Approximately. Sometimes the terrain, a bridge and/or a river for example, will require that the mile marker be "kicked out" or moved a little to accomodate it.
This varies by state. Some states post one for each 1/10 of a mile. Others post mile markers by the 1/4 mile. Most however, especially along the interstate highway system mark only the miles, one per.
A square mile is a measure of area, such as the acreage of a plot of land. A mile is a measure of length or distance, such as from one city to another.
Imagine that you are a balloon floating around in the air. Lets say the wind is blowing at 100 miles per hour and you are floating along with the wind. In this case you will not be travelling any air...