A trial was just rolled out in Westminster (London) enabling you to use the app to find parking spaces easily. Not sure if any US cities have it but London is the first in Europe it seems.
The tech behind this is pretty interesting given that they haven't had to dig up the whole road to add the sensors. Each set has a battery that lasts around 5 years. They also use ZigBee so they can communicate with each other through a mesh network.
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