MBTA used to calculate the reliability of a given transport mode using something called On-Time Performance(OTP). Now they have started using a new metric called Excess Trip Time (ETT) since December 2024 for Rail and subway.
This is calculated as a percentage of on-time trips that took place out of total trips that happened on that day. The way the count this is based on two different tests as outlined in their service delivery policy page 23 Table 9.
For non-frequent services(longer than 15 minutes), they measure the on timeness in 3 different points(called timepoints) accross the journey:
- At origin: service should start within 3 minutes
- Mid route: depart 1 minute early or within 6 minutes
- At destination: arrive within 5 minutes.
For frequent services,
- At origin: headway(time between two schedule buses) + 3 minutes.
- At destination: actual trip time shouldn't be more that 1.2 times the expected trip time
The final percentage has two components
- numerator: number of time points within standards
- denominator: total number of time points.
Note that the MBTA reliability metric dataset still publishes OTP.
To quote the Service policy page 19:
Percent of passenger trips where trip time is within 5 minutes of benchmark trip time, calculated
as the actual trip time minus benchmark trip time, summed for all passengers.
Now this metric is currently planned to use only for Commuter Rail. But we can calculate this for buses as well using the datasets in this portal search.