Traffic (48 page)

“global optical flow”: Not all of our sense of motion comes from visual inputs, of course. The reason I, like many other people, experienced bouts of “simulator sickness” in the various driving simulators in which I drove is that the picture of the moving road I was looking at did not correspond to what my vestibular system (the “balance” system of the inner ear) was experiencing.

our “target”: In an interesting experiment at Brown University, researchers used virtual reality to create an optically impossible situation in which subjects had to walk toward something without the use of optical flow, instead of merely walking toward the object via its egocentric direction (its direction in space relative to the subject). Subjects were less accurate in their approach without the optic flow. See W. H. Warren, Bruce Kay, Wendy Zosh, Andrew Duchon, and Stephanie Sahue, “Optic Flow Is Used to Control Human Walking,”
Nature Neuroscience,
vol. 4, no. 2 (2001), pp. 213–16.

a kind of radial pattern: This is not an entirely resolved issue and is still being debated. Gibson, for example, observed: “The behavior involved in steering an automobile, for instance, has usually been misunderstood. It is less a matter of aligning the car with the road than it is a matter of keeping the focus of expansion in the direction one must go.” But as vision researcher Michael Land has pointed out, this argument may not account for a driver’s behavior around curves: “On a curved trajectory the locations of the stationary points in the flow-field vary with distance, generating a curved line across the ground plane, not a single focus of expansion.” Land notes that we rely instead of the inner edge of the road in driving around curves, with some 80 percent of driver’s glances being directed in that region. See Michael F. Land, “Does Steering a Car Involve Perception of the Velocity Flow Field?” in
Motion Vision–Computational, Neural, and Ecological Constraints,
ed. Johannes M. Zanker and Jochen Zeil (New York: Springer Verlag, 2001).

our sense of speed: It has also been argued that optic flow influences our estimates of distance while driving as well. See M. Lappe, A. Grigo, F. Bremmer, H. Frenz, R. J. V. Bertin, and I. Israel, “Perception of Heading and Driving Distance from Optic Flow,”
Driving Simulation Conference 2000
(Paris), pp. 25–31.

tree-lined roads: This information comes from T. Triggs, “Speed Estimation,” in
Automotive Engineering and Litigations,
vol. 2, ed. G. A. Peters and B. Peters (New York: Garland Law Publishing), pp. 569–98.

flow at the same speed: Christopher Wickens,
Engineering Psychology and Human Performance
(Upper Saddle River, N.J.: Prentice Hall, 2000), p. 162.

those at lower heights: See, for example, Christina M. Rudin-Brown, “The Effect of Driver Eye Height on Speed Choice, Lane-Keeping, and Car-Following Behavior: Results of Two Driving Simulator Studies,”
Traffic Injury Prevention,
vol. 7, no. 4 (December 2006), pp. 365–72; or B. R. Fajen and R. S. David, “Speed Information and the Visual Control of Braking to Avoid a Collision,”
Journal of Vision,
vol. 3, no. 9 (2003), pp. 555–555a.

than they intend to: See C. M. Rudin-Brown, “Vehicle Height Affects Drivers’ Speed Perception: Implications for Rollover Risk,”
Transportation Research Record No. 1899: Driver and Vehicle Simulation, Human Performance, and Information Systems for Highways; Railroad Safety; and Visualization in Transportation
(Washington, D.C.: National Research Council, 2004), pp. 84–89.

speed more than others: See, for example, Allan F. Williams, Sergey Y. Kyrchenko, and Richard A. Retting, “Characteristics of Speeders,”
Journal of Safety Research,
vol. 37 (2006), pp. 227–32. Of course, any findings that drivers of SUVs and pickups drove faster than other vehicles brings up other “confounding” factors, such as a higher rate of male drivers for those vehicle categories, or the idea that people who choose to drive SUVs and pickups may be more prone to speeding or feel safer and thus are more likely to drive at a higher speed—instead of the vehicle making them more prone to speeding.

slowly than they really were: N Harré, “Discrepancy Between Actual and Estimated Speeds of Drivers in the Presence of Child Pedestrians,”
Injury Prevention,
vol. 9 (2003), pp. 38–41.

slow down slightly: See “Research Shows Speed Trailers Improve Safety in Temporary Work Zones,”
Texas Transportation Researcher,
vol. 36, no. 3 (2000).

Some highway agencies:
Minnesota Tailgating Pilot Project
(St. Paul, Mn: Department of Public Safety, 2006). The Pac-Man information comes from the
Star Tribune,
December 20, 2006.

how fast they’re going: For a good roundup of research, see Leonard Evans,
Traffic Safety
(Bloomfield Hills, Mich.: Science Serving Society, 2004), p. 173.

279 feet: I am using the example provided by crash investigator and human factors researcher Marc Green, available at
http://www.visualexpert.com/Resources/reactiontime.htm
.

directly at a fielder: For a fascinating discussion of the complexities of catching a ball, among other things, see Mike Stadler,
The Psychology of Baseball
(New York: Gotham Books, 2007).

as much as several seconds: Robert Dewar and Paul Olson note that drivers “often perceive a stationary vehicle as moving, even with five seconds’ viewing.” Dewar and Olson,
Human Factors in Traffic Safety
(Tuscon: Lawyers and Judges Publishing, 2002), p. 23.

no idea of the rate: For a good discussion of this, see Olson and Farber,
Forensic Aspects of Driver Perception and Response
(Tucson: Lawyers and Judges Publishing Co., 2003), p. 112.

overtaking crashes: The psychologists Rob Gray and David Regan suggest that what is going on here is that as we stare for a while at things like the white stripes on the road, or trees on the side of the road, our brains quickly adapt; they compare the effect to the well-known “waterfall effect”: You stare at water rushing down a waterfall for a while, and then look at a nearby rock—it will seem to be moving upward. When we come off the highway, something similar happens, and it may look to us as if the stop sign at the end of the ramp is farther away than it really is, which is why engineers have tested chevrons and other patterns on off-ramps: to break up the illusion of those white stripes. Rob Gray and David Regan, “Risky Driving Behavior: A Consequence of Motion Adaptation for Visually Guided Motor Action,”
Journal of Experimental Psychology: Human Perception and Performance,
vol. 26, no. 6 (2000), pp. 1721–32.

really tell the difference: This has long been known to people who study driving. In
Human Limitations in Automobile Driving
(Garden City: Doubleday, Doran & Company, 1938), authors J. R. Hamilton and Louis L. Thurstone (psychologists at Harvard University) observed: “From eight hundred feet right down to where the other car is almost on top of you, the average eye will not have any idea of the
rapidity
of motion, or speed, of the oncoming car. It will perceive motion, and that is all. The distance at which motion is first perceived, as we have said above, does not depend very much on the speed of either car.
But the distance at which rapidity of motion is perceived depends entirely upon the speed of each car.
[italics in original] With two cars traveling 40 miles an hour, that distance where the average eye suddenly perceives rapidity of motion is about 145 feet between cars. When two cars are traveling at 50 miles an hour, that distance is about 70 feet. Now we begin to have some understanding of the reason for the frightful collision accidents on the highway.”

speed of the opposing car: See D. A. Gordon and T. M. Mast, “Driver’s Decisions in Overtaking and Passing,”
Highway Research Record,
no. 247, Highway Research Board, 1968.

your attempted passing: One study remarked on a “conundrum” about passing difficulty and passing risk, noting that drivers were found “to be somewhat poor at making the judgments required for passing maneuvers, particularly judgments about opposing vehicle speed, but the safety record of passing maneuvers is very good. This suggests that passing maneuvers occur in a relatively forgiving environment. First, while drivers are relatively poor in making passing judgments, many drivers may inherently understand this and make very conservative decisions about passing. Second, the buffer area provided downstream of each passing zone provides a margin of safety against collisions resulting from poor driver judgments.” From “Passing Sight Distance Criteria,” NCHRP Project 15-26, MRI Project 110348, prepared for the National Cooperative Highway Research Program, Transportation Research Board National Research Council, Midwest Research Institute, March 2000.

up by only 30 percent: L. Staplin, “Simulator and Field Measure of Driver Age Differences in Left-Turn Gap Judgments,”
Transportation Research Board Record,
no. 1485, Transportation Research Board, National Research Council, 1995.

to actually see: R. E. Eberts and A. G. MacMillan, “Misperception of Small Cars,” in
Trends in Ergonomics/Human Factors,
vol 2, ed. R. E. Ebert and C. G. Eberts (North Holland: Elsevier Science Publishers, 1985).

slower the object seems: H. W. Leibowitz, “Grade Crossing Accidents and Human Factors Engineering,”
American Scientist,
vol. 73, no. 6 (November–December 1985), pp. 558–62. Leibowitz also noted another potential reason—the “deceptive geometry of collisions”—for overestimating the distance of an approaching train, similar to the problem mentioned with drivers trying to judge the distance of an approaching car. A car and a train that are approaching each other will retain consistent positions. He wrote, “There is no lateral motion, and thus the principal cue to velocity is the increase in size of the visual angle subtended or the expansion pattern…. The rate of increases of the expansion pattern is not linear but rather is described by a hyperbolic function. For distant objects, the rate of change in the expansion is low. As the distance decreases, the visual angle subtended increases at an accelerated rate.” This is somewhat similar to a phenomenon known as “motion camouflage,” which has been observed in the natural world—male hoverflies, for example, move in a way to conceal the fact that they are moving when they are tracking female hoverflies. They do so, it has been argued, by “approaching along a path such that its image projected onto the prey’s eye emulates that of a distant stationary object (a
fixed point
). During its attack, the predator must ensure that it is always positioned directly between the current position of the prey and this fixed point.” Humans, research has suggested, are also susceptible to this effect. See Andrew James Anderson and Peter William McOwan, “Humans Deceived by Predatory Stealth Strategy Camouflaging Motion,”
Proceedings of the Royal Society B: Biological Sciences,
vol. 270, Supp. 1 (August 7, 2003), pp. S18–S20.

latter was moving faster: Joseph E. Barton and Theodore E. Cohn, “A 3D Computer Simulation Test of the Leibowitz Hypothesis,” U.C. Berkeley Traffic Safety Center, Paper UCB-TSC-TR-2007-10, April 1, 2007;
http://repositories.cdlib.org/its/tsc/UCB-TSC-TR-2007-10
.

human vision is an illusion: See Sandra J. Ackerman, “Optical Illusions: Why Do We See the Way We Do?”
HHMI Bulletin,
June 2003, p. 37.

(much more at night): Dewar and Olson,
Human Factors in Traffic Safety
, p. 88.

remember more at night): D. Shinar and A. Drory, “Sign Registration in Daytime and Night Time Driving,”
Human Factors,
vol. 25 (1983), pp. 117–22.

blind to our blindness: See H. W. Leibowitz, “Nighttime Driving Accidents and Selective Visual Degradation,”
Science,
vol. 197 (July 29, 1977), pp. 422–23.

as drivers actually do: M. J. Allen, R. D. Hazlett, H. L. Tacker, and B. L. Graham, “Actual Pedestrian Visibility and the Pedestrian’s Estimate of His Own Visibility,”
American Journal of Optometry and Archives of the American Academy of Optometry,
vol. 47 (1970), pp. 44–49, and David Shinar, “Actual Versus Estimated Night-time Pedestrian Visibility,”
Ergonomics,
vol. 27, no. 8 (1984), pp. 863–71, and Richard Tyrrel, Joanne Wood, and Trent Carberry, “On-road Measures of Pedestrians’ Estimates of Their Own Nighttime Conspicuity,”
Journal of Safety Research,
vol. 35, no. 5 (December 2004), pp. 483–90.

drive 20 miles per hour: See Olsen,
Forensic Aspects of Driver Perception and Response,
p. 157.

through the landscape: The contrast experiment discussed can be viewed at
http://www.psy.ucsd.edu/~sanstis/Foot.htm
. For an interesting discussion of the experiment and the traffic implications, see Stuart Anstis, “Moving in a Fog: Contrast Affects the Perceived Speed and Direction of Motion,”
Proceedings of the Conference on Neural Networks,
Portland, Ore., 2003.

signs have been set up: See C. Arthur MacCarley, Christopher Ackles, and Tabber Watts, “A Study of the Response of Highway Traffic to Dynamic Fog Warning and Speed Advisory Messages,” TRB 06-3086, Transportation Research Record, National Research Council, Washington, D.C., February 2007.

not brake accordingly: For an excellent discussion of snowplow visibility, see Albert Yonas and Lee Zimmerman, “Improving the Ability of Drivers to Avoid Collisions with Snowplows in Fog and Snow,” Minnesota Department of Transportation, St. Paul, Minn., July 2006.

glances over the shoulder: The rearview mirror information is drawn from Thomas Ayres, Li Li, Doris Trachtman, and Douglas Young, “Passenger-Side Rear-View Mirrors: Driver Behavior and Safety,”
International Journal of Industrial Ergonomics,
vol. 35 (2005), pp. 157–62.

actually it is
half:
This example was proposed by the art historian E. H. Gombrich in
Art and Illusion
(Oxford: Phaidon Press, 1961) and was later confirmed and studied further by Marco Bertamini and Theodore E. Parks in “On What People Know About Images on Mirrors,”
Cognition,
vol. 98 (2005), pp. 85–104. Their use of the phrase “on mirrors” immediately reveals one of the disconnects we tend to have with mirrors, as we tend to say “in mirrors,” as if the image lurked behind the glass. The authors note, “Both the fact that our image is half the physical size, and the fact that this relationship is independent of how far we are from the mirror, are counterintuitive. However, they become clearer as soon as we realize that a mirror is always located halfway between oneself and our virtual self.”