This is a nearly impossible intersection, with traffic coming up Mass Ave (from below your picture) needing to merge with traffic coming in from JFK St. The latter sometimes want to turn and go through the tunnel, while the former (apparently including yourself) want to go left and continue on Mass Ave toward Porter Square. The traffic entering the mess from Garden, and wanting to go either on Mass Ave toward Porter Square or through the tunnel toward Cambridge St, Broadway, etc, does not help much either. While I think relatively recent changes have made things a bit better, the reality is that almost any change make some transitions easier and others harder.

Having been sideswiped once and had many near-misses, when I'm driving up Mass Ave from Central toward Porter, I tend to avoid the problem entirely by taking a right on Trowbridge, going to Broadway, and then going around and through the tunnel and onto Mass Ave there. That route probably takes longer and has its own disadvantages, but I end up feeling much safer.

That, of course, doesn't make your questions any less important but, unless the rules are made more clear and then enforced, it is not clear where they lead.

Thank you for reporting your concern to City of Cambridge via Commonwealth Connect. We have reviewed the information you provided our staff and will be investigating this issue further.

I'm so glad someone brought this up. The lane markings make it look as if you need to be in the right lane to go left and the left lane to go right. That's got to be wrong, but I've been honked at for being in the left lane when going left toward Mass. Ave. north.