Since a few of you shared my interest in why Pennsylvania and Maryland have different distances to D.C. and Baltimore on their highway signs, I figured I would ask the Maryland State Highway Administration how they measure the distance, and how it might differ from PA’s method.
Here’s what they sent me:
Good Afternoon Mr. Swift:
This email is in response to your question regarding how Maryland determines the mileage for post interchange distance signs. In the case of Baltimore City, Maryland State Highway Administration (SHA) measures from the location of the sign to the Town Hall. In the case of Washington D.C., the SHA measures from the sign location to the center of the Elipse. The mileage is generally rounded up so as not to display decimals or fractions, particularly when the distance is great such as the distance from the Maryland / Pennsylvania State Line to the Baltimore and DC destinations.
Interchange guide signs, which do display fractions, are typically rounded down to the nearest ¼ mile so that the motorist is aware that their exit is eminent and that time to make necessary lane changes is limited.
Thank you for allowing the SHA the opportunity to respond to your concerns.
Asst. Chief, Traffic Engineering Design Division