IS 85TH PERCENTILE SPEED USED TO SET SPEET LIMITS

Most agencies responding to a recent survey stated that the 85th percentile speed is the predominant factor used in setting speed limits. While 85th percentile speed remains the goal for posted speed limits, a majority of sites are being set at more than 5 mph less than the measured 85th percentile speed. Reviewing the operating speeds on a sample of roads found that the posted speed was equal to only 23-52% of the drivers on suburban/urban collectors and local streets, respectively. On rural roads, the speed limits were equal to as high as 72% of the drivers. Data for 128 speed zone study surveys was available for analysis. At only 10% of the sites did recommended posted speed limit reflect a rounding up to the nearest 5 mph increment. At about 33% of the sites, the posted speed limit was rounded to the nearest 5 mph increment. For the remaining 67% of sites, the recommended posted speed limit was greater than 3.6 mph below the 85th percentile speed. A review of these studies shows that the 85th percentile speed is used only as a starting point, with the posted speed limit being almost always set below the 85th percentile value by as much as 8-12 mph.