Python (Euro)DOCSIS (3.0) Traffic Meter
This tool uses a DVB-C capable video card (e.g. a cheap USB stick) to measure the EuroDOCSIS 3.0 traffic per frequency, allowing you to venture an educated guess about your local segment's utilization.
Data is written to a UDP socket in graphite format.
This was tested with cPython 2.7.13 and cPython 3.5.3 using a Hauppauge WinTV soloHD USB stick on a Raspberry Pi 3 running Raspbian/stretch.
It should however work with most Python versions as long as the DVB-C card is supported by your kernel and it's driver complies with the DVBv5 API.
EuroDOCSIS 3.0 uses standard DVB-C mechanisms to transport it's data: It's encoded as a standard MPEG Transport Stream on PID 8190 with either 64- or 256- QAM modulation with a symbol rate of 6952ksyms/s. Since cable is a shared medium, determining the total amount of data transferred and comparing this to the total amount possible after FEC (which is about 51Mbit/s for 256-QAM and 34 MBit/s for 64-QAM) will show you how much capacity is used.
Take a look at your cable modem's management pages.
No, you can't.
I wanted to learn Python.
You can easily use tools like netcat to capture the data.
Yes. You would probably not see other cable modems upstream signals due to... erm, "electrical" stuff.