Friday, September 8, 2017

The Route Productivity Problem

Spontaneous Accessibility measurements concern themselves with properties of transit networks as a whole. This is most evident with Network Accessibility, but even Time Qualified Point Accessibility, though localized to a single center point and starting time, has this property. Though viewed through a narrow aperture, it measures the ability of the transit network as a whole to provide service. Though the blog has touched on the many advantages of Spontaneous Accessibility measurement, it is not without downsides. It can feel far removed from the techniques that planners can actually use to improve a transit network. The realm of planners is one of transit corridors, routes, and their respective frequencies and spans; these are the controls that can be manipulated in network design.

Spontaneous Accessibility measurements evaluate the outcomes from these manipulations, but on their own do not offer much guidance into how the controls can be manipulated to achieve a positive outcome. This is problematic because when planners make changes, they are rarely overhauling the entire transit network. While drawing out a new network from scratch and measuring its Network Accessibility could result in a vastly improved transit network, it is a costly endeavor that is incredibly disruptive to current transit users. For this reason, I consulted service planning documents from King County Metro in Western Washington and the Massachusetts Bay Transportation Authority (MBTA) of the Metro-Boston area, to demonstrate how Spontaneous Accessibility measurement could best improve and supplement tried and tested service planning processes.

Both documents establish procedures for evaluating the benefit that an individual transit route provides. King County Metro calls this route productivity in their 2016 System Evaluation document. Route productivity is based on two measurements, riders per platform hour and passenger miles per platform mile (where the "platform" qualifier indicates that the measurement includes time when the bus is out of service, such as driver breaks or deadheading). While route productivity is a secondary consideration to crowding and lateness when allocating additional service, for removing service it is the primary signal. Routes are divided into urban and suburban categories based on their characteristics. Then each route is measured on both measurements for peak, off-peak, and night timeframes. Routes that fall within the bottom 25% of these categories are candidates for service reductions when funding is imperiled.

The MBTA's Service Delivery Policy describes a Bus Route Cost-Benefit Ratio. This measurement is a weighted combination of the ridership of the route, the ratio of those on board who are transit dependent, and a Value to the Network measurement. The latter includes the catchment area: a count of people uniquely serviced by the route, the number of jobs near the path of the service, and the proportion of passengers who make use of the service to connect to additional service. Though a low score is not a trigger for making service cuts to a route, it may be used to make other modifications.

Both route productivity and Bus Route Cost-Benefit Ratio are fairly complex measurements. Ridership measurements, which inform all of route productivity and the vast majority of Bus Route Cost-Benefit Ratio, require that passenger boardings and deboardings are properly recorded. This can be difficult because equipment may be present on only a subset of buses, the recording may not work accurately, or, due to shortages, buses with malfunctioning sensors may still be used, polluting the data. Furthermore, ridership variation may have causes outside of the transit network itself. Weather, extended road closures, and special events may influence rider behavior enough to distort the six month data collection timeframe that King County Metro uses to measure route productivity. Measuring and utilizing catchment area is also complicated. While a route might appear to have a small catchment area because other transit service might exist in the proximity of a route's path, that service may or may not allow the same destinations to be reached as the route that is being evaluated.

Nevertheless, the ability to assign a value to a single route is clearly an important part of a practical transit planning process. While Spontaneous Accessibility is a network-level measurement, it is possible to apply its principles to the measurement of a single route. Each route contributes some amount of Spontaneous Accessibility to the whole network. A valuable route contributes Spontaneous Accessibility in a unique way, connecting origins and destinations at times that no other route does, whether directly or through the connections that it enables. To measure this, first, a Network Accessibility or Sampled Network Accessibility measurement is computed for the entire network. Then, using the same collection of Sectors if Sampled Network Utility was used, a single transit line is marked as ineligible and the measurement is made again. The proportion of change between the ratios, called the Sampled Network Accessibility Contribution (SNAC) or the Network Accessibility Contribution (NAC), is used to evaluate the impact of the route on the network's Spontaneous Accessibility. If the transit line was largely redundant, the network will hardly show an effect, as riders have alternate paths to the Sectors that the line served. Otherwise, the ratio may be substantially changed reflecting that many unanticipated trips have become more difficult.

Where NAR' and SNAR' indicate the ratios calculated with the proposed change in effect.

To demonstrate, SNACs modeling the elimination of several King County Metro routes that were considered unproductive by route productivity were calculated. A thirty minute isochrone and 2000 samples were used. Each route ranked in the bottom 25% in at least half of the timeframe and measurement type pairs for which it was eligible (some routes do not operate at night or off-peak, and thus do not have measurements from these periods) and did not have any measurement in the top 25%. As SNAC measures the Spontaneous Accessibility improvement of some change, routes that are more valuable have lower scores. The results indicate that though Metro's measurements view these routes as comparably unproductive, their contributions to the network's Spontaneous Accessibility vary widely, suggesting a variety of causes, and thus solutions, to their deficiencies.

Route Route Map SNAC Comp. Map (Common scale) Comp. Map (Normalized scale)
4 Link -0.00111 Link Link
24 Link -0.00455 Link Link
33 Link -0.00393 Link Link
37 Link -0.00045 Link Link
47 Link -0.00003 Link Link
99 Link -0.00011 Link Link

Routes 47 and 99 appear to be largely redundant with other service. For these routes, more frequent service is available on streets that are very close (Broadway for the 47 and 3rd Avenue for the 99), and thus for most trips, it is a better option to take this more frequent service and walk to destinations on the paths of these less frequent routes. They appear to be the strongest candidates for complete elimination. Route 37 provides value along the coastline of West Seattle, but the route's tail into the interior provides little benefit from a Spontaneous Accessibility standpoint. As such, costs can be reduced by truncating it. Route 4 provides most of its value on the path that it shares with route 3. It provides additional Spontaneous Accessibility value to the Sectors north of Mount Baker in the I-90 corridor. Perhaps this area would be better served by service to frequent transit on Rainier Avenue rather than a meandering path towards downtown. Routes 24 and 33 provide considerable value throughout Magnolia; reducing their service would make unanticipated trips to and from there considerably more difficult. However, route 24 is very circuitous and route 33 does not get to northern Magnolia in a particularly direct way. In this case, it may only be possible to preserve Spontaneous Accessibility and reduce hours by fundamentally restructuring service to Magnolia.

Conducting an analysis like this one takes mere days from conception, to execution, to visualization and evaluation. A more thorough study would consider a variety of walking speeds, to ensure the eliminated routes are not critical to maintaining Spontaneous Accessibility for riders with mobility issues. In addition, it would also be useful to know the SNAC of eliminating route per some cost of its operation. This would ensure that short routes are not disproportionately targeted. Also it would be best to study a variety of SNACs with different isochrone times. These variations would not extend the study time greatly, allowing much faster understanding of the value of routes than studying ridership for six months. It is also a more direct measurement. Low ridership is a symptom of many diseases afflicting transit networks; elimination is not always the proper cure. By measuring the properties of the network directly, it is more evident whether and how a route is providing value.

For both King County Metro and the MBTA, using Spontaneous Accessibility Contribution measurements could improve their route evaluation processes in a natural and non-disruptive way. Its extra insights allow any agency that must evaluate its routes to make more nuanced decisions without upending existing processes.