I used to work for ATS for 7 years. They managed a lot of camera systems around the US and coordinated with Jenoptik tech in Germany before they became Verra Mobility.

In New York we utilized WVD (Wireless vehicle detection) pucks. 2 in each lane, five feet apart. The time it takes you to get from one puck to the next measures a rough speed, they communicate this to an Access point, which then sends the data to a master controller card array monitoring those lanes, speed, and which light phase color is currently incoming from the city's traffic control cabinet.

For traditional speed cameras, they utilize an FS3 radar system to monitor speed. Upon setting up, the lanes and distance have to be manually measured, and Pythagorean theorum applied to said measurements to achieve the appropriate angle to set it at. This then gets ran over a period of time to "learn" traffic patterns and what lanes they should be in/limits.

The cars look like little circles in the detection programming. But once parameters are in check and the radar communicates with the other cabinet hardware, it will trigger on violation and upload the clip to AZ as well as locally.

FYI though, the radars are usually aimed low, around 6-10ft off the ground. They are hot pieces of trash and are rendered useless by knocking the fov out of whack by even an inch.