Normally when we suggest routes and state "Use (say) MO/AR-5", we mean use MO-5 until you cross into Arkansas and then use AR-5 (or vice-versa). But if I tell you to use DE/MD-54, I am telling you to use exactly the same road. Since this road follows the state line, the centerline of the road demarcates the state boundary (also known as the 'Transpeninsular Line'), so that if you pass somebody along this highway, you have changed states twice, once when crossing the centerline to pass and once when re-crossing it back into your travel lane. If you are in the west-bound lane, it is marked as DE-54. If you are in the east-bound lane, it is marked as MD-54. BTW, this is not Delaware's only boundary quirk.