Where I grew up, in Berkeley, there were streets with these big parking barriers that diverted the flow of traffic from streets where they wanted to reduce through traffic, presumably for kids playing in the street and such. Here's a fancy version of one of these:
Here in Florence there is a much better version of this: an electronic barrier. This thing is genius because it blocks traffic when you need to (busy days, parades, etc.) but allows it when you don't.
The little lights around the top of the Street Droid flash red when it comes up from the ground, which is really a sight to behold.
Again, this thing is great because you can raise it when you need to close off the streets, but lower it when you don't. So in Berkeley, for instance, you could have them up during the day, and lower them at night when kids aren't out. There are tons of good uses for these barriers, and we probably ought to have them in the US.