Rock Spring Avenue north of Bel Air was improved as a macadam road starting in 1929. The improved road extended from the county seat north to the location of the Bel Air Bypass in the community of Frogtown in 1930 and was extended to about south of Forest Hill by 1933. The gap south of Forest Hill remained under county control until the highway was resurfaced with macadam around 1938. MD 24 was extended south from Forest Hill through Bel Air to MD 7 at Van Bibber in 1938. Through Bel Air, MD 24 followed Main Street, which was widened to in width around 1940.