Challenges of Scheduling Frequent Transit in Mixed-Traffic

It is no longer strange for contemporary transit professionals to promote frequent transit as a solution to increase transit patronage due to the resulting connective network effects between high frequency lines. Nonetheless, scheduling for frequent transit in mixed-traffic conditions is easier said than done, especially along busy urban (and dense suburban) corridors where auto-to-transit shift is desired.

Schedule-less Service vs Schedule-less Timetable

The idea behind high frequency is to induce the positive forget-the-timetable effect and experience for the more discerning choice users, but creating a schedule-less high frequent service can't simply be translated as adding more buses (or streetcars) on the road. Schedule-less (read: improperly scheduled) service leads to bunching, which leads to unpredictability, which curtails much of the forget-the-timetable freedom that the service intends to create in the first place.

High Frequency Scheduling

In the ideal world, it would be easy to assume headway will remain the same for the subsequent trips. The table below explains how a service route headway deteriorates, when headway/frequency planning does not consider changes in travel time.

In reality, in order to achieve targeted headway/frequency, one has to plan for subsequent trips with the longest expected travel time, and work backwards, such as explained in the table below.

One often-touted solution is to afford extra layover schedule buffer times at the terminus. But then, too much reliance on schedule buffers results in poor operational productivity, as extra vehicles and drivers that will be required to serve during peak would be severely underutilized during off-peak. Furthermore, the higher the travel time variance between peak and off-peak, the harder would it be to maintain headway standard without the need for additional shifts and vehicles.

Striking the Right Balance

The most practical way for any cash-strapped transit agency to avoid this problem is to allow drivers and vehicles to be utilized on a part-time basis (such as portrayed in an Excel-generated schedule above - the columns labeled A and B represent peak-only runs (left-side) and headways (right-side), and the thin in-between columns represent manual layover buffer adjustments), but this can be pretty challenging, considering the rigidity of employment contracts (in North America) and operating permits of transit fleet (almost everywhere).

Buses would be much easier than trams to be chartered out for private uses during off-peak. Smaller buses (and vans) are much easier to be chartered out than longer buses due to the limitation of most commercial drivers from driving heavier vehicle classes. But then, assuming constant ridership contexts, smaller buses (and vans) have higher per pax operating cost than larger vehicles.

On the other hand, the non-permeable grid yet high-density (oxymoronic terms to North American ears) development pattern seen in many city-regions across the developing world provides fertile ground for experimentation of high frequency, low capacity buses (or vans), carving previously-unexplored but efficient paths between regional rapid transit nodes and urban activity centres. A 6-meter long, 2-meter wide van may provide a much more linear, time-saving route that serves cul-de-sac mixed use complexes sandwiched between squiggling-cookie-cutter arterials in a way that a conventional 12-meter long, 2.5 meter wide bus can't.