A positively judged soundscape generally includes both natural and social/cultural sounds in the sonic environment. Road traffic noise (RTN) is a major source of sound that may impact the both, such as the case of West Lake. Many studies examine soundscapes contexts with RTN based on physical descriptors, and subjective social scientific assessments by their descriptors mainly using onsite questionnaires to develop an understanding of the situation. By application of an objective evaluation method borrowed from speech intelligibility measurement techniques defined as the signal-to-noise-ratio-loss in the presence of RTN, research of the correlation between background RTN and environmental soundscape is developed by a self contained and evident proof derivation, proposed an objective evaluation method for protecting the soundscape from RTN, and presented the design and performance of an experiment to verify the method at two roads where RTN is propagated a distance into two roadside urban parks at West Lake. Our goal is the assessment and protection of the environmental soundscape from RTN using a convenient objective evaluation method that supplements cumbersome subjective investigations, provides an early warning concerning the RTN impact to the soundscape, and a tool how to improve the soundscape within the RTN impacted areas.