With the green paint, it's as safe as it can be with a limited budget and without removing parking. I actually think the execution is quite good, it's not just a random line in traffic which many people wouldn't know the meaning of, but a clearly defined space with a different color with cyclists actually using it! So the fact that it is being used already probably shows that most people won't consider their lives at risk if they use the bike lane, and besides, Boston traffic isn't that bad, it's mostly exaggerated, and the main problem area is downtown where slow moving crowds of tourists anger mostly taxicab drivers.