Driving terminology. At an intersection, when drivers making a left turn across oncoming traffic have right-of-way. In general, a protected left occurs with a green arrow pointed left traffic signal, or occurs when a green dot shifts through amber to red.

Except. of course, in the Boston area. Evidence to the contrary:

Left arrow == no right-of-way
Curtis & Powderhouse, Park & Beacon (both in Somerville, MA. At an intersection where a two-way street becomes a one-way street pointed into the intersection, a green light for the two-way side is replaced with two green arrows, pointed right and left. This indicates the drivers cannot go straight, not that they have any special right-of-way.
Green dot == protected left
Broadway & Alewife Brook Parkway, Somerville, MA. Each direction on Broadway has its own green light, out of phase with the other. A protected left exists with each green light, but is not denoted in any special fashion.
Left turn at the end of a cycle
Portland & Broadway, Cambridge, MA. While northbound and southbound traffic on Portland share a green light, this ends first for northbound traffic. A northbound driver, expecting to make a left turn as southbound traffic stops, is often surprised by the lack of deceleration in oncoming traffic.

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