Last week, Jarrett Walker had a great post illuminating the basic reasons for ‘frequency mapping,’ where a transit agency maps out transit routes that meet some threshold for frequent service (such as buses every 10 minutes, or 15 minutes, etc).

There are many degrees of frequency and span, but in general, most transit agencies’ service can be sorted into three categories of usefulness based on these variables:

• The Frequent Network runs often enough that you don’t have to plan your trip around a timetable.  That typically means every 15 minutes or better all day, but it needs to be more frequent than that where aiming to serve relatively short trips — as in the case of downtown shuttles for example.  If you aren’t willing to plan your life around a bus schedule, you are interested only in the Frequent Network.
• Infrequent All-day services are the rest of the service that runs all day.  This network often relies on timed connections.
• Peak-only service exists only during the peak period.  It mostly takes the form of long commuter-express routes that add lots of complexity to a system map but represent very specialized services for limited markets.

These three categories are useful in such completely different ways that I would argue they are at least as fundamental as the three basic categories of urban road — freeway, arterial, and local — that virtually all street maps clearly distinguish.

We have some great examples of this in DC.  The entirety of the Circulator network is, in essence, a Frequent Network.  The Circulator aims for 10 minute headways, the routes are fairly simple and easy to understand, and thus people can look at the map and understand where the bus is and where it’s going.

WMATA’s bus map for DC, however, doesn’t make this distinction.  While there is a extra color designation for Metro Extra service (meeting the Frequent Network threshold), the other color distinctions merely show which jurisdiction the bus route operates in.

The distinction between which services operate only in DC (in red) and those which cross into Maryland (green) isn’t really important for a rider.  Furthermore, the overwhelming use of red for the DC routes makes it hard to follow those routes across the map, seeing where they turn and what streets they travel down.

Blue services with dashed lines, however, is indicative of MetroExtra (for some reason, a separate brand from Metro Express), and at least makes a effort at differentiation based on frequency – but that tends to get lost in the visual complexity of the overall map.

There’s a common phenomenon of ‘rail bias,’ (hat tip to The Overhead Wire) where riders will opt for riding a train rather than a bus.  However, rail systems tend to have several key attributes that make them more attractive – the investment in the infrastructure both enables and requires a high frequency of service, and the route structure is almost always simple enough to convey in an easily-understood diagram or map.

The lesson from Jarrett’s post is that simple mapping based on frequency can help address some of the perceived shortcomings of buses.  Even without addressing route structure, this is a relatively simple improvement in communication that helps riders a great deal.