area.

However, if you care about people getting where they're going, the one-way split reduces the area served by a transit line. That's because for a two-way line to be useful, you have to be able to walk to both directions of a service. The further apart the two directions are, the smaller the area (light blue) that will have a reasonable walk to both of them.

If you're wondering whether a project is about getting people where they're going or just appearing to do so, the handling of one-way splits is often a clue.

Obviously, one-way splits for transit are often required by a one-way street pattern, but even in these cases, when we're planning for both legibility and ease of use, planners sometimes suggest combining the two directions on one street. This can be done by giving transit a lane that allows it to travel against the main traffic direction (called a contraflow lane), so that although traffic is split between two streets all transit is on one. That maximizes the area actually covered by the line, but of course, it may reduce the area that has symbols of being covered!