(OT) Traffic Speed Detecting Signs

[Paintedcow]

That makes sense....

I have also noted that those signs are always showing about 2mph slower than my speedometer on my pickup truck. But vehicle speedos are never

100% accurate because of factors in the vehicle, gear ratios and so on, but tire size is a huge factor. I have heard of guys changing the speedo gear, just because they put on larger or smaller tires. As long as I know that my speedo is showing "30" when I'm actually doing 28, I know I can legally go a little faster. I actually like those signs, because they do "test my speedometer.

I dont know what size tires came with my truck, but I tend to run heavier tires than most people, because I haul farm equipment, hay bales, etc. I'm sure that makes up the difference. My tires are all

10ply, and the quarter or half inch of extra thickness does matter in the long run.

[Paintedcow]

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....

[Paintedcow]

That's about right.... plus/minus 5mph is about right

[Tekkie®]

DerbyDad03 posted for all of us...

Good one

[Tekkie®]

Pat posted for all of us...

A negative speed? Huh? NO It's doesn't care what direction you are going. The beam pattern is narrowed so it's only measuring the oncoming traffic. The ones we have recorders are installed so traffic studies can be performed. When citizens complain about speeding in their neighborhood then a study is done. If warranted then enforcement is started. Usually the studies are done in nearby areas too. Catches the complainers speeding in someone else's neighborhood. Enforcement catches the stop sign violators.

[Ed Pawlowski]

Your speedo is off. Manufacturers would rather it be that way the the opposite so you don't get tickets.

§ 393.82 Speedometer. Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).

Tire size and pressure will affect the readings too.

[James Wilkinson]

How much does it jump about? I could believe a line of cars varying by something like 28mph to 34mph.

[James Wilkinson]

A lot of drivers don't stay at the same speed, they're impatient and/or stupid and speed up, then brake as they approach the car in front. That's why you get those ridiculous rolling slow bits on motorways sometimes. I always delete them by just driving at the average speed of the cars in front.

[clare]

Try it and find out. Make sure the same cop is there to catch you - trhen you can say you were just trying to find the answer to the question he could not answer - that way you both learn something - - -

[clare]

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.

[clare]

Possibly LIDAR instead of Radar

>

[clare]

Most metric units have 3 digit readout. Imperial units for urban use generally have only 2 digits - 100mph in urban areas is not "expected" but 100kph is far from unheard of.

[clare]

Only if it is a rather old car. Most cars today with electronic speedos will register mileage in both directions. Some even register reverse speeds.

[clare]

because your speedo, like most, is inaccurate.. Perhaps your tires are undersized??

[Pat]

The physics cares. A car appraoching at 35 is very different from a car going the other way. The software might be programmed to display the absolute value of the speed, but internally, the device knows toward (positive) from away (negative).

That may be the intent, but real world roads have curves and cars on the other side are sometimes detected.

[Pat]

That could be it, or it might be working as designed. I believe that cars in the US must have speedometers that are accurate to within minus 0% and plus 5%. I don't recall the exact number, but the idea is they can allowed read fast but never slow. If that is true, I would think the average speedometer would read a liitle fast. At 60 mph, a designer working with a + or - 2 mph device would set it to be

2 mph fast thus turning -2 to +2 into -0 to +4 with an average of 2.

[Tekkie®]

Pat posted for all of us...

So what is your point?

So. They are not being used for enforcement. To be used properly they just aren't hung on a fence nail in the pole. Are you looking in the mirror when you pass one going the "negative" direction; if so what is the reading?

[Unquestionably Confused]

+1

Not mention that if it was done intentionally, they might want it reading your speed as 3 mph higher to slow you down.

With my portable GPS I will occasionally check my car's speedo. Set the cruise control at 60mph on a flat stretch and see what the GPS tells me. Rarely is there a difference between the two.

[Unquestionably Confused]

Doubtful as the Valentine One picks up LIDAR/Laser as well. Trips everytime I run through a toll plaza where the cameras are equipped with IR Illuminators to light up the plates at night.

One explanation I read was frequencies of the mobile units, since they aren't used for enforcement, per se, are not calibrated/recalibrated and the detectors won't pick them up.

No attribution was provided with that explanation so I rather doubt it for that and several other reasons.

[Paintedcow]

I always have a tough time with most metric stuff, but if I'm right,

100kph is roughly 62mph (62.137mph).

(Yea, I cheated, I used an old program called convert.exe which was made for Windows3.x, or was it Win95. One of the most useful pieces of software ever made. It converts darn near everything).

I'm glad they never changed the traffic signs in the US to metric. I'd probably always be so confused that I'd have to have a computer on the seat next to me at all times that I am driving.