alt Hacker NewsA 25mph school zone? That seems fast. 15mph would be more the norm, which is in line with the 17mph the car believed itself to be traveling.
FYI, unless you are a commerical truck, a cop, or a racer, your speedometer will read slightly fast, sometimes as much as 5 to 10%. This is normal practice for cars as it limits manufacturer liability. You can check this using independant gps, ie not an in-dash unit. (Just imagine the court cases if a speedo read slower than the actual speed and you can understand why this started.)
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Also, a different wheel diameter than the speedometer was calibrated with and you will have a larger difference between actual velocity and speedometer reading. The odometer will also not record actual distance traveled.
It does seem fast to me -- school zones are 20 mph in Seattle, at least when children are present. But Google suggests 25 is the norm in Santa Monica, where the incident occurred.
In Encinitas, California, that sign would have no more than 20 MPH. In adjacent Carlsbad, I believe 25 is normal.
I mostly see 25 mph for school zones, though I'm in NC. Checking California, it sounds like 25 is standard there as well.[0] Some will drop to 15, but 25 is the norm as far as I can find.
[0] https://www.dmv.ca.gov/portal/handbook/california-driver-han...