[throwaway193948]

my car has these features and sometimes thinks the speed limit is 100mph in residential neighborhoods

[pornel]

I've seen 30 recognized as 80mph. Clearly the system didn't even have a plausibility check: in the UK 80mph isn't a legal limit!

It also gets confused when there are overpasses or tunnels crossing the road, and the nav-based speed limit flips from 70mph to 15mph to 70mph.

[throwaway193948]

ah i have seen 80 before, that could explain it it was never clear to me if it was reading the signs or just basing it off of map data

[bzzzt]

In some neighbourhoods in the Netherlands people put fake '30km/h' signs on their roadside garbage bins. Those are not the official limits, but will be picked up by traffic sign recognition algorithms...