Abstract: In order to analyze rationality of post speed limit, authors select the first grade highway Xinjiang Saibo road as a study object.Three study sites were posted in the vicinity of speed limit sign.By exploring relationship between actual speed and post speed limit inside and outside speed zone, the rationality of post limit sign can be evaluated.The analysis results lays foundation for post speed determination and also for informs decision makers to be aware where speeding is most prevalent.It is concluded that on the premise of traffic safety, the PSL should optimize the use of limited road resources.