Many cars now come with a database of speed limits and display it on the dashboard. Although this shouldn't be relied on for self driving, it could be used to d

Many cars now come with a database of speed limits and display it on the dashboard. Although this shouldn't be relied on for self driving, it could be used to double check. OSM data for types of roads and areas is pretty accurate too, so you could use that ('an 85mph speed limit for a single lane within a residential area doesn't sound correct'). This doesn't need AI, it just needs someone with common sense to come up with a few basic rules about speed limits. If there is some doubt you could prompt the driver to confirm the speed limit change, and use that to build up a database of real world data. It sounds like the Tesla is just using what the camera sees - which is bad if true. Admittedly this was tested on a race track, so maybe there is no data saying otherwise (or even the opposite).