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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 SPEED PERCEPTION AND DRIVING SPEED Introduction Speed perception refers to a driver's judgment of how fast he or she is traveling. While direct speed information is available from the speedometer, drivers still rely heavily on cues from the environment to judge how fast they are traveling. Auditory (engine noise) and tactile (vibrations) information can influence speed perception; however, drivers' primary basis for estimating their speed is the visual sensation provided by the highway geometrics and other information about objects in their immediate environment streaming through their visual field. If drivers underestimate their travel speed, they are traveling faster than they expect, and if they overestimate their travel speed, they will travel slower than they expect. Design Guidelines The driver's perceptual experience of the roadway should be consistent with intended travel speed. There should be some consistency between relevant roadway cues and posted speeds. FACTORS THAT AFFECT SPEED PERCEPTION Factors that May Cause Drivers to Factors that May Cause Drivers to UNDERESTIMATE Their Travel Speed OVERESTIMATE Their Travel Speed Higher design standard Two-lane narrow urban roads Greater roadway width Roads densely lined with trees Divided, walled urban roads Transverse pavement markings Rural roads without roadside trees Daylight compared to nighttime illumination conditions GRAPHICAL EXAMPLE OF OPTIC FLOW FROM A CENTRAL FOCAL POINT, WHICH IS INDICATED AS THE RED BOX (FROM CNS VISION LAB (1)) Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-4
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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion In Fildes, Fletcher, and Corrigan (2), subjects viewed film presentation of moving scenes in a laboratory setting. The study was conducted to develop a suitable means of assessing the sensory perception of speed on the road and to evaluate the effects of several road and roadside features on the speed judgments of drivers. Among other findings, the researchers reported that drivers underestimated their travel speeds on roads with higher design standards, on roads with a greater width, on divided and wide urban roads, and on rural roads without roadside trees (compared to those with many trees). They tended to overestimate their speeds on two-lane narrow urban roads. In Triggs and Berenyi (3), subjects estimated speed under day and night conditions as passengers driving in a car on an unlit freeway. Speed was underestimated in both day and night conditions; however, judgments were more accurate at night than during the day. Importantly, centerline pavement-mounted reflectors provided a highly visible feature that was unavailable during the day. From three types of speed estimation--(1) a driver's estimate of his/her own vehicle speed, (2) the estimation of approaching vehicle speed, and (3) detection of relative velocity when car-following--Triggs (4), a broad review of speed estimation studies, shows the following trends: Speed perception increases when transverse stripes are painted across the road with their separation progressively decreasing (though they may be effective only for drivers who are unfamiliar with the site). Speed judgments tend to be higher when a rural road is lined with trees. Speed judgments tend to be higher in low light conditions. During car-following, judgments of relative speed tend to be made more accurately when the gap between the two vehicles is closing rather than when it is opening. When car-following, observers in the following car tend to underestimate the relative speed difference between their car and the one in front of it. The figure on the previous page illustrates two important sources of information that underlie drivers' speed perception. The first is the point of expansion, which is denoted by the red square, and the second is the optic flow, which is shown as the blue arrows. During forward motion, the point of expansion indicates the observers' destination and appears stationary relative to the observer. All other points are seen as moving away from the point of expansion, and the relative motion of the optic flow points forms the basis for speed perception. Points that are closer to the observer appear to move faster than points closer to the point of expansion. Stronger and more consistent optic-flow cues (e.g., dense/cluttered visual environments, salient pavement marking, etc.) can amplify the sensation of speed through the environment and cause higher speed judgments. Design Issues Speed adaptation, which occurs for drivers who continue at a constant speed for an extended period of time, leads to drivers generally underestimating their speed in latter sections of extended tangent sections (4). This adaptation effect has implications for design elements requiring speed changes, such as horizontal curves, because drivers may be traveling faster than expected. Additionally, this effect may also carry over to nearby roadways (5). Milosevic and Milic (6) investigated the accuracy of speed estimation in sharp curves and the effect of advisory signs on speed estimation and found that drivers with over 11 years of experience significantly underestimated their speeds. Cross References Behavioral Framework for Speeding, 17-2 Effects of Roadway Factors on Speed, 17-6 Effects of Posted Speed Limits on Speed Decisions, 17-8 Key References 1. CNS Vision Lab (n.d.) Heading Perception: Where Am I Going? Retrieved November 24, 2009 from http://cns.bu.edu/visionlab/projects/buk/ 2. Fildes, B.N., Fletcher, M.R., and Corrigan, J.M. (1987). Speed Perception 1: Drivers' Judgements of Safety and Speed on Urban and Rural Straight Roads. (CR 54). Canberra, Australia: Federal Office of Road Safety. 3. Triggs, T.J., and Berenyi, J.S. (1982). Estimation of automobile speed under day and night conditions. Human Factors, 24(1), 111-114. 4. Triggs, T.J. (1986). Speed estimation. In G.A. Peters & B.J. Peters (Eds.), Automotive Engineering and Litigation (pp. 95-124). New York: Garland Press. 5. Casey, S.M., and Lund, A.K. (1987). Three field studies of driver speed adaptation. Human Factors, 29(5), 541-550. 6. Milosevic, S., and Milic, J. (1990). Speed perception in road curves. Journal of Safety Research, 21(1), 19-23. 17-5