Re. SCATS, I used to work at the RTA as a consultant for another company back in ~2000 when it was still based as Eveleigh. My understanding is and has always been that they were the ~first to put loops under the road to count traffic but the other elements of the system were not that amazing. They still had a PDP live, and also ran the Sydney portion of the emergency services radio network from the same data center. A control system would show real time traffic densities. We were adding a PTZ-capable remote video on demand component circa 2000 using a combination of 128K ISDN and fiber. I wrote some kind of last minute multiplexing PTZ data control bridge kludge in perl. The idea was that, after the traffic system detected an anomaly, the control system operators could "dial in" and "look around" to "determine" the cause of traffic issues and create an appropriate response using their bridgedeck mounted joysticks straight out of the 80s. We sold some to the Philippines and I got to go, as an 18 year old, and party hard with the locals after we signed a deal with the mayor of Manila. Fun times. Corrupt as knifespoon.

These days I assume most people/companies/governments source their traffic flow info from Google because it has more 'active' sensors in so far as every car has at least one mobile phone. Telcos could and should also sell this type of data in an anonymous format. Large toll roads track individual vehicles with a combination of wireless billing systems and numberplate ID so they are effectively able to provide a similar solution for those high value roads.

> it's basically impossible to sync green traffic lights in both directions.

For a bi-directional green wave, intersections have to be spaced at intervals consistent half the cycle time (times the intended travel speed) of the traffic lights. In that case opposing vehicle streams will always meet each other at an intersection, thereby minimising the green time required along the green wave and maximising the green time available for handling crossing and turning traffic.

If an intersection deviates from that mathematically optimal location, it means that there'll be less of an overlap between the green times for the two opposing direction, meaning that there's less green time left over for cross traffic.

And since in real life, intersections are rarely spaced at the mathematically optimal interval, in practice this means that eventually you have to give up and just favour one direction depending on the time of the day. (To some extent you can try to salvage things by varying the assumed speed between intersections in order to keep the travel time from intersection to intersection constant, but there's only a limited usable speed range between too slow and the legal speed limit.)

And if you attempt to have multiple green waves crossing each other, you introduce even more scheduling constraints which might leave no room for a perfect solution.

Germany does it very well. Until I make a turn, I do not wait at red lights more than once or twice.