Improvements to US 40 began shortly after its designation. The highway was widened to in width from Baltimore to Rosedale and from Aberdeen to Havre de Grace, and to between Rosedale and Aberdeen, by 1930. US 40 was also widened to from Perryville to the Delaware state line by 1934. The Cecil County section of the highway was widened again with a pair of shoulders in 1937 and 1938. The U.S. Highway was relocated to its present alignment through Aberdeen to eliminate a pair of crossings of the Pennsylvania Railroad between 1930 and 1933; the old highway south of the railroad tracks became an extension of MD 22. However, the very high levels of traffic due to being the main highway between Baltimore and Philadelphia led to calls for the highway to be expanded to in width along its entire length by 1934.