What are the two cables in some roads?

kaiowas said:
Traffic flow monitoring, they tend to be set up in clusters to measure traffic flows in to, out of and inside a particular area. Usually done to assist with planning traffic management and new developments etc.
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this

my housemate used to work as a traffic planner, or 'roundabout designer' as he claimed his job title was :p
 
bam0 said:
No, that's my point they don't need to know how long the car is at all, the two cables are about a foot apart (it'll be a precise measured distance), start a timer when you receive a signal that the first cable has been hit by a wheel, stop the timer when the second cable is hit by the same wheel. You've got the distance between the two points and the time it took one set of wheels to go that distance, that gives you the speed. Imagine the car was a unicycle, it'd work fine. Imagine there are two unicyclists holding a pole that keeps one 2.5m behind the other, you'd get a reading for each of the unicyclists, that'd be how it'd work with a car.
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Ah I completely missed the point there are 2 cables and that there would be timers on each one :o

Thanks for being patient with me :o

/me shreds engineering degree, applies for acting school.
 
A45 - Birmingham to Coventry.

They recently had a load of these set up by the speed cameras. Over a period of a few weeks, they placed them before and after the cams.

Two sets per cam, so two cables about 100ft before the cam and two cables about 100ft after the cam.

Suggests to me they do measure speed as it would be pointless having two sets wouldn't it. Also, it would highlight that people just slow down for the cam, as indicated by the speeds recorded before and after the cam.

:confused:
 
markf said:
Front wheel goes over 1st cable - timer starts, Front wheel goes over 2nd cable - timer stops.
Rear wheel goes over 1st cable - timer starts, Rear wheel goes over 2nd cable - timer stops.

Not, front wheel goes over cable timer starts, Rear wheel goes over cable timer stops.
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A large wheel with lowish pressure (lorry) will have more contact with the ground than a smaller wheel (mini etc). The lorry will appear to be going faster due to the shorter amount of time between the wheel releasing the first cable and pressing the second one (due to the increased contact area).

NOT used for speed ;)
 
Tyre pressure wouldn't have any effect. The timer will start as soon as there is sufficient pressure on the first cable and then stop when the same wheel (therefore, same pressure) hits the second cable.

Its not counting the number of seconds that each cable is depressed for, but the time between both of the cables registering a hit.
 
Code: 
O     . ,

O is the wheel
. is first cable
, is second cable

I'm sure we're all aware that S = D/T

T1 will register when wheel (O) hits first cable (.)
T2 will register when wheel (O) hits second cable (,)
T = T2 - T1

To keep things simple, we'll assume T1 = 3pm and T2 = 4pm, therefore, T = 1 hour

Next, we'll assume for the purpose of this calculation that the cables are 1 mile apart. So, D = 1

Therefore...
S = D/T
S = 1/1
S = 1mph.

Now, change the distance to feet and the times to milliseconds.

SIMPLES!
 
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Amoeba said:
A large wheel with lowish pressure (lorry) will have more contact with the ground than a smaller wheel (mini etc). The lorry will appear to be going faster due to the shorter amount of time between the wheel releasing the first cable and pressing the second one (due to the increased contact area).

NOT used for speed ;)
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attempted rebuttal fail :)
 
Meeko said:
Code: 
O     . ,

O is the wheel
. is first cable
, is second cable

I'm sure we're all aware that S = D/T

T1 will register when wheel (O) hits first cable (.)
T2 will register when wheel (O) hits second cable (,)
T = T2 - T1

To keep things simple, we'll assume T1 = 3pm and T2 = 4pm, therefore, T = 1 hour

Next, we'll assume for the purpose of this calculation that the cables are 1 mile apart. So, D = 1

Therefore...
S = D/T
S = 1/1
S = 1mph.

Now, change the distance to feet and the times to milliseconds.

SIMPLES!
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It is theoretically possible and it is simple, but the distance between them is so small (a few feet at most) compared to the flex in them (a few cm either way) that they couldnt give an accurate result.
 
iaind said:
It is theoretically possible and it is simple, but the distance between them is so small (a few feet at most) compared to the flex in them (a few cm either way) that they couldnt give an accurate result.
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They are laid down quite taught, but suppose the two cables were 50cm apart and moved upto 3cm each way, a car travelling at 30mph would read 26.5mph or 33.5 mph at the extreme limits, I think 50cm is probably a little small, at 80cm the same extremes would give 27.7mph to 32.2 mph. Considering these examples are the extremes of movement and that what would cause one cable to move forward would likely make the other move forward and vice versa, also that these devices would be looking at average speeds you would probably get a very accurate average.
 
iaind said:
I'm an ex transport minister and I design speed camera systems in my free time. Common sense says they arent accurate enough to monitor speed. Has that changed your mind? :)
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Of course they can be used for speed. They know how far apart the two wires are set, pressure on one wire starts the timer, pressure on second wire stops it. You know the time and distance, so you can calculate speed from that.

Doesn't matter what away the traffic is flowing either, first hit is on, second hit is off.
 
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