A few years ago we assisted San Francisco Municipal Transportation Agency in rethinking how they talk about the various services they operate. Our key idea was to classify services by tiers of frequency, while also distinguishing, at the highest frequency only, between faster and slower services. In extensive workshops with staff, we helped the agency think through these categories and the names to be used for them.
It's great to see the result coming out on the street. The old-fashioned term "limited," for example, been replaced by "Rapid," a new brand that emphasizes speed and reliability improvements as well as frequency and widely spaced stops.
But the biggest news is that a new network map, by Jay Primus and David Wiggins, is about to debut. Don't open it yet!! Before you do, look at it in a fuzzy small image:

Notice how much information you can get just from this fuzzy picture. Most transit maps are total nonsense at this resolution, but in this one, even though you can't even see the legend, you can see the structure. All you need to know is that bigger, brigher lines are more useful lines, because they tend to be faster or more frequent. In other words, this works just like any coherent street or map (paper or online) in which the faster roads are more visually prominent. Any good map is legible at multiple levels of attention, including very zoomed out like this, and in loving detail of every right and left, which you'll also admire if you zoom into the massive PDF.
Why has it taken so long for transit maps to get this clear? Well, first of all, you have to figure out that frequency, not speed, is the primary equivalent of speed in a highway map. Highways and streets can all be ranked by their design speed, but in transit, frequency trumps speed in determining most kinds of utility, and speed distinctions matter most where frequency is already high.
(The exception, high speed but low frequency service, tends to be commuter rail and commuter express bus service. That service is so intrinsically specialized and complex that it makes a complete mess if you put it on the map with the all-day frequent routes. These ephemeral routes must be faded out; on this new map they are the weakest lines of all. The previous map was chaotic precisely because it used the strongest color — red — for these most specialized and ephemeral services, concealing the structure of interdependent service that is running all the time and that vastly more people will use.)
What are the other barriers to maps of this clarity? Well, you have to decide whether your goal is information (helping people understand their options) or marketing (which at its worst means deliberately confusing people so that they do you want them to do). I have always argued that in transit, clear and beautiful information is the best marketing, but many professional marketers disagree.
This map is glorious because it's 100% information. Services aren't highlighted because someone thinks that they serve "target markets" or "more important demographics", for example. Everything is mapped, and named, according to its potential usefulness to anyone. The more diverse the range of people who'll find a service useful, the brighter the line is.
Of course, it doesn't show everything, but that's also why it's clear. I'm sure I will be bombarded with comments pointing out that they don't show how San Francisco's network connects to the wider region's, and that they don't show how transit integrates with cycling, walking, private transit, Segways, and whatever else. Including too much non-transit information is also a great way to make transit maps confusing. This map is just what it is, a map of San Francisco's fixed route transit network. It's also, in my experience, one of the best in the world, something even the world's best transit systems could learn from.