My wife is good friends with one of the guys who works for Metro Traffic. They have about six functioning cameras left, and they're never, never going to be repaired. The whole system was very expensive when it was installed, and the prices on those kinds of cameras haven't come down much. The original installation was, I believe, paid for by a one-shot grant...and neither Metro nor anyone else has the money to maintain the cameras; they're just that expensive.
That said, the cameras were nice but they rely more on reports from a cadre of many "semi-official" spotters they have all over the region that call in reports. Plus calls and reports from just random Joes who report stuff via the SmarTraveler cellphone interface (*1 on your Cingular phone...or 617-374-1234 for regular diallers). You'd be surprised how much good intel they get that way. And there's the reports from the overhead choppers and even the Red Sox blimp sometimes.
In terms of how often they update...it's pretty continuous from about 6am to 8pm. Figure reports are updated every 5-15 minutes (mostly around every 10 minutes). But the problem, if you can call it that, is that traffic is SO dynamic that a "free-flowing" area now could be at a standstill within 30 seconds. And once a slowdown happens, it starts rippling back along the highway very fast; cars travelling at 55MPH cover 50 ft. every second. So that ripple is travelling backwards often just that fast as well. No wonder a nice commute can change to a nightmare so quickly!
Plus you've got to add in delay time from when the traffic reporter analyzing the situation, writes the script (usually just a quick outline), records it for WBZ, and then for WBZ to air it. I don't believe they do those reports live...although I could be wrong. If I'm right, then the report you're hearing now could be only 5 minutes old or as much as 20 minutes old. Big difference there.
The whole "traffic on the 3's" thing is more a marketing gimmick than anything else; traffic is far too dynamic/chaotic a system for anyone to really report on in a meaningful manner. In theory MassHighway could manage the traffic better if they had a huge system of monitoring coupled with millions of drivers willing to have "guidance" systems (GPS Mappers) that would dynamic update their route suggestions based on feedback from MassHighway to better alleviate overcrowded intersections. But that's very expensive and very dependant on millions of horrible Boston drivers buying in to the system. Not too likely to happen. :-\
