Sources report that Massachusetts has deployed blinking green traffic signals at some intersections. My drivers' education, at least, was entirely silent on the question of what one does at a flashing green. If red means "stop" and flashing red means "stop, and then proceed with caution," does that then make the flashing green a signal to go through the intersection normally, and then proceed with caution? If a yellow means "slow down" and flashing yellow means "slow down even more," does that make a flashing green your command to go faster?
It turns out (courtesy of the ODP and rec.travel), perhaps unsurpsingly, that there is no uniform agreement on the meaning of a blinking green light. In a bunch of Canadian provinces, it has the same general meaning that a regular green light does, with the added modifier that you are the undisputed master of all you survey. All other traffic entering the intersection has a stop sign or a red light, and must bow down before your awesome cosmic powers. On the other hand, if you're in Massachusetts or British Columbia and you try a no-look Ontario-style left turn on a blinking green, you're liable to get into a smackup, since the blinking green means only that cross traffic is seeing red, with no guarantees about oncoming traffic. One might observe that this makes a blinking green light entirely equivalent to a regular green light. In this, one would be correct, except that the flashing green is usually only triggered by pedestrians, which means the pedestrian who triggered it is probably somewhere nearby, and you really ought to be careful not to wrap your fender around them.
To recap, in Alberta, Manitoba, Ontario, Quebec, and New Brunswick, the flashing green light is your license to be more than usually reckless, but in Massachusetts and British Columbia, it means you should be more than usually careful. The usability gurus would have a field day, I'm sure. Messing with the traffic signals is a sure sign of a society in turmoil: during the Cultural Revolution, a Red Guards contingent observed that the Beijing traffic lights were reactionary, since glorious revolutionary red meant "stop." The proposal got as far as Zhou Enlai's desk before being tabled.