mile marker vs milepost

mile marker

noun
  • A numbered milepost or sign along a highway, or some other road, used to determine the location of a motor vehicle. 

milepost

noun
  • A post on a highway, often with one or more fingerposts, showing the distance in miles to nearby places 

  • A sign or post beside a railway marking the distance from the (actual or nominal) start of a line (usually the principal terminus or junction with a more major line) 

verb
  • To place mileposts along (a road, etc.). 

How often have the words mile marker and milepost occurred in a corpus of books? (source: Google Ngram Viewer )