The theory of the 85th percentile states that 85% of drivers on the roadways will drive at a reasonable and appropriate speed on a street or highway. However, several factors have been steadily increasing and affecting the 85th percentile. Drivers across the country have taken the attitude that the speed limit is a minimum rather than a maximum. Another very important factor is the enforcement and judicature of speeding. The majority of police departments will not issue citations at an absolute speed limit. In light of this, drivers rely on a 5-10 mph (8-16 kph) cushion they know will not get ticketed. Arterial streets signed at 40 mph (64 kph) have typical travel speeds of 47-50 mph (76-80 kph) for the average speed and the 85th percentile speeds. The city of Glendale, Arizona, commonly sets arterial speed limits at 35, 40, or 45 mph (56, 64, or 72 kph) and collector and residential streets at 25 mph (40 kph), with the majority of the arterials being set at 40 mph (64 kph). With the 40 mph (64 kph) speed limit, Glendale has found that it enables the signal system to be set at its most efficient progression. Because of this, the signal progression is set at 40 mph (64 kph) throughout most of the city. Glendale has done numerous studies to determine the speed of travel on residential streets signed at 25 mph (40 kph). Almost without exception, the average travel speed is 31-32 mph (50-51 kph). The 85th percentile is traveling at 35 mph (56 kph). The police department in Glendale usually enforces speeds at 7-11 mph (11-18 kph) over the speed limit. Therefore, the average travel speeds on residential streets, collectors, and arterials are 5-10 mph (8-16 kph) over the posted speed. This then raises the question of whether the 85th percentile will continue to rise if the speed limit on arterial streets is raised to reflect the new 85th percentile.


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Media Info

  • Pagination: 10p

Subject/Index Terms

Filing Info

  • Accession Number: 00770273
  • Record Type: Publication
  • Report/Paper Numbers: Publication No. CD-006
  • Files: TRIS
  • Created Date: Oct 21 1999 12:00AM