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77 GLOSSARY OF TERMS Combs: Rhomboid pavement markings placed adjacent to the edges of the travel lane. The combs visually reduce lane width and increase the optical flow for drivers. Converging chevrons: A series of pavement markings where chevrons (with the point oriented in the direction of travel) are marked along a section of road where the spac- ing between successive markings decreases as one moves downstream. Countdown markers: A series of consecutive signs (usually three) that are posted beneath a primary sign that is used to convey the message to the driver of an impending downstream condition. The countdown markers are gen- erally rectangular with the height of the sign greater than the width. The marker has a geometric symbol (usually a âslashâ) across the sign; the upstream sign has three slashes separated vertically, the next sign has two slashes, and the final sign has one slash. The reduction in the num- ber of slashes is coded information intended to resemble a countdown. Dragonâs teeth: Pavement markings placed adjacent to both edges of the travelled lane and directly opposite for any length of roadway such that they resemble a row of teeth. Also known as sharkâs teeth. Gateway: Any device or change in the road character or installation, located at the threshold between the high speed and low speed areas, that is meant to inform the driver of the speed reduction. Hamlet: A community of people usually smaller than a vil- lage, but often used interchangeably with small town, vil- lage, or settlement. Optical flow: The pattern of apparent motion of objects, sur- faces, and edges in a visual scene caused by the relative motion between an observer (the driver) and the scene (the road). Optical speed bars: Transverse pavement markings placed in or across a lane that are intended to increase the flow of information in the visual field and cause drivers to slow down. Speed bars may be placed at a uniform spacing or may use an exponential spacing where the distance between successive bars decreases as one travels down- stream, creating a sense of increasing speed. Optical width: The relation of the height of the vertical ele- ments at the roadside to the distance between the vertical elements on opposite side of the road when measured in cross-section. When the distance between the vertical elements exceeds the height of the vertical elements, the optical width is large, and promotes faster operating speeds. Portal: See gateway. Psychological traffic calming: Calming measures that aim to increase the mental load on drivers and thereby encour- age them to reduce speed. This is in contrast to physical traffic calming measures that require drivers to slow down in order to safely negotiate the measure. For exam- ple, a speed hump is physical traffic calming; optical speed bars are psychological traffic calming. Speed roundel: A pavement marking symbol that shows the speed limit (expressed as a numeral without units of mea- sure) in a concentric ring. Wundt illusion: An optical illusion in which the lane/edge line markings or pavement edges appear to converging when chevrons pointed in the direction of travel are marked in the lane.