Speeding is a common behavior of drivers all over the world. Traditional speed-controlling measures fail to reduce speeding effectively. With advances in technology, new devices have been developed for such a purpose, including a speed-monitoring display (SMD) that detects the speed of vehicles and displays it on an electronic board, informing drivers of their current speed. It has been hypothesized that SMDs coupled with speed cameras to catch violators may deter drivers from speeding. This study examined the effectiveness of SMDs with enforcement cameras on speed reduction. Speed data of free-flowing vehicles were collected before and after the SMD and enforcement cameras were installed. Speed data were also collected with the SMD turned off and then on again. The results showed that mean speeds of vehicles were significantly reduced after SMD and camera installation. Even with the SMD turned off, the speed reduction was not significantly affected, but the proportion of speeding vehicles became higher. It was found that the presence of SMDs could alert drivers of enforcement cameras downstream, making speeders adjust their speed to comply with the limit.