The SL3 route is subject to frequent delays due to the opening of the Chelsea Street Bridge – as many as ten times per day – for ships serving the upstream oil terminals. Each bridge opening causes a delay of up to 20 minutes, and the only alternate route involves a lengthy detour on Route 1A. Federal regulations give priority to marine traffic.[125] In December 2018, MassDOT officials sought to create a six-month pilot program to reduce peak-hour openings of the bridge, as well as the nearby Meridian Street Bridge used by the busy 116 and 117 bus routes.[81][126]
The CNG buses bought for the Washington Street service caused disruptive 35 Hz vibrations in nearby residential buildings.[127] It took a year to retrofit the fleet with new mufflers; during that time, older buses which did not have that problem were used at night.[8]: 3.5
