Humans lay road to gain fast access to distant places until aeroplane is invented, I would presume the mermaids living underwater can make use of the strong water current to get around the ocean quickly. The issue is unlike road these currents changes shape and direction frequently so how can the mermaids get to its destination accurately without trial and error using our 21st century technology?
One could rely on rules of thumb to determine where they go, but if you consider the challenge of mapping changing tides similar to the challenge of tracking traffic speeds to find the fastest route (not just the shortest route), then a Waze like tool could crowd-source the constant monitoring of current directions. It might even help them identify where predators like sharks might be lurking on the side of the road... I mean current.
Temperature and salinity should be sufficient to track currents. (see https://www.noaa.gov/education/resource-collections/ocean-coasts-education-resources/ocean-currents)
From a strictly biological sense it may be that Mermaids have keen-enough chemosensory facilities to detect currents on the basis of "smell" (i.e the relative concentrations of distinct chemical components in a parcel of water).
Each current has a distinct smell or chemical signature that correlates not only with its source, but also, as mixing of water parcels occurs, it's previous path.