Conference Interactive Program
To view the 2013 Annual Meeting Interactive Program
TRB 90th Annual Meeting (January 23-27, 2011)
Advances in Traffic Control Devices
Jan 25 2011 7:30PM- 9:30PM
Marriott, Salon 2
Unintended Consequences of Improper Speed Zoning (11-1561)
1 ABSTRACT 2 Discussions of speed zoning focus almost exclusively on inducing the desired response by 3 drivers. However, speed limits also get used for a variety of other applications over which 4 the posting agency has no control and of which it may be ignorant. Posted limits may be 5 used to determine sight distances, traffic signal clearance times, and the selection and 6 placement of warning signs. However, to assume that they represent or control actual travel 7 speeds is usually incorrect and can lead to the creation of adverse conditions inimical to road 8 safety. 9 A survey was made of the available literature, codes, and manuals to get an idea of the 10 extent of the potential problems. Several earlier reports acknowledge posted speed limits are 11 generally well below the prevailing (85th percentile) speed. Many found them to be near to, 12 or lower than, the 50th percentile. Others indicate that the average for urban/suburban roads 13 is below the 30th percentile and for rural highways below the 60th percentile. 14 The posting or lowering of speed limits is not a safety measure and unless speed limits 15 are posted at or above the 85th percentile speed, they misinform engineers, planners and the 16 general public by indicating that travel speeds on a road are slower than they actually are. 17 Bad information leads to bad results. Their potential to create unsafe conditions should not 18 be minimized. 19 Though the consequences of establishing improper speed limits may be unintended, they 20 are not unforeseeable.
Ford, Stephen H., Mendocino County Department of Transportation
Transportation Research Board. 500 Fifth St. NW, Washington, D.C. 20001
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