My commute home is almost entirely in bike lanes on Commonwealth and Massachusetts Avenues. Both are decent bicycle facilities on busy roads, but the Comm Av lane is rather unique in that it is a left-side lane. It's very nice in that there are no issues with open car doors, and it runs alongside the Comm Ave median, so there are no pedestrians nearby. This facility was studied extensively: Google has a lot of information on it; the first five links here are a good primer.
(In the long run, this might work very well as a raised, separated and wider bike lane along the Comm Ave median would be a further improvement, which could be made eight feet wide by taking a foot from the median and a foot from each of the 11-foot travel lanes, which could be narrower since Comm Ave is closed to trucks and buses.)
There are even bike boxes, so that bicyclists traveling on the left and wishing to turn right can cross in front of stopped traffic without impeding the crosswalks. It's quite nice. All of the roads are one-way, so when you're biking the street there are only four crossings in each direction where you have to worry about any cars turning in front of you from the left-hand travel lane across your travel path.
However, at those four locations, turning traffic can be dicey or dangerous. The left-hand lane is the first of its kind in the area, so drivers are not accustomed to how to behave. And state law is not particularly applicable, as bike lanes are defined as lanes and the driving manual states that you must "turn from the lane closest to the lane you want to enter" while the statute tells you to be as close to the curb as practicable. And a bike lane is defined as a lane. So drivers might be supposed to signal, merge safely in to the bike lane and make their turn, rather than passing a cyclist and abruptly hooking in front of them. But cars don't do that, and no one really knows what drivers should do. And that's for right turns—no one has ever seen a left turn, which exists at less than a dozen locations on Comm Ave anyway.
Common sense dictates that a driver should, if they are going to make a turn, yield to cyclists if they are present, letting them cross the intersection and then make the turn. Which would be fine, but I've had multiple occasions of left-hook turns in front of me in the lane, causing me to have to slam on my brakes and nearly hit the vehicle. All of these cars had passed me and not noticed or not cared. I chased down and chastised two of these (it wasn't much of chase; they encounter a red light in the Comm Av median) and both—one a civilian and one a taxicab—said I didn't have right-of-way.
So I wrote to the city. "These are great lanes," I wrote, "but at these intersections we should have signs telling motorists to yield to bicyclists on turns. I copied the bicycle coordinator (Nicole Freedman has moved on to white pastures) on an email to the Taxi police. The last time I'd sent an email about a dangerous road, the pothole was filled. But I didn't expect much.
Until I was riding today and noticed the signs.
WOW. That's perfect. I have no idea if it's thanks to my email, but it gets the point across. It's not foolproof, but it certainly lets vehicles know that they do not have right-of-way, and should think twice before cutting in front of a cyclist.
I'm impressed, Boston Bikes.
Quick note on the sign picture. I tried taking it with my phone with the flash on, and the reflectivity washed the sign out. I tried taking it without, and it was dark. So, finally, I used the headlights of the passing vehicles to get the right exposure, and it came out, well, good enough.