The aim of the present paper is a critical review of the effect of traffic flow to road accidents. Firstly, an introduction is provided, including macroscopic as well as microscopic studies. Afterwards, relevant studies are discussed regarding the effect of traffic flow to accident frequency and rates, inside and outside urban areas and in junctions as well. A review of studies which examine accident severity is also illustrated and discussed. Lastly, a critical assessment of the studies reviewed is provided. Accident frequency seems to have a linear positive relationship with Annual Average Daily Traffic (AADT). Concerning accident rates, a non-linear relationship exists. Lastly, the effect of traffic flow on accident severity is not clear and needs further research. In general, the great majority of existing studies are macroscopic. Consequently, it is suggested that further microscopic investigation of the effect of traffic flow to road safety is performed where existing studies are few. Keywords: Traffic flow, accidents, severity, road safety.