This photo is taken at the top of Polish Mountain, MD, which at its highest elevation is 1,800 feet. The road starts in Baltimore, MD, and at its inception was the eastern start of the National Road, US 40 (MD 140). The Old Motor photo above shows a young mother and child who have posed with their Dodge at the top of the mountain for this 1923 photo. The first photo below shows a garage that is on the very left side of the photo above.
The second photo below is about two miles east of Flintstone, Maryland and shows the trip down the mountain road and the reason for the warning sign. This photograph is from the Maryland State Roads Commission, and is found on a great site, 125 M to B where there are many more photos and information about the road. The third photo below is taken from Down the Road which also has many great photos, along with more information about the Nation Road.