Are you asking for literally the rate at which pedestrians and vehicles pass on these streets (for example, number passing a given spot in an hour), or just want an idea where the busiest streets are? Also, are you seeking info on highways, or just the city streets? The city of Boston proper, or the whole metro area? I don't mean to seem curt. It's just that I think people outside your field might need more detail to understand what you're really asking.
For precise, up-to-date info on population, you can check out the website of the U.S. Census Bureau:
Census Bureau Home Page. The approximate population figures would be, for the city of Boston, a bit under 600k (maybe 570?). As for the metro area, it gets a little complicated because the census bureau now keeps statistics on at least two levels of metropolitan areas. There's the traditional metro area, meaning a city and its commuter suburbs, but in the last few years, for densely populated areas with a lot of interconnected cities and towns that cover a wide area, such as most of the Northeast Corridor, they also have kept stats on something they call a Consolidated Metropolitan Area, which involves several interconnected metro areas. In the case of Boston, the approximate population figure for the more traditional metro area is 4-1/2 million. The consolidated metro area, which extends out to central MA, up into NH well beyond the towns that border MA, into southern ME, etc. is something like six to 6-1/2 million. Those are round figures, so check the Census Bureu's site for mor precise figures.