They are installed on pretty much on every bridge over major roads these days: Traffic jam sensors. Recently, I got to see one close up and found it quite interesting that they are totally autonomous, no network cable (of course) but also no power cable. Have a look at the picture on the left. According to the website of the company that has deployed those sensors, data is sent over the GSM network when necessary, i.e. only when the traffic flow changes. No mention whether SMS or GPRS is used for the data exchange.
Then I wondered how much power the sensor draws and if the solar panel could supply enough power for a round the clock operation? The module has a size of about a quarter of a square meter so how much power could it deliver per day? From experience with a solar panel that had a size of about 4 square meters that could deliver about 500 watts under best conditions this module could perhaps deliver around 30 watts under the best conditions. Now the sun is not always shining so perhaps the module delivers 100 Wh per day. Is that enough?
That depends on how much power the device draws. The radio module itself is probably pretty efficient. Idle mode power consumption of embedded modules is typically less than 10 mW. So letting it run around the clock would result in a power consumption of the radio module of around 0.24 Wh per day plus the increased power consumption for communication. I assume the device only communicates rarely so this value should not increase much. And then there's the application processor board and the sensors themselves. As their main job is to count and analyze impulses it gets when cars move under the sensor I can imagine that its power consumption is equally low. But even if it takes a full watt, the solar panel and a battery to store energy for nighttime should be way sufficient to drive the device. And a full watt is already quite a lot, even a Raspberry Pi Model A under full load is specified to take only 1.5 Watts.