I have a work research topic due Monday - in time for an afternoon meeting, actually. What was the FHWA's justification in determining the currently specified mile marker designs (reference marker and intermediate reference marker)? Not the fancy "enhanced" ones, just the plain "MILE 345". In particular I'm looking for reasons that the digits are 10"/12" (with 12" applying to freeways), which drives the overall sign dimensions. I'm sure this is an upgrade over a previous standard - the 2003 MUTCD has both, the 2000 MUTCD has only the mile markers and no tenth-miles, so hopefully there is some reference document that can be pulled out... Thanks!
I am not aware an engineering justification has ever been published (although one may have been referred to in a past rulemaking notice, e.g. the one that gave rise to the 2003 MUTCD). Here is what I have been able to find out:
1961 MUTCD--Unbordered, no word "MILE," digits 4" to 6"; Interstates must have 6" digits.
1967 Insurance Institute of Highway Safety report Mileposting--Marker designs used in the various states, generally with digits 4" to 6" (smaller digits used for mile equations in the states which explicitly note these on mileposts)
1978 MUTCD--Bordered, word "MILE," digits 6" on conventional roads, 10" on freeways, option to use unbordered mileposts (no "MILE") on minor roads, with digits 4" high
There is an element of arbitrariness in setting letter and digit height standards for signs (such as mileposts) which are not advance guide or exit direction signs, because it is not as critical that they be read in full.