They measure speed (how long between compression of each hose), direction (which hose was hit first), weight (how much was each hose squeezed), and axel count (how many pairs of wheels went over).
When you are calculating road wear, number of axels (and weight) is more important than number of vehicles.
You can often derive the number and type of vehicles by the pattern of “hits”. A passenger car will have a different pattern than an 18-wheeler; a van will have a longer wheelbase than a motorcycle, etc.