Non-steep berms, particularly with flat tops, were more effective at shielding road noise than noise walls of the same height due to wind effects. Berms with a forest-floor type cover reduced sound by 0.9 to 2.7 decibels more than grassy berms.
Van Renterghem, Timothy, and Dick Botteldooren. (2012). On the choice between walls and berms for road traffic noise shielding including wind effects. Landscape and Urban Planning, 105(3), 199-210.