I too have wondered about this line. Here's what I've found via internet searches.
The mileage of roads in the United States is so vast and the traffic on many of the country roads is so light that it is out of the question to improve more than a small percentage of these roads with a hard surface. This does not mean, however, that that all other roads must be neglected. They should be improved just as far as their importance and the traffic will warrant. The common clay roads may be vastly improved by a little judicious grading and systematic maintenance.
In many cases, especially in the vast regions of the South, most of the common roads may be improved, for all practical purposes, merely by the addition of sand or clay, as the case may require, and incorporating this with the surface soil of the road. Thousands of miles of sand-clay roads have already been built in the Southern States at an average cost of about $750 per mile. These roads are almost without exception answering the purposes as well as a far more expensive form of construction would do.
The above excerpt comes from: Descriptive Catalogue of the Road Models of the Office of Public Roads. (1913). United States: U.S. Department of Agriculture, Office of Public Roads. (Link to the scanned book.)
The above text also appears in this Scientific American post from 1913, the same year Frost's A Boy's Will was published (though this poem was published elsewhere years before). That Scientific American article has pictures of various forms of road, but I fell asleep before reading it entirely
The 1920s were a "golden age" for road building. In 1922 alone, federal-aid projects totaling 16,500 km were completed at a cost of $189 million, three times as much roadway as had been improved since the start of the federal-aid highway program in 1916. The projects usually involved providing graded earth, sand-clay, or gravel surfaces.
As noted in a comment by @GarethRees, "Spreading sand to suppress dust doesn't seem as if it would work on its own: there would need to be a binder like oil or bitumen." The clay served as a binding agent, as I've read here.
According to that excerpt from highways.dot.gov, it looks like sand-clay roads were a smoother convenience than just gravel. It seems like some sort of wheel would distribute sand onto the roads to improve them. So perhaps helps us understand the speakers fearlessness about never finding "highway where the slow wheel pours the sand."