First Look: Google Fi is "increasing" spectrum by ~20%. WTF?
In effect, T-Mobile and Sprint are sharing spectrum and that increases capacity. Large spectrum blocks are less likely to be congested, partly a result of queueing theory. Google Fi uses both Sprint & T-Mobile - and lots of Wi-Fi - choosing whichever has capacity when the phone needs it. With Sprint itself, when capacity is used up the data doesn't get through, even when there is spare capacity on T-Mobile. There's no way in practice to hand over the connection. The same, of course, applies to T-Mobile and every other network.
Until now. When Sprint is congested but T-Mobile isn't, the connection automatically switches to the T-Mobile network and gets through. The two networks have different user bases, peak areas, and physical networks. The peaks are often at different times.
There's no reason Sprint and T-Mobile couldn't do the same with all their customers. The result would be the same as adding spectrum: with cooperation and sharing, you can handle more traffic.
My take: 4G gear is now cheap enough it's time to stop building 3G except in the poorest countries. 4G is so much cheaper to run essentially every telco in the developed world is quickly migrating. That's a very good thing because carriers inefficiently using spectrum limit everyone's capacity.
30 MPH traffic would require a prohibitive number of small cells for carriers to do it themselves. 
When Michael Marcus and others at the FCC created the "unlicensed bands," they had little idea about how it would go. Wi-Fi has been a major game changer but that wasn't clear at the beginning. It's yet one more example of what creativity makes possible. Today, we see some congestion in the "commons," as well as proposals to squat on a large share. We need some "rules of the road" to manage the congestion fairly and efficiently. But they need careful design to have minimal impact on innovation. 
