Silver Line SL4 Named Boston's Slowest Bus Route
Data from October 2025 shows the route between South Station and Nubian Station moving at just 6.09 mph.
Updated on Sept. 29, 2026 in Remote Work
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An annual review by Transit Matters identified the MBTA Silver Line SL4 as Boston's slowest bus route during October 2025. The route averaged only 6.09 miles per hour, contributing to broader concerns about transit reliability in the city.
Why it matters
Traffic congestion and blocked bus lanes continue to limit speed and reliability for daily riders. With seven of the 10 slowest routes serving the Roxbury neighborhood, these delays disproportionately affect residents who rely on public transit.
The 10 slowest bus routes in Boston averaged 6.42 miles per hour in October 2025, an improvement from 6.83 miles per hour in October 2023. Additionally, the 32 route recorded the highest bunching rate at 17.1%.
The players
Transit Matters
This organization tracks and publishes performance data regarding the Massachusetts Bay Transportation Authority.
MBTA
The Massachusetts Bay Transportation Authority is the primary public transit operator for the Greater Boston area.
The details
Transit Matters analyzed public performance data to evaluate route speed and bus bunching rates, finding that the bunching rate for the 10 worst-performing routes reached 13.9% in October 2025. Boston plans to address these issues by expanding bus-priority measures, including dedicated lanes and enforcement cameras, though infrastructure project delays could blunt expected gains.
Timeline
October 2023: The 10 slowest bus routes averaged 6.83 mph.
October 2024: The 10 slowest bus routes saw higher bunching rates.
October 2025: The SL4 route averaged 6.09 mph.
Market Landscape
This performance analysis follows the trend established by the Transit Matters annual bus performance review to track transit reliability across the city. The data highlights how local infrastructure constraints continue to limit public transit efficiency compared to the broader goal of improved urban mobility.
Reliability issues on these routes directly impact daily commuters, particularly in the Roxbury neighborhood where 39% of residents lack car access. Ongoing infrastructure projects, if delayed, may prevent expected improvements in daily transit times.
The takeaway
Public transit speed and reliability remain significant hurdles for city residents reliant on bus service. Improving these metrics will require consistent progress on planned infrastructure priorities to overcome chronic traffic congestion.
Further reading
Learn more about the latest developments in transit efficiency on the Remote Work section.
Source note: This article includes information reported by The Cool Down.
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