What it is: Bus Rapid Transit
Bus Rapid Transit, henceforth referred to as BRT, is the concept of turning street running, mixed traffic buses into something closely resembling Boston's Green line. The central concept in BRT, which is NOT obeyed in Boston's Silver Line, is the concept of dedicated lanes. Why should a bus with 80 riders be forced to stay behind a car with one? By taking the "high speed" lanes in the middle of roads like Atlantic Avenue or Mass Ave, you can *add capacity*, by replacing space inefficient cars with massively space efficient buses. In addition, you create a transit line for the cost of a few buckets of paint. It won't be as fast as the Red Line, but with priority at traffic signals (something the Green Line lacks, for reasons that baffle me), it can get pretty close.
The biggest benefit: cost
For various reasons, all transit in America is expensive. Instead of wishing for a land of $50m/mile subways (It's current an order of magnitude above that), lets work with what we have now and create 5m/mile busways.
5 million per mile?
Yes, you read that correctly. For 5 million dollars a mile, we can create rapid transit with similar speeds and frequencies to Boston's Green Line. With costs like that, the equation changes from "Why?" to "Why not?". Why shouldn't we turn all of the 66, 1 and 77 to BRT? Why shouldn't we have dedicated bus lanes all over Harvard and Kendall? Why do we dedicate 80sf of street space to one car when we could give it to 20, 30, 40 people on a bus? And who cares about Red Line reliability when you have five transit routes running Boston -> Cambridge, all with 5 minute headways?
The answer is, unfortunately, a political one. It order to get lanes for thousands of people on buses, you need to take it away from hundreds of people in cars, and those users are highly resistant to giving up lane space. It's worth noting that there is a common fallacy involved here: I won't stop driving my car, so traffic will just get worse with fewer lanes. A correct response, though not exactly an obvious one, is that even if you will never switch away from your car to the new, faster bus, someone else will. This solves the problem of congestion on roadways, even with fewer car lanes. In Boston, these challenges have defeated all useful BRT proposals, but in Cambridge, with the master plan on a large number of brains, I'm hopeful we can see the math behind BRT and go on an all-out war for better transit, cheaply.