Maryland Transportation Section 21-301
(a)   On every roadway that is wide enough, a vehicle shall be driven on the right half of the roadway, except:
(1)   While overtaking and passing another vehicle going in the same direction, under the rules governing this movement;
(2)   Where there is an obstruction that makes it necessary to drive to the left of the center of the highway, but the driver of any vehicle doing so shall yield the right-of-way to any other vehicle that is traveling in the proper direction on the unobstructed part of the highway and is so near as to be an immediate danger;
(3)   On a roadway that is divided into three or more clearly marked lanes for vehicular traffic, subject to the rules applicable to these roadways;
(4)   On a roadway designated and signposted for one-way traffic; or
(5)   On a roadway that is marked or signposted in a manner indicating that a contrary rule exists.
(b)   On every roadway, except while overtaking and passing another vehicle going in the same direction or when preparing for a lawful left turn, any vehicle going 10 miles an hour or more below the applicable maximum speed limit or, if any existing conditions reasonably require a speed below that of the applicable maximum, at less than the normal speed of traffic under these conditions, shall be driven in the right-hand lane then available for traffic or as close as practicable to the right-hand curb or edge of the roadway.
(c)   (1)   On any roadway that is divided into four or more clearly marked lanes for vehicular traffic and that provides for two-way movement of traffic, a vehicle may not be driven on the left of the centerline of the roadway, except:
(i)   Where authorized by a traffic control device designating a lane to the left of the center of the roadway for use by traffic not otherwise permitted to use this lane; or
(ii)   As permitted under subsection (a)(2) of this section.
(2)   This subsection does not prohibit the crossing of the centerline of a roadway while making a left turn into or from an alley or a private road or driveway.