Having seen, on many occasions, signs labeled "12 mph", I suspect that the "19 kph" signs are of the same ilk, and meant to focus the attention of the driver toward maintaining a very low speed. Perhaps "10 mph" is interpreted to mean "around 10 mph", which is then interpreted to mean "as fast as one would like to travel", while "12 mph", being more specific, may bring about the intended behavior.
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From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of Stefan Jeglinski [jeglin@4pi.com]
Sent: Friday, January 14, 2011 1:39 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Will the US ever switch to the metric system?
This reminds me... there is a local "supermall" in my area, and the
mini beltway that surrounds it (a mall road, not the main road) is
posted throughout with 19kph signs. I have long wondered about the
purpose of this speed. It's not rounded in the normal speed-limit
sense (15 or 20). I next thought maybe they were misprinted with kph
but 19mph doesn't make much sense either (and is rather fast anyway
for that road). Then I thought maybe a mischievous student was in
charge of manufacturing the mph signs and decided to convert to kph,
but 11.8mph (or 12mph) also makes no sense. For common definitions of
sense, anyway.
Perhaps it was just a mfg mistake and they decided to use them anyway
to avoid additional costs. Or perhaps they saved .0000001% of the
mall development cost when someone offered these particular signs
from their "mistaken speed limit sign graveyard." My best theory is
they meant 10mph, the student got a hold of it and converted to
16kph, but them accidently inverted the 6. A stretch if you ask me.
Anyone ever seen 19kph signs? Any ideas on why that speed?