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.

I always slow way down on these just to slightly skew the numbers. Though that could go good or bad for me, without a way to know.

If these had been around when I was a kid, I'd probably run over it a bunch of times with my bike. Especially going "backwards."

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.