Maryland has an extensive system of state highways, exclusive of the national Interstate and U.S. highway systems, that serves all 23 counties and the independent city of Baltimore, almost every incorporated city, town, and village, and most unincorporated places in the state.

There are a few instances where a county department of transportation maintains sections of a state highway.

Two unique situations are that MD 200 is maintained by the Maryland Transportation Authority, which controls all of the publicly owned, tolled highways and bridges in the state, and part of MD 295, also known as the Baltimore–Washington Parkway, is maintained by the National Park Service as a National Parkway.

The only signed example of duplicate numbers is MD 222, which is the old alignment of US 222 before the latter was rolled back from Perryville to Conowingo.

Also, MD 68 intersects I-70 in close proximity to I-68 that signs are posted on I-70 advising drivers of the difference between the roads.