The environmental impact assessment (EIA) is an acknowledged method in a world for the prognosis of the possible changes of environment caused by proposed development, including linear infrastructures. The EIA of planned projects for building or reconstruction of the motorways in Latvia usually are carried out under busy time frame, therefore the data characterizing current environmental conditions sometimes are not provided in required quality. That kind of data gaps caused inaccurate prognoses on possible changes on environmental quality after project implementation. Our research demonstrated that this was a case also concerning miss-evaluation of sound level on Saulkrasti bypass. Within the frames of EIA the prognosis of sound level was elaborated, proofing that generally the traffic sound will be within officially allowed levels, although the problem points are recognized and building of sound barriers are proposed. But after construction of bypass the noise monitoring haven't been carried out, to suppose that prognosis are true. However the results of experimental measurements in 4 points on Saulkrasti bypass are showed opposite – the noise level in daytime (Ld) exceeded the acceptable value in all points of measurements, including places with installed sound barriers. The task of our experimental measurements on bypass was also to find the correlation between the noise level and traffic intensity. The results are showed that correlation between those factors are weak, and the noise level isn't directly depending from traffic intensity, but from total impact of several factors.