Our town recently got one of those Traffic Speed Detecting Signs, that
tell you how fast you're going. I've always wondered how they work, and
why they dont detect cars going the opposite direction.(I assume they
work on radar). I also noticed that they only have TWO digits.
Therefore, if you exceed 99mph, what does the sign do?
Last night I was in a convenvience store, when one of the local cops
walked in. That was my chance to ask him what happens if someone exceeds
99mph. His reply was "I'll give you a speeding ticket". I laughed and
said "that's what I thought you'd say". Then I said, "besides the
ticket, what would the sign do?" He said he never tried it, so he dont
If a cop dont know, who does? I'm wondering if they go back to zero, or
just keep showing 99?????
Dudes! This is not a stupid question. This is a fun
question. When did you guys lose your sense of curiosity?
My guess is that the display is driven by a BCD (binary coded
decimal) counter and that it just rolls over to 00.
I know this cop and he's a decent guy. I think I caught him off guard
with this question, but he was having a little fun with it, as well as I
was. But it really is a question which has an answer (somewhere). I
assume those signs have user manuals, and there must be a mention of
using them for speeds over 99. Normal drivers wont experience that
speed, but if those same signs were used at a race track, they would not
work, since many race cars exceed 99mph. Since truck and tractor pulls
uses these signs, I would think that race tracks would use a similar
device, but obviously need one that allows for 3 digits. (I assume they
make a 3 digit one for such uses).
I wish I knew the manufacturer of the sign, so i could see if there is a
user manual online.....
Till then, I'll go along with the "roll over to 00" !!!
It happens that firstname.lastname@example.org formulated :
Two and a 'half' digits would be good enough for displaying speeds up
to 199 MPH. It cuts down on the circuitry needed in the display.
I would think of it as a two and a half digit display with the 'half'
being left to the reader. It could well be that a log could be kept
which logged speeds well above 100 MPH rather than a rolling over,
leaving only the display limited to two digits.
K-band single directional radar unit
Latest generation AlInGaP high intensity LED’s
12” or 18” Full Matrix (rounded) Characters for quick recognition
Minimum Speed Display, High Speed Blanking & Flashing Digit Violator Alert
Automatic intensity adjustment to ambient light conditions
12 VDC operation, 12 VDC or 100-277 VAC, 50/60 Hz input
Power consumption: 5.76W Nominal, 14.88W Peak
Keyed On/O switch
Single cycle ON/OFF clock
Sign defaults to last settings upon power up
Built in on-screen diagnostics
On-board Options & Diagnostics
LED Operation Diagnostics
LED Intensity Diagnostics
MPH / KPH operation
True enough. My point was that the counter wouldn't have to roll over
just because of the limits of the display.
You can display Pi on a calculator and only get so many decimal places,
yet you can then subtract 3.14159 from it and retrieve six more decimal
places in many cases. The limits of the display does not necessarily
limit the underlying calculation (or, in this case, the count).
Only by looking at the circuit or code used could I determine if the
thing actually counts modulo 100 (0 through 99 and 'rolling over'
dropping anything over 100) or just not displaying the most significant
On 8/10/2016 3:09 AM, email@example.com wrote:
He's mixing the sign and physical presence of cop.
There are a couple of signs I frequently go through here and they blink
off when your car has passed. They do not keep a record of your car and
In Europe over 25 years ago we were told that trucks were required to
have a recorded record of their trip and when cops pulled them over
would check the record and could give them a speeding ticket even though
they exceeded the speed hours ago.
One day with gps's and computers recording they could do this to anybody.
I rarely see a cop near those signs. I know they signal is not being
sent to the cops, recorded, or anything. The sign is programmed to flash
if I'm exceeding the speed by more than 3mph or something like that. In
town the speed limit is 20, and it wont flash at 21 or 22, but I think
it starts to flash at 23 or 24 (and up).
Ive seen those same signs used at tractor and truck pulls to record the
speed of the puller....
The infamous "tachograph"
OBD2 computers on today's cars can apparently be queried to determine
speed and whether the brakes were applied in accident investigation.
Only the last few seconds are apparently available.
On Wed, 10 Aug 2016 03:09:05 -0400, firstname.lastname@example.org wrote:
Yes, they work on radar. Cars going the opposite direction DO get
detected, but they show up as going a negative speed. The built-in
software just ignores those signals and displays only positive speeds.
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