PA Residents? what does this sign mean?

Question

http://www.flickr.com/photos/28467467@N04/2812477150/

i live off of Milford Road in a town in Pennsylvania, that road is also known as State Road 2001 (SR 2001), in the sign shown at the intersection has a 90 at the bottom. a few miles down the road the number at the bottom changes to 40. this holds true for most SR signs, the numbers keep changing at the bottom of the signs. one will say SR 2120 and a 70 at the bottom and a few miles later it will be a 10 at the bottom of sign. any ideas what the secondary number means?

Answer 1

It looks like a mile marker  SR 2001 90 miles from where it started