They are for counting vehicles. You cannot determine speed accurately without knowing the tire circumference
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By that logic, you cannot tell how fast I'm walking without my circumference.
They're usually not used for speed where I live. They're simply for measuring traffic.
I can think of a few situations where it is used on small roads only to check the traffic without it necessarily meaning that any changes will be made to the small road.
If someone owns a small private road which is open to public traffic and it gets used a lot by people who don't live on that road, it's possible to request a reading of how much traffic passes through to determine if the municipality should take over responsibility of maintaining the road.
If there recently was built a larger bypass it makes sense to check if people still use the smaller roads instead of the bypass for whatever reason.
These things can be used to figure out lots of other stuff about traffic dependent on how and where they're set up, but generally it's that kind of thing. Just traffic measurement.
Speed readings would be incredibly annoying to do this way. While it is technically possible, most of the data would be useless. Sorting through data from several weeks only to be able to determine that someone probably speeded there last week doesn't really give any meaningful information to anyone.
Speed checks are better done with the sort of doppler measurement devices that you might know from the signs that show your speed and blink when you're speeding.
It's for traffic count. IIRC the double track to is differentiate the count between passenger cars and semi-trucks (aka tractor trailers, 18 wheelers). The double track can differentiate when there are multiple axles like you see on semi-trucks. Sometimes you want separate counts.
You could calculate speed for shits and giggles but it's not enforceable obviously.
It's not for enforcement...this will tell them if they can 'make profit' by putting up auto ticketing systems that fine you when speeding.
They generally only use a single one for counts. You use the double setup here for speed tracking.
I don't know for sure, but I think theoretically you could calculate the time it takes between a tire driving over each line to determine the vehicle speed. I imagine that's why there are two lines and not just one.