Why don’t Boston and Cambridge just close more streets to vehicles altogether?

Boston Mayor Marty Walsh recently proposed lowering the city’s speed limit to 20 miles an hour from 25 along an unspecified number of neighborhood streets. Presumably that would include many of Boston’s more heavily trafficked enclaves and cut-throughs.

In that, the city would join neighboring Cambridge in bringing down the speed limit to a more pedestrian- and bicyclist-friendly 20 m.p.h. in a lot of areas.

Cambridge since March 2018 has enforced that limit in busy places such as Harvard, Central, Porter, and Kendall squares, where commutes are nearly as likely to be by foot (usually to the Red Line) or by bike as by car.

While data does show that lowering speed limits can save lives, the bigger question might be why worry about the speed limits in these areas at all: Why not just close them to private and commercial (but not emergency) vehicles altogether?

There are already certain areas of Cambridge and Boston that shut out vehicular traffic at certain times. We’re thinking Cambridge’s Memorial Drive on Sundays during the warmer months. Or that experiment with car-free Sundays along Back Bay’s Newbury Street. And much of Downtown Crossing has been closed to vehicles for years.

But just as there is earnest discussion about making the T gradually fare-free for certain populations, perhaps it’s time to start talking about cordoning off certain stretches of street permanently. Such a move would really make things safer, and, if the experience of some cities that have already done so are any guide, it would also be a boon to retailers.

And this discussion is especially necessary as more and more bike-shares—dockless, electric battery-powered, and otherwise—hit the Boston area’s streets and it becomes clearer and clearer that e-scooters will inevitably roll out too. Just a couple of years ago, after all, the region had only one bike-share and it was limited in its scope. That has changed dramatically.

Vehicles will not only have to share more of the road more often—for safety’s sake and for the sake of what has become some of the worst traffic on Earth, they may have to cede some roads entirely.

Which streets and enclaves, though, should be closed to cars and trucks? We open up the comments section below to your suggestions.