I’ve never spent much time in the Eastern part of the country.  I was 15 years old before I ever crossed the Mississippi river, and since then I’ve crossed it maybe a dozen times.  I’m a Westerner by birth and by choice.  So every time I go east, I find something that surprises me.  Today I flew into Philadelphia on my way to a client’s site for a meeting tomorrow in Newtown, PA—about 40 miles north of the airport up I-95.

In Western states, mile marker numbers and exit numbers exactly correspond.  Exit 254 on I-35 is at mile marker 254.  It’s that way in every Western state that I can recall.  Certainly in Texas, New Mexico, Colorado, Utah, Arizona, Nevada, California, Wyoming, and the Dakotas.

The Philadelphia airport is at mile marker 13 on I-95.  Since my driving directions to Newtown said that I needed to head north to exit 30, I figured it’d be a 20-minute trip.  30 minutes later I was at exit 25 and starting to get suspicious.  It was then that I noticed mile marker 42 coming up.

Why did they do that?  Did somebody invent the mile marker system after the I-95 exit numbers had already been assigned?  Or are mile marker signs a convention that’s implemented by the individual states?  One would think that the Interstate Highway system would use the same convention nationwide.