Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Appendix E: Glossary
Appendix E: Glossary
Pages 391-418
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From page 391... ...
. 10-mph Pace The 10-mph pace is the 10-mph range encompassing the greatest percentage of all the measured speeds in a spot speed study.
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From page 392... ...
Rather than lowering the regulatory speed limits at each of these locations, traffic engineers often place standard warning signs accompanied by a square black-and-yellow advisory speed Figure E-2 Advisory plate as shown in Figure E-2. Although speed plate.
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Using data from a spot speed study, the average is calculated by summing all the measured speeds and dividing by the sample size, n. Basic Speed Law The Uniform Vehicle Code (National Committee on Uniform Traffic Laws and Ordinances 1992)
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(Note: 1 m = 3.28 ft and 1 km/h = 0.62 mph.) Business District For the purpose of establishing statutory speed limits, the Uniform Vehicle Code (National Committee on Uniform Traffic Laws and Ordinances 1992)
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From page 395... ...
Motorists traveling less than the posted speed limit might appear to be in compliance, but under certain weather, visibility, or traffic conditions, they may be violating the basic speed law. In the more general case of free-flowing vehicles under favorable environmental conditions, measures of compliance (actually, noncompliance)
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From page 396... ...
. Table E-1 National Safety Council and FHWA Traffic Crash Costs Type of Accident Cost ($)
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· Lane width -- must be sufficient to accommodate the design vehicle, allow for imprecise steering maneuvers, and provide clearance for traffic flow in adjacent lanes. It is dependent on the design vehicle, design speed, volume, the presence or absence of shoulders, horizontal alignment, and the presence of oncoming traffic.
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Design Speed AASHTO defines a roadway's design speed as "the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the high
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From page 399... ...
Although it appears reasonable that the posted speed limit should not exceed a highway's design speed, the existing roadway system includes countless horizontal curves with safe speeds below the design speed or posted speed limit; these situations are routinely handled with curve warning signs and advisory speed plates (see Figure E-2)
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From page 400... ...
; · Existing traffic controls (regulatory and warning) ; · Design features, including horizontal and vertical alignment, sight distance, and lane width; · Pavement surface condition; · Parking; · Presence and usage of driveways; · Roadside hazards; · Pedestrians and bicycles; · Speed limits on adjacent roadway sections; and · Existing level of speed enforcement.
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From page 401... ...
In other words, the driver of a free-flowing vehicle chooses a speed that he or she finds comfortable on the basis of the appearance of the road. In conducting a spot speed study, the field observer detects and records the speed of free-flowing vehicles.
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Design speeds are similar to those of rural freeways, but urban freeways often have three or four lanes in each direction and interchanges spaced at less than 1.2 mi (2 km)
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Highway Functional Classification In designing a highway facility, the engineer initially defines the function that the facility will serve. The level of service required for the anticipated volume and composition of traffic determines the subsequent selection of design speed and geometric criteria.
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The engineer selects R as a function of the highway's design speed and the superelevation. Level of Service A roadway's operational condition as perceived by motorists is referred to as the level of service.
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Local urban streets provide access to property and connections to roadways of higher functional class. Design speeds are typically 37 mph (60 km/h)
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Residence District For the purpose of establishing statutory speed limits, the Uniform Vehicle Code (National Committee on Uniform Traffic Laws and Ordinances 1992) defines a residence district as the territory contiguous to and including a highway not comprising a business district when the property on such highway for a distance of 90 m or more is in the main improved with residences or residences and buildings in use for business.
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From page 407... ...
, and traffic barriers to shield traffic from steep slopes or other potentially hazardous objects. Safe Curve Speed The safe speed through horizontal curves is often less than the design speed on adjacent sections of tangent roadway.
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Table E-2 indicates decision sight distances on rural highways where the expected maneuvers are a controlled stop and a speed or path change. Sight Distance, Intersection AASHTO identifies several intersection sight distance criteria that must be considered by the designer (AASHTO 1994)
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From page 409... ...
By contrast, passing sight distances for operational purposes assume that a partially completed passing maneuver may be aborted if an opposing vehicle comes into view while the passing vehicle is in the left lane. This assumption shortens the necessary sight distance considerably.
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From page 410... ...
100 670 320 110 728 360 120 792 Note: 1 m = 3.28 ft and 1 km/h = 0.62 mph. Sight Distance, Stopping Stopping sight distance is the minimum distance for a vehicle traveling at or near a highway's design speed on wet pavement to come to a complete stop before reaching a stationary object (150 mm high)
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From page 411... ...
Prima facie speed limits are the alternative to absolute limits. Speed Limit, Differential The motor vehicle codes in some states prescribe different speed limits for different classes of vehicles.
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Speed Limit, Statutory State motor vehicle laws specify numerical values for speed limits on specific categories of streets and highways. For example, a code might limit speeds to 25 mph (40 km/h)
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From page 413... ...
One method of addressing these types of situations is through the use of variable speed limits. An urban freeway variable speed limit system would operate in the following manner.
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From page 414... ...
Speed Variance Speed variance for a spot speed study is calculated by summing the squares of the differences between each measured speed and the average speed, and dividing the total by the sample size minus one (n 1)
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Speed Zone Speed zoning is the process of establishing a reasonable and safe speed limit for a section of roadway where the statutory speed limits given in the motor vehicle laws [e.g., 30 mph (48 km/h) in business districts]
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From page 416... ...
Traffic Calming Traffic calming is a term used to identify various engineering techniques to physically control vehicle speeds and/or volumes on local streets. The techniques, which include speed humps, traffic diverters, narrow roadways, and staggered alignment, are deployed in response to complaints by adjacent property owners of speeding traffic or excessive traffic volumes.
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From page 417... ...
VMT is commonly used to characterize the amount of travel on different classes of roadway and as a normalizing factor in calculating crash or fatality rates. REFERENCES ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials NSC National Safety Council AASHTO.1994.
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