Speed is one of the main causes of severe traffic accidents, especially of those involving vulnerable road users. Speed display radars are one of the engineering solutions that aim to reduce the speed of traffic flow at locations where it is crucial. A before and after study was conducted at six locations on district roads in the Republic of Croatia. As part of the research, vehicles’ speeds were measured for two weeks, during which the devices’ displays were switched off for one week and switched on for another, however, the devices’ radars recorded and collected the speeds of all vehicles which passed during the research period. The research collected a total of 182,352 speeds of recorded vehicles. The test was performed to determine the effect of speed display radar on the potential vehicles’ speed reduction. The results did not show a statistically significant difference in average hourly speeds at most locations, however, they showed a statistically significant difference in 85th percentile hourly speeds of approximately 1 to 3 km/h. Therefore, it can be concluded that the speed display radars at the locations had a positive effect on reducing the vehicles’ speeds. Additionally, at some locations a decrease in the number of speed limit violators was found which proved to be a statistically significant difference between the share of speed limit violators in total traffic flow.