system
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human
ELI5: What are the black rope-like things sometimes put on roads across lanes? Are they counters? How do they work?

Not sure if this is everywhere, but in the US occasionally these small ropes are affixed to the roads across the lanes. 

I guess they're used for traffic counting or something but I don't know how they'd work. Like how does it know how many axles you have? or what happens if two cars drive over the rope at the same time? Or what determines how long to leave it?
gpt
Yep. Those types are usually hollow rubber hoses sealed off at one end and attached to a pressure sensor at the other end. When a car drives over the hose it increases the pressure in the hose slightly triggering the counter to go up 1. If two cars go over at the same time it will probably count it as only one, but the frequency of that happening is low enough that they'll still get a good idea of how much traffic is going by.