Boston and surrounding areas have been working on that for years, with good results. Here's some info if you're interested: http://www.mwra.state.ma.us/03sewer/html/sewcso.htm#16southd...

The specific project I linked to involved installing over 25 miles of storm drain to avoid combined sewer/storm overflows into the ocean. We lived on one of the impacted streets.

Older houses had gutter downspouts that dumped all rainwater into the combined storm drain/sewer system. The MWRA came to neighborhood association meetings, and helped residents to re-aim downspouts onto green space.

This article takes a really really long time to get to the point. It raves and raves about how great the system is and then the final page it says oh BTW it doesn't work, too much flow, should have diverted that rainwater.

I think it basically is a given. To segregate storm water from sewer would require a rebuild of the system. Pretty impractical at this point. It can be done in some cases. I live in the west suburbs of Chicago, and the storm water from my (less than 5 year old) complex drains into the Salt Creek. I don't think my city is connected to the big dig, but we do have the capability to dump into a local quarry. The quarry has also been used to dump snow into for large winter storms. We've also been through a large expansion in recent years of green areas and detention drainage areas to alleviate flooding. Living right on the Salt Creek, I routinely see the water level fluctuate by around 10 feet or so after even a decent storm. We're probably about 5-6 feet above normal right now after recent storms.

Presumably that requires work everywhere in the city instead of at a few key points like this project which makes it harder, even if more effective.

Yeah, I never noticed them mention that possibility, even to dismiss it.