While Franklin was Postmaster [off and on between 1753 and the start of the revolution] he laid thousands of miles of post roads all up and down the colonies and into Canada and ordered them marked them with milestones. These stones were pretty consistent in shape and content. Every one that I've seen has been sandstone and they haven't weathered well. Many of those old post roads were upgraded over time and quite a few of those stones have gone missing during that construction [at least in the northeast].
Many other colonial roads [especially the "King's Highways" going from major city to major city] were marked with milestones and these run the gamut. Often they were just the closest, tallest rock with the directions and mileage hand-chiseled into them [and often the name or initials of the person setting the stone]. There is no conformity whatsoever to these types of stones and they are the hardest to find [you have to find the road first...]. These old roads also have all kinds of other ancient markers on them [directions, boundaries, "half-way" indicators, survey, etc.] and most of these are in surprisingly good condition [with some going on 300 years now].
The turnpike companies [active from the 1790's until the railroads killed them off] marked their roads with stones and these were pretty consistent in shape and content most of the time [but unique to each company - and there were 100's of turnpikes].
When the automobile came along, and before the numbered highway system, there were "Named Roads" and many of these were marked with milestones and directions by the organizations that named and promoted the road, like the Lincoln Highway, the Yellowstone Trail, the Mohawk Trail and some of these were coast-to-coast trails. There were several hundred "Named Roads".
I do have lots of pictures, but many were taken before digital cameras so I'll have to scan some examples. But first, I'll have to find them as I recently moved and haven't unpacked everything yet. Give me a few weeks.
PS Tubeman, I like your photo - do you always wear a lab coat?