Introduction
In December 2024, the MBTA adopted an updated Service Delivery Policy (SDP) that includes new methods of calculating service reliability. For the T’s heavy rail service (the Blue Line, Orange Line, and Red Line), reliability is now measured using Excess Trip Time (ETT). This post will discuss what ETT is and the methodology behind the measure, how ETT compares to the formerly used On-time Performance metric (OTP), and how to use the new ETT dashboard on mbta.com/performance. For more information on the new SDP, please visit mbta.com/SDP to view the policy, and view our OPMI Data Blogpost on how service evaluation differs under the new policy (coming soon). Formore information about other recent updates to the MBTA Performance Dashboards, please view our OPMI Data Blog posts on Ridership Dashboard Updates and Trip Times Post-Track Improvement Plan (coming soon).
What is Excess Trip Time?
ETT is a passenger-weighted metric that measures the amount of time that a passenger spends waiting for and riding public transit in excess of a baseline. In other words, ETT tries to answer the question - what is the passenger experience on transit? This is fundamentally different from the metric previously used to measure heavy rail reliability, OTP. OTP is based on the scheduled headway for service, and calculates the percent of passengers that wait less than the scheduled headway at stations. This enables the T to determine performance using the resources we have available in a given rating[i],which in general, is typically above 85% for heavy rail. However, adherence toa schedule does not necessarily reflect good customer experience. If a customer experiences, for example, a 15-minute headway during the morning peak and travels over multiple sections of track with restricted speeds implemented, the trip may be precisely as scheduled but is still a poor experience for the customer. ETT seeks to better measure the passenger experience by comparing experienced wait and travel times to a baseline trip time. In simpler terms, ETT measures how long a rider’s journey actually takes, versus how long that rider’s journey should take. While this concept is new for the MBTA, a version of ETT is already in use by other transit agencies, such as the MTA in New York City, and WMATA in Washington, DC.
[i]A “rating” is essentially a season, and is the frequency at which service schedules are updated to reflect changing resources.
Methodology
The three main components of this metric are passenger loads, passenger wait times, and travel times for each OD pair on the T’s heavy rail system. Actual passenger loads and wait times are derived from the Origin, Destination, and Transfer (ODX) model in use at the T. The model, developed by Korbato, infers a passenger’s origin, destination, and any associated transfers based on card validations throughout the day. The algorithm therefore places passengers on specific vehicles, allowing us to calculate headways associated with each passenger’s trip and the amount of time spent onboard the vehicles before alighting. Both of these components are calculated at the trip level.
Travel times, both actual and baseline, are calculated using data from the Lightweight Application for Measuring Performance (LAMP), which provides trip-level data on subway travel times. The actual travel times are calculated by service period by day (e.g., AM Peak, PM Peak, Off-peak) and applied to any trip made during the respective service period. The baseline is calculated by finding the fastest monthly median travel time by segment since April 2021, with each possible origin/destination runtime equaling the sum of the segment runtimes. If the origin stop is not a terminal, the runtime is calculated from the vehicle arrival at the origin stop to the vehicle arrival at the destination stop. If the origin stop is a terminal, the runtime is calculated from the vehicle departure at the origin stop to the arrival at the destination stop. The track condition varies significantly across the system and receives upgrades and repairs on different timelines, so this method ensures that the baseline travel time is theoretically attainable but also a stretch.
The calculation is:
(Actual Wait Time + Actual Travel Time (includes Dwell)) - (Baseline Wait Time + Baseline Travel Time) = Excess Trip Time
The actual wait time is the actual headway for each trip divided by 2. This uses the imperfect assumption that passengers arrive at a random cadence during the time between trains, so each passenger’s expected wait time is half of the headway. Although imperfect, this method does allow us to penalize for uneven headways because fewer passengers arrive during the shorter headway. For example, imagine there are two trains scheduled 10 minutes apart. The first arrives after only 5 minutes and the second arrives 15 minutes later. If 100 passengers walked up to the platform during those 20 minutes, 25 of them wait an average of 2.5 minutes(62.5 passenger-weighted minutes), while the remaining 75 passengers wait an average of 7.5 minutes (562 passenger-weighted minutes). Combined, the 100passengers spent 625 minutes waiting. The baseline would have been 5*100 = 500passenger-weighted minutes.
