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Comment by woodruffw | Hacker News Reader

Posted limit detection seems reliable in existing cars, both self-driving and not; is there some reason to believe this will be any less reliable than other safety technologies?

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I live in Boston and my car’s HUD displays this information (speed limit and such). Sometimes it gets it wrong despite living in a major city center. Once my car even alerted me when I was going the wrong way on a one-way — except that I wasn’t. I can only imagine in deep suburbs / rural it is even less reliable.

Boston can really confuse a GPS. While driving in the underground freeway, the GPS was giving me turn-by-turn instructions for the city streets above.

It sounds like you're describing a system based on GPS and map data, not one of the systems that read the actual signs posted as you drive by them. The only problems I've had with the speed limit sign reader in my car are conditional speed limits (e.g. school zones during certain hours, truck-specific limits). Otherwise, they're just limited by the prevalence of the physical signs.

Even reading the signs is non-trivial, I drive several stretches of road where an access road runs parallel to a highway, and the 20mph lower speed limit signs for the access road are clearly visible from the highway and only slightly farther off the shoulder than the correct signs.

This seems like a useful service for cities to provide: street maps and traffic rules for each segment in a standard format.

With clear conditions and no obstructions, yes, but my model Y frequently doesn't pick up the right speed limit, because: fog/rain/snow, glare, obstructed by another vehicle, defaced sign, no sign, non standard sign(eg variable speed limit sign in school zone).

There's a sign indicating the default speed limit in my town, 25mph, with some extra text that anyone who can read will realize it's indicating the default speed and is not a speed limit sign. But otherwise it looks exactly like a speed limit sign, and it sits in the middle of a 50mph zone. Apple and Google Maps both tell me the speed limit is 25mph for about 1/4mile until the next sign indicating 50mph shows up. I have been told it trips up Teslas (but don't have one and haven't driven one here so cannot personally confirm).

It'll be fun to have everyone's car slow down suddenly to match this non-existent speed limit, going from 50-60mph down to 25-35mph.

My understanding is that these systems use a combination of posted sign identification and GPS data.

Right. Here the databases are wrong so the GPS data will indicate 25mph and the sign looks like a speed limit sign so posted sign identification will likely be tripped up (again, based on reports from Tesla owner friends and colleagues). So for 1/4 of a mile on a 50mph road cars that automatically enforce the speed limit will be screwing up.

And I doubt this is a unique situation.

That sounds pretty unique to me; I've done a few 1000+ mile road trips across the US and have never run into a sign that looks like a speed limit but isn't actually one. The closest thing would be a yellow advisory sign, but those don't look like speed limits.

(This sounds like the kind of thing you should report to either your state or county -- AFAIK every state maintains a DB of posted limits with geofences, and correcting them is generally a high priority for ticketing reasons.)

Loads of towns around Boston have the exact setup described above.

Do you have an example? Not that I don't believe you; it just seems nuts to me that any state would have a sign that looks almost exactly like a speed limit but that isn't one.

For the record: I've gone through the length of MA a couple of times (NY to Boston), and have never noticed these. But maybe that's because I thought they were all speed limits.

For a road that I commuted on a lot: At the town line entering Lexington, MA, they have posted the default Lexington-wide speed limit: https://www.google.com/maps/@42.4098312,-71.2076201,3a,75y,3...

That road, Concord Ave, is posted as a 35 or 40 mph road for its length. (The MassDOT GIS database lists that first segment in Lexington as having a 40 mph speed limit, object_id = 395024.)

At 94 (about 1500' ahead), Concord Ave is posted as 35 mph: https://www.google.com/maps/@42.4131439,-71.2130807,3a,75y,3...

At 190, posted as 35: https://www.google.com/maps/@42.4156383,-71.2191302,3a,75y,3...

At 72 (in the other direction) posted as 40: https://www.google.com/maps/@42.4126782,-71.2123193,3a,75y,1...

Mass Ave, entering Lexington: https://www.google.com/maps/@42.425451,-71.1924309,3a,75y,28...

Mass Ave shortly ahead, posted at 30: https://www.google.com/maps/@42.4277977,-71.2025678,3a,75y,2...

Rt 30, entering Newton: https://www.google.com/maps/@42.3398206,-71.1676001,3a,75y,2...

500-ish feet ahead: 30 mph: https://www.google.com/maps/@42.3394727,-71.1686319,3a,75y,2...

I believe a casual sign-reader could easily see the default sign and mistakenly conclude the speed limit on the pavement they are on is 25 mph. If I was stopped for 30 in the vicinity of any of the default limit signs I posted, I really, really like my chances to have it dismissed.

Given that GPS gets confused and thinks you're on surface roads when you're on the highway and vice versa, I don't see how this will always work. In dense urban areas there are often multiple layers of roadways going the same direction with very different speed limits. Also basic issues like GPS errors because of tall buildings or foliage cover can easily confuse systems such that you look like you're 100's of feet away from your actual position.

Its wrong all the time on my Tesla. Its always fun when the autopilot brakes to drop to 55 mph on the freeway when the speed limit is 65.

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