Back in the late 2000’s I wanted to start a company that broadcasted the current speed limit near signs. Like a BT beacon, sorta, but obvs not BT. It turns out this requires too much coordination. Specifying a protocol, getting manufacturers to build it in, lawmakers to pay for it, sign makers to embed it, etc.
Good point. Also, M25 (London orbital motorway) has large sections with variable speed limit. Google maps navigation just doesn't show a limit for those areas, since it does not have timely data.
Also the 20mph boroughs really confuse the systems (cars and satnav), since they very often say 30 in 20 and also 20 in a 30. The various limits are, admittedly, not well signed, and I suspect that's deliberate to keep drivers cautious.
One problem is "map matching". That is, given your lat/lng (or, more realistically, a time-series of lat/lngs) and a map, figure out which road you're on.
The thing is that most consumer devices with GPS (like phones and even standalone GPS navigators) have a lot of internal "magic" to hide the raw noise in location fixes from the GPS signal.
For example, certain Garmin devices (intended to be used off road) will snap your location to an Interstate if you are going more than 55 mph and heading roughly in the direction of the road (which is evident when you run into traffic and your location keeps switching between the frontage road and expressway--the error distribution of the noise doesn't have mean=0!).
Basically, they haven't, or at least they hadn't 4 years ago when the car I have with it was made.
Diving along a free flowing 50 and having it decide it's now a 30 because it saw a sideroad and drop a gear is semi-regular. The second most dangerous thing it does after having a touchscreen.
It also manages to read 45 sometimes but I have literally never seen a 45 sign.
Our prototype just screwed onto the sign post and could be aimed that way. We never added it to the prototype, but having an accelerometer added to detect movement and shutting down/calling home would help with damage or vandalism.
The tech is the easy part.