In this research the analysis of the impact of the new road traffic safety law (new RTSL) in Serbia to the decrease of the number of killed in traffic accidents in Serbia is shown. The effects of a new legislation and larger enforcement made by new RTSL implementation to the decrease of the number of killed and injured in traffic have been analysed by comparing the number of killed and injured in traffic in Belgrade (the capital city of Serbia) during the period from 2002 to 2009, to the data for the year 2010, the first year of new RTSL implementation. Bearing in mind the severity of the consequences, the casualties in traffic was classified into: fatalities, serious injuries and minor injuries and also the analyses of the effects were based on the sorts of traffic participants: drivers, pedestrians, cyclists and passengers. Noticing the effects has been conducted by comparing the number of each class of the casualties in the year 2010 to the annual average number for the period of 2002 to 2009, minimal annual number of killed for the period of 2002 to 2009 and the forecasted trend value for the year 2010 that was obtained by using ARIMA modelling. The analyses of the effects of the new RTSL in Belgrade and Serbia conditioned by new legislation decrees and increased penalties has shown that there have been positive effects, manifested in the decreased number of killed in traffic accidents in 26.5%, but effects of new RTSL weakened as the year was finishing.