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The Ramsey County lettered roads are a special situation. They do not get abbreviated with the letter. For example County J should not be named CR-J or CH-J.
These roads have a long history. They were laid out by Ramsey County on unincorporated land north of St
pail before most of the cities we know today were created. [http://www.historicmapworks.com/Map/US/487761/Index+Map+001/Ramsey+County+1931/Minnesota/ See a historical 1931 map here]. As cities incorporated land, many of them kept these names even though they were no longer county owned and maintained roads. As far as Ramsey County is concerned, if it is a county owned road, it will have a #. For example County J is Ramsey County 1. All the official blue 5-sided Ramsey county sign's only have the numbered routes. Nowhere is there a letter on a blue shield sign (in Ramsey County). So that's why we don't abbreviate them. CR- should eventually make the white county sign, and CH- should eventually make the blue county sign, The lettered roads never appear on a blue or white sign, so they do not get the CR- or CH- abbreviation. Also some of these are not even county owned roads so they don't get upgraded to primary street by default either. For example, [https://www.waze.com/editor/?env=usa&lon=-93.13445&lat=45.03588&layers=5&zoom=9 Count D at this link] is a city road maintained by the cities of Roseville and Shoreview who have decided to leave the road named as County Rd D. It is not a Ramsey County owned or maintained road and has no white or blue county highway signs. So it is named "County D" to match the street signs and is classed as a street classification. It is not upgraded to a primary street because it is not a real "County Road."
One bit of trivia since it is often asked, where are the missing letters? County A was where Larpenter Ave is today and County G was where Highway 96 is today.