Last week was our annual clean-up week, a time when we get our facilities prepared for the new year. During this cleaning, I uncovered a large stone marker tucked away behind some boxes. It's far too heavy to safely move it right now, but I did manage to get a few photos of it. It appears to read "18 M to P 1 M to Sp H".
Based on the design, it seems to be a milestone. The "M" most likely stands for "miles". So this sign means 18 miles to "P", which I would imagine stands for "Philadelphia". The "Sp H" probably stands for "Spring House", as that town is roughly 18 to 19 miles from Philadelphia.
Milestones like this one are not unique to the United States. Other examples of road markings date back to early empires such as Rome and Byzantine, just to name a few. Markers were placed along roads connecting various cities. They were used to help travelers identify where they were and ensure they were still travelling in the right direction. Depending on the civilization, culture, and available resources, designs of these markers varied. While some were made of stone and used numbers and letters, others were made of wood and used roman numerals.
We still use mile markers today, although generally they are made of metal. They are most commonly seen on major highways and are often used to report traffic incidents.
As for this particular milestone, I'm not entirely sure where it was originally placed. I would imagine the most likely location would be roughly 1 mile south of Spring House on Bethlehem Pike. If you are driving along major local roads such as Germantown and Bethlehem Pikes, you can still see some milestones that have survived to this day.