The have dense network of road

plains

Explanation:

It has been noted that the density of road infrastructure is higher in the plains.

This is because the terrain is leveled and easier for building, which makes it more likely for cities and settlements to be built here. If the population of the region is higher, there is more need for the roads and traffic to be built.

Additionally, it is easier to drive through the plains than to through the mountains, which is why roads will be built in the plains more likely. When the terrain is leveled, roads can also be built through the forests and deserts.

81 is a composite number.