Road speed limits are to set the maximum speed of vehicles on roadways. Because the speed limit is closely related to driver’s driving behaviors, this has been a dispute within drivers for decades. For example, people might say that the speed limit is too low even though the width of road is enough to drive fast, or people rather say that the speed limit is too high while the road is narrow. This is drivers’ desires in a subjective point of view, but it implies that speed limits need to be decided by taking the road features such as road width, number of lanes, traffic volume, the proportion of heavy vehicles, and land-uses into consideration. In other words, there is a demand for reasonable speed limits.
Speed limits in the Republic of Korea (hereafter ‘Korea’) are regulated by the Korea’s ROAD TRAFFIC ACT, and the speed limits have been usually set to 60 km/h in most urban roads and the speed limit equal or higher than 80 km/h has been allowed for the two-lane roads. However, this speed limit regulation is limited to comprehensively consider the surroundings of roads such as land-uses, population, and the proportion of heaby vehicles, which define the functionality of roads. Ultimately, this limitation in law hinders to decide reasonable speed limits. In addition, the national police agency in Korea recently announced that the speed limits in urban roads will be lowered to 50 km/h or 30 km/h (called ‘speed limit changes 5030’), based on the claim that the speed limits in urban roads in Korea are higher than the many cases in Europe and north America.
With these backgrounds in mind, This study investigated the effectiveness of the speed limit changes in urban roads in terms of safety (i.e., accidents) and mobility (i.e., travel speed and time). The study results indicated that lowering speed limits decrease accidents and mitigate the severity of accidents. Although lowering speed limits slightly increased travel time, it was not a significant impact in urban road because the travel times in urban roads are significantly affected by traffic signals. With this in mind, lowering speed limits will carry a positive impact in urban road, based on that the accident reduction impact (benefit) was greater than the travel time increases impact. This finding justifies the speed limit changes policy in Korea. In addition, this study suggested the road characteristic indexes to be considered when setting speed limits in urban roads, and also proposed a reasonable procedure for determining speed limits. These study results will help to set reasonable speed limits, and we hope that this study enhances traffic safety in urban roads.