Roads dept speed cables

Ozboy · 24-08-2008, 05:04 PM

Anyone noticed the amount of cables the roads dept are putting down this time of year.
I may not be right here, but those 2 cables they lay across the road approx 300mm apart are for speed detection, right? They monitor the average speed in an area so speed cameras can be set up according to the data supplied by these cables.

I've seen no fewer than 6 sets of them down Marmion ave.Anyone know their real purpose and why on earth have so many on one route?

Sprint · 24-08-2008, 05:05 PM

Traffic volume auditing.

S.

thro · 24-08-2008, 05:06 PM

^^ yeah whs.

they'd never work for speed calcs, as you'd need to know the distance between the wheels of the vehicle crossing it, wouldn't work with 6 wheelers, trailers, etc...

FireStorm · 24-08-2008, 05:06 PM

Quote:

Originally Posted by Sprint

Traffic volume auditing.

S.

+1

Ozboy · 24-08-2008, 05:08 PM

Quote:

Originally Posted by Sprint

Traffic volume auditing.

S.

Well thats what I thought Sprint, but surely you only need one cable to register a vehicle. Is the second cable used to determine what type of vehicle has travelled over it? ie wheelbase distance.

thro · 24-08-2008, 05:10 PM

Quote:

Originally Posted by Ozboy

Well thats what I thought Sprint, but surely you only need one cable to register a vehicle. Is the second cable used to determine what type of vehicle has travelled over it? ie wheelbase distance.

I think its more to validate the results.

If you were to get a kid/someone jumping up and down on one cable, and it doesn't register on the other cable, that "trap" is invalid.

Well, thats my guess anwyay.

Ozboy · 24-08-2008, 05:12 PM

Sounds credible to me Thro. Thanks guys

Cam · 24-08-2008, 05:13 PM

Quote:

Originally Posted by thro

I think its more to validate the results.

If you were to get a kid/someone jumping up and down on one cable, and it doesn't register on the other cable, that "trap" is invalid.

Well, thats my guess anwyay.

Yup, that's how I'd set it up.

Unless both are set off within 'x' timeframe (allowing for an average spread of wheeltracks and weights) then it will not be counted.

Johnno · 24-08-2008, 05:27 PM

They are for speed reading, they read the time the front wheel takes to cross both. The wheel base doesnt matter cause its not reading between front and rear.
The single strips are for traffic volume.

fer0x · 24-08-2008, 05:31 PM

I always thought for the traffic auditing too, a lot of the time it's to see (afaik) how much traffic will be affected if road works are carried out on that area and to make best plans for road work times and detours

I guess speed reading would be a possibility, but till now hadn't heard of that theory

semi · 24-08-2008, 05:33 PM

I've never seen a single one, i dont think theyre for speed

karen · 24-08-2008, 05:59 PM

But expect a round-a-bout or speed humps soon.

-Stevo- · 24-08-2008, 06:03 PM

They are 'Pneumatic Road Tubes' and are used for traffic volumes...

jonnyking · 24-08-2008, 06:13 PM

i always wondered about these things.

figured they were for road volume but could see the use for speeding possibly

Hornet · 24-08-2008, 06:16 PM

Single ones will record numbers of axles and the time that each axle/wheel crossed.

The dual ones can calculate the time taken for a wheel to hit the first then the second, from which a speed can be determined. If the 2nd axle/wheel crosses faster than the first, then the vehicle was accelerating. If slower, decelerating. They can often be set up with a 2nd one for dual lanes and they can then determine which lane the vehicle was in.
So the dual ones can record time, speed, accelerating or decelerating.

The Saint · 24-08-2008, 06:16 PM

I thought they were sewers for the mice community

Cam · 24-08-2008, 06:17 PM

Quote:

Originally Posted by Johnno

They are for speed reading, they read the time the front wheel takes to cross both. The wheel base doesnt matter cause its not reading between front and rear.
The single strips are for traffic volume.

There is no way to remotely accurately measure speed of vehicles with these devices, seriously.

You could use them to take a rough guestimate, but yeah...

Stormrider · 24-08-2008, 06:27 PM

they had one on broom street in cottesloe to find out how fast cars where acctually going over a blind rise, the maximum was done at 1:30 in the morning and was 157.8 (something like that). not only would that have meant the car got airborne but it means that the device is fairly sophisticated.

Cam · 24-08-2008, 06:29 PM

Quote:

Originally Posted by Stormrider

not only would that have meant the car got airborne but it means that the device is fairly sophisticated.

Or wildly inaccurate.

I just don't buy that accurate readings could be plucked from these things at all.

Simonpink · 24-08-2008, 06:44 PM

Quote:

Originally Posted by Johnno

They are for speed reading, they read the time the front wheel takes to cross both. The wheel base doesnt matter cause its not reading between front and rear.
The single strips are for traffic volume.

Quote:

Originally Posted by Cam

There is no way to remotely accurately measure speed of vehicles with these devices, seriously.

You could use them to take a rough guestimate, but yeah...

6 Detectors
If we want to collect information about the flow of road traffic, we have to do measurements. The
only devices in the beginning of road traffic study were a stopwatch and manual counters.
This chapter discusses the equipment to measure road vehicle data, the detectors. Traffic detectors
are used to detect the presence and/or the passing of a vehicle. In these notes, emphasis is given to the
measurement of individual vehicles. Detection is used to adapt traffic control to the traffic flow, to
count the number of traffic units, and to measure traffic flow parameters.
The detection of pedestrians and cyclists is mostly done with a push button (street crossings). A push
button cannot be used to count the number of users. Detection of the weight of persons is hardly used.
6.1
Pneumatic road tube
A rubber tube with a diameter of about 1 cm is placed on the
surface of the road. When a vehicle passes, the wheel presses
the tube and the air inside the tube is pushed away. One end
of the tube is connected to a box that contains a membrane
and an electrical switch. The air pressure moves the
membrane and engages the switch. The other end of the tube
has a small opening, to prevent reflection of the air wave.
The tube is cheap and reliable, but because the tube is on the surface of the road it wears out rapidly
and must be checked regularly. The pressure box and the electrical contacts are susceptible to
pollution. Also bouncing relay contacts can cause inaccurate counting. It is also possible to use a
piezoelectric element instead of a contact, to detect the air pressure wave. This sensor is not sensitive
to pollution, but the electrical pulse is very weak and has to be amplified before it can be used. The
amplifier also suppresses bouncing, and delivers a digital signal. This makes the detector more
reliable, but also more expensive.
When two tubes are places at a short distance from each other it is possible to calculate the speed of
the passing vehicle. The profile of the tires has influence on the accuracy of this measurement.
Features:
mechanical pressure, axle detection, passing cars, no vehicle separation.
Application: simple counting, speed measurement.