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94 GLOSSARY A number of the terms commonly used in speed literature have formal legal or mathematical definitions. For brevity, the definitions provided in this document are limited to two sentences. These definitions are intended to serve as a quick conceptual reminder and may not capture all of the nuances and caveats associated with certain terms. Consequently, other references may use or define these terms somewhat differently than this publication. 85th Percentile Speed: The speed at or below which 85% of vehicles travel on a specific roadway segment. Active law enforcement: Police officers are present in or near the work zone with the intent of identifying and pulling over (intercepting) violators. Advisory speed limit: A recommended (but not mandatory) speed for a roadway segment. In the United States advisory speed limits are marked using black-on-orange signs in work zones and black-on-yellow signs elsewhere. Automated speed enforcement (ASE): A system for detect- ing speeding vehicles and issuing citations to the registered owner of the vehicle by mail, without pulling over or inter- cepting the vehicle. Casualty: Any injury or fatality. Chicane: A roadway segment designed to limit vehicle speeds by requiring vehicles to navigate two or more relatively sharp horizontal curves; an S curve. Contraflow: A lane arrangement where traffic operates on the opposite of its usual side of the roadway; for example, a work zone that uses a median crossover followed by two- way operation on one side of a divided highway. Design speed: The speed established for a specific roadway segment as part of the geometric design process. Excessive speed: Exceeding the posted or statutory legal speed limit. Free-flow speed: The speed that vehicles on a roadway seg- ment tend to travel when the traffic volume is low, law enforcement is not present, and there are no adverse weather conditions. Inappropriate speed: Driving too fast for the prevailing road and traffic conditions, but within the posted or statutory limits. Mean (average) speed: A measure of the typical speeds on a roadway segment, computed by summing the instantaneous speeds of individual vehicles and dividing by the number of vehicles observed. Median (50th percentile) speed: A measure of the typical speeds on a roadway segment, computed by listing the instantaneous speeds of individual vehicles in rank order (lowest to highest) and finding the value that is halfway down the list. Multi-modal distribution: A statistical distribution with two or more clusters of frequently observed values. Normal distribution: A bell-shaped statistical distribution that is symmetrical around the mean value. Objective safety: The expected number of crashes or casualties (by type and severity) or (in some studies) the actual number of crashes observed over a multi-year period. Ordinary speed limit: The speed limit without road work; the pre-construction speed limit. Overtaking (or passing): Changing lanes to move past a slower vehicle. Passive law enforcement: Police officers are visibly present in or near the work zone to remind drivers to slow down, but are not favorably positioned for pulling over (intercepting) violators. Pilot car (or pilot vehicle): A designated vehicle that leads a platoon of other traffic through the work zone. Posted speed limit: The maximum lawful vehicle speed for a roadway segment as displayed on a regulatory sign. (In the United States regulatory speed limits are displayed on white-on-black signs.) Skew distribution: A statistical distribution that is not a normal distribution, but instead tends to be clustered at values above or below the mean. Speed variance: A measure of how much a set of vehicle speed observations is spread out; specifically the square of the standard deviation. A variance of zero indicates that all vehicles in the traffic stream are travelling at the same speed. Speed variation: The extent to which all vehicles on a road- way segment travel at different speeds. A low value indicates that all drivers choose similar speeds; a high value indicates that individual drivers select a wide range of speeds. Standard deviation: A statistical measure of the amount of variation from the average. A low standard deviation indi- cates that the data points tend to be very close to the mean; a high standard deviation indicates that the data points are spread out over a large range of values. Statutory speed limit: The maximum lawful vehicle speed for a category of roadways (such as freeways, rural two- lane highways, or residential streets), as defined in state, provincial, or local laws. Strategy: An overall plan to achieve an objective; including the decision making that leads to tactical execution. Subjective safety: The feeling or perception of safety; for example, how people subjectively experience traffic crash risks. Tactic: The means and methods used to accomplish an objec- tive; an element of a strategy. Traffic calming: Any set of physical or visual devices (except traffic signs alone) that is designed to reduce traffic speeds and encourage greater driver awareness of other road users. Two-way one-lane operation: A lane arrangement where only one lane is available for traffic and the lane is time-shared between opposing directions. In the United States, this is also called a flagging operation; in the United Kingdom, it is known as âshuttle working.â
95 equivalents have been included in parentheses in this report. When values quoted in source material appear to be approxi- mations (such as speeds rounded to the nearest 10 km/h), the equivalencies are also approximated, in most cases by making an exact conversion and then rounding to the nearest 5 mph. Example 1: Using radar observations, Researcher A deter- mined that median work zone speed decreased by 9.9 km/h (6.2 mph) when treatment X was deployed. Example 2: In an interview, Policymaker B stated that in her jurisdiction the use of enforcement technique Y typically slows traffic by 10 to 20 km/h (5 to 10 mph). Unstable traffic flow: Traffic conditions characterized by fre- quent speed changes, commonly referred to as stop-and-go driving. Work zone speed limit: A temporary speed limit imposed during roadway construction or maintenance. NOTE ON THE USE OF METRIC UNITS IN THIS SYNTHESIS REPORT This document summarizes work zone speed management research and agency practices from various countries. When metric units were used in the original source material, U.S.
