A summary of this analysis was published on the Seattle Transit Blog, but here is a lengthier version of the write-up. Data, analysis, and writing by Zach Stednick and Mike Logsdon.
Although coverage of transit often revolves around Metro or Sound Transit, we wanted to shift the focus a bit to look at another method – ferries. It seems that the majority of the coverage of WSDOT ferries focuses on the fiscal side such as costs of ferry maintenance and replacement as well was critical examinations of ferry employee salaries. We wanted to change the conversation slightly and focus on the logistical aspect of the ferry system. Are the ferries generally on time? Are certain routes or vessels unusually punctual or unusually late? Are some days or times better or worse than others? To achieve this we filed a Public Records Request to gain access to this data. Here we present findings for on-time rates for calendar year 2014, where we specifically investigated the difference between actual and scheduled departure times. For simplicity this report considers only Puget Sound ferry routes. This work considers the data in an exploratory fashion – investigating patterns in the data but not considering statistical models or prediction algorithms.
Overall we find broad reliability of the WSDOT Puget Sound ferries, with a few minor trouble spots. Across the approximately 133,000 sailings throughout Puget Sound in 2014, average departure occurred 2.9 minutes after the scheduled departure. The three legs of the Fauntleroy – Southworth – Vashon triangle showed the lowest reliability, with average departure occurring 3.9 minutes after scheduled departure, a minute slower than the system average. Those routes experienced delays of at least five minutes on approximately one quarter of all sailings. The most punctual routes, in order, were Point Defiance – Tahlequah, Mukilteo – Clinton, and Edmonds – Kingston. Those routes were delayed approximately two minutes on average, and left within five minutes of schedule on 89 – 95% of sailings. While minor delays of five or ten minutes were somewhat common (15% of all sailings were delayed by at least five minutes), longer delays proved extremely rare. Only 19 out of the 133,158 sailings we investigated were delayed by more than an hour.
In addition to route, we also explored trends by vessel, month, weekday, and time of day. Not surprisingly, there was qualitatively a strong link between well-perfoming vessels and well-performing routes, and vice versa, and attempting to pull apart features of the route and dock from features of the vessel was beyond the scope of this write-up. The monthly pattern was also fairly intuitive, with the largest delays occurring in July and August around five minutes per sailing, and the smallest delays occurring in the depth of winter around two minutes per sailing. By day of week the delays peaked on Friday and Saturday at over three minutes per sailing, and were the lowest on Mondays at 2.4 minutes per sailing. Finally, by time of day, morning sailings were most reliable, followed by nighttime sailings, then daytime and evening sailings.
Preliminaries – Data Oddities
As is the custom, daylight savings time tangles up the recording a few hours of the year. For example, a sailing from Seattle to Bainbridge island was scheduled for 2:10 AM on the morning of November 2, 2014, right at the end of daylight savings time. The actual departure time was recorded at 1:11 AM. This likely reflects the manner in which sailing times were reported during the daylight savings switch, rather than an instance of a boat leaving one hour ahead of schedule. For simplicity the small number of late night sailings at the margin of daylight savings time were excluded from the analysis.
In addition there were several records of boats leaving inexplicably early. The most extreme was a sailing from Vashon to Fauntleroy, scheduled for 8AM on the morning of April 26, 2014. The actual departure time was recorded at 1:21 AM, over six hours before the scheduled sailing. These records were attributed as data entry errors and also excluded from analysis. (Wouldn’t you be disgusted to show up to the dock at 7:30 AM, only to be told that the 8AM boat already left… in the middle of the night?)
Routes and Vessels
Of primary interest is a look into whether certain routes or vessels are exceptionally reliable or frequently delayed. It’s a bit tricky to separate routes and vessels, as they are intertwined by scheduling, and either one could plausibly influence on-time rates. One could imagine logistical complications from either a high-traffic route, or poorly functioning vessel. Table 1 shows the number of sailings by route and by vessel for 2014. You can click on the table to bring up a full-sized version in a new tab.