Studying Characteristics of Operating Speed for Proper Speed Limit: An Age-Old Subject with New Evidence from a Developing Country

Using design speed as a primary consideration for freeway speed limit selection has been a common practice since the opening of the first freeway in China about 20 years ago. The public, freeway operators, and law enforcement have widely questioned and challenged this practice in recent years. Particularly at locations where operating speeds are much higher or lower than speed limits, speed limits based solely on location specific design speeds in many newly completed freeways in the country have been under intensive public scrutiny. The operating speed that closely relates to driver behavior, vehicles’ mechanical properties, and the design speed is one key consideration in establishing a speed limit. Driver behaviors vary greatly in regard to a host of factors, including cultural background and stage of economic development. Undoubtedly different social, economic, and cultural environments contribute to significant driver behavior variations. The methods used by developed countries in speed limit selection may not be suitable or entirely applicable to situations in developing countries. Thus, the goal of this study, as part of a project sponsored by the Chinese Ministry of Communication, is to investigate the relationship between speed limit and operating speed under different freeway environments. The principal study findings are: (a) operating speed is only loosely associated with speed limit, (b) for a given speed limit, the average speed varies greatly depending on location characteristics, (c) standard deviation of operating speed distribution on Chinese freeways is generally much bigger than that on a typical freeway in developed countries, and (d) there are huge speed gaps between small and large vehicles. The last two findings represent big freeway transportation safety problems. The combination of speeding and slow running vehicles on freeways also significantly lowers freeway capacity. A few suggestions on speed limit and speed management are prompted by the unique speed characteristics observed by this study.