What are the black strips laid across a road measuring
They are connected to traffic counter devices. Or actually, more correctly, axle counters.
As wheels travel across the tubes they compress the air in the tubes which actuates a counter in a device connected to the ends of the tubes. That then gives the owner of the road a count of how many vehicles (actually, axles) have traveled down the road over a number of days or weeks. The presumption is that two axles = one vehicle.
Why is this important? Sometimes the traffic count can be used to calculate payments due to a private road owne. Sometimes they are used to count the number of vehicles using a side street to see if traffic diversions need to be installed.
Usually though, they are used to gather information to inform the design of a new or upgraded road, or whether the amount of traffic on a road warrants an upgrade to the road. This is why black strips laid across a road measuring.