Characteristics of Operating Speed for Proper Speed Limit

Using design speed as a primary consideration for selecting the speed limit of a freeway has been a common practice since the opening of the first freeway in China about twenty years ago. This practice has been widely questioned and challenged in recent years by the public, freeway operators, and law enforcement. Speed limits solely based on the location specific design speed in many newly completed freeways in the country have been under intensive public scrutiny, particularly at locations where the operating speeds are much higher or lower than the speed limits. One key consideration in establishing speed limit is the operating speed that closely relates to driver behavior, vehicles' mechanic properties, and the design speed. Driver behavior varies greatly by factors including cultural background and stage of economic development. Undoubtedly different social, economic, and cultural environments contribute to significant variation in driver behavior. The methods used by developed countries in speed limit selection may not be suitable, or at least not entirely applicable, to situations in developing countries. Thus, as part of a project sponsored by the Chinese Ministry of Communication, the goal of this study is to investigate the relationship between speed limit and operating speed in different freeway environments. The principal findings from the study are: (a) operating speed is only loosely associated with speed limit, (b) for a given speed limit, the average speed varies greatly depending on the characteristics of the location, (c) standard deviation of operating speed distribution on freeways in China is generally much bigger than on a typical freeway in developed countries, and (d) there are huge speed gaps between small and large vehicles. The last two findings represent a big safety problem for freeway transportation. The combination of speeding and slow running vehicles on freeways significantly lowers freeway capacity, as well. The unique speed characteristics observed by this study prompts a few suggestions on speed limit and speed management.