Authors: Tom Vanderbilt
“once the prediction is broadcast”: Inrix, for example, predicted, ahead of the big I-5 highway closure in Seattle, that traffic would not be as bad as people were making it out to be (for the “disappearing traffic” reasons already mentioned). And it was not. Not everyone heard Inrix’s prediction, however, or at least they did not have enough faith in it against the wall of dire predictions of traffic mayhem. See Danny Westneat, “Math Whiz Had I-5’s number,”
Seattle Times,
August 22, 2007.
real-time, the better: See I. Kaysi, “Frameworks and Models for the Provision of Real-Time Driver Information” (Ph.D. thesis, Department of Civil Engineering, Massachusetts Institute of Technology, 1992).
travel times and congestion: See, for example, Daniel Florian, “Simulation-Based Evaluation of Advanced Traveler Information Services (ATIS)” (dissertation, Massachusetts Institute of Technology, 2004). For a useful review of previous studies, see David Levinson, “The Value of Advanced Traveler Information Systems for Route Choice,”
Transportation Research Part C,
vol. 11 (2003), pp. 75–87.
as more people have it: See Levinson, ibid.
for the savvy taxi driver: Other studies, however, have suggested that as more people have information about traffic conditions, traffic can actually get
worse.
The reason goes back to the noncooperative nature of the traffic network. If everyone is told at once that route A is better than route B, and people self-interestedly and immediately all move to route A, it will no longer be good. People who study networks call these “concentration” and overreaction problems. This is where imperfect information can be worse than no information at all: If no one is told anything, the outcome will be random—each route might be good or bad. It all depends on how quickly people get the information and the choices they make. Ideally, the roads would then be like, for instance, the rows of customs inspectors’ queues at an airport. Everyone can see how much each window is being used at once. If a new window opens up, people can exit every other queue and fill up the new one so that the new queue is as long as the others. The system is in equilibrium. Does it always work so well for the individual, however? You may have moved to the line a bit too slowly and found yourself farther back than you were in the queue you left. You had the information, but did you make the right decision? See H. S. Mahmassani and R. Jayakrishnan, “System Performance and User Response Under Real-Time Information in a Congested Traffic Corridor,”
Transportation Research A,
vol. 25, no. 5 (1991), pp. 293–307. See also R. Arnott, A. de Palma, and R. Lindsey, “Does Providing Information to Drivers Reduce Traffic Congestion?”
Transportation Research A,
vol. 25, no. 5 (1991), 309–18, and A. M. Bell, W. A. Sethares, and J. A. Bucklew, “Coordination Failure as a Source of Congestion,”
IEEE Transactions on Signal Processing,
vol. 51. no. 3, March 2003.
congestion has been passed: In a simulation by David Levinson, a professor of civil engineering at the University of Minnesota, travelers could save the most time through real-time information when traffic conditions were at 95 percent of the available capacity. This is the moment, he suggested, before queues have begun to form and the options begin to dwindle. From Levinson, “Value,” op cit.
huge majority of the traffic: In the Puget Sound study, interestingly, it was found that 5 percent of the tolled networks generated 50 percent of the hypothetical revenue for the study. Data from an e-mail exchange with Matthew Kitchen.
10 percent of the roads: See S. Lammer, B. Gehlsen, and Dirk Helbing, “Scaling Laws in the Spatial Structure of Urban Road Networks,”
Physica A,
vol. 363, no. 1 (2006), pp. 89–95.
because they are the fastest: A similar dynamic, interestingly, exists in ant-trail formation. As noted in the book
Self-Organization in Biological Systems,
ants tend to congregate on the paths that lead to the richest food sources or are the fastest: “The shortest path enables ants to minimize the time spent traveling between nest and food source, takes less time to complete, and therefore allows ants to consume their food more quickly, minimizing the risk that a good source of food will be discovered and monopolized by a larger or more aggressive neighboring colony. Shorter paths also mean lower transportation costs.” Attractive trails are visited by more ants, who lay more pheromones, which attracts even more ants, in a “feedback mechanism.” When a trail branches, ants will choose the branch that has been chosen by more ants. See Scott Camazine, Jean-Louis Denéoubourg, Nigel R. Franks, et al.,
Self-Organization in Biological Systems
(Princeton: Princeton University Press, 2001), particularly Chapter 13.
havoc with local roads: In England, for example, rural towns have seen traffic surge on roads that are essentially one-lane tracks, as SatNav-equipped drivers looking for shortcuts are sent on routes that “look good on paper,” as it were, but are ill-prepared to deal with a large influx of new drivers. See David Mill-ward, “End of the Road for Unreliable SatNavs,”
Daily Telegraph,
June 11, 2006.
was still the best: I kept having this experience. In Phoenix, I tried repeatedly to find alternate routes when I ran into congestion, and the phone, always pleasant, kept advising, “No alternate routes available.”
traffic in things: As pointed out by transportation researcher G. F. Newell, many people are resistant to treating vehicle transportation like any other good. “Economic theory is seriously flawed as applied to transportation,” he wrote, “because most economists treat transportation like a consumer good that can be sold to the highest bidder, but they don’t ask: ‘What does society want?’” He added, “I don’t know either.” See G. F. Newell, “Memoirs on Highway Traffic Flow Theory in the 1950s,”
Operations Research,
vol. 50, no. 1 (January–February 2002), pp. 173–78.
Chapter Seven: Why Dangerous Roads Are Safer
New York Times
observed darkly: Paul J. K. Friedlanden, “H-Day Is Coming to Sweden,”
New York Times,
August 20, 1967. See also “Sweden May Shift Road Traffic to the Right to Curb Accidents,”
New York Times,
November 12, 1961; “All Goes Right as Sweden Shifts Her Traffic Pattern,”
New York Times,
September 4, 1967; “Swedes Face the Trauma of Shifting to Right Side,”
New York Times,
April 10, 1966; and “Swedes Adjust, Some Grumpily, to Switching Traffic to the Right,”
New York Times,
September 5, 1967.
year before the changeover: See R. Näätänen and H. Summala,
Road-User Behavior and Traffic Accidents
(New York: Elsevier, 1976), pp. 139–40.
half
that of conventional intersections: The speed and conflict information for roundabouts comes from Timothy J. Gates and Robert E. Maki, “Converting Old Traffic Circles to Modern Roundabouts: Michigan State University Case Study,” in
ITE Annual Meeting Compendium
(Washington, D.C.: Institue for Transportation Engines, 2000).
about 90 percent: R. A. Retting, B. N. Persaud, P. E. Garder, and D. Lord, “Crash and Injury Reduction Following Installation of Roundabouts in the United States,”
American Journal of Public Health,
vol. 91, no. 4 (April 2001), pp. 628–31.
about to hit: See Kenneth Todd, “Traffic Control: An Exercise in Self-Defeat,”
Regulation Magazine,
vol. 27, no. 3 (Fall 2004).
free of junctions): See “The Impact of Driver Inattention on Crash/Near-Crash Risk: An Analysis Using the 100-Car Naturalistic Driving Study Data,” DOT HS 810-594, U.S. Department of Transportation, April 2006, p. 118.
“allow it on the roads”: Jake Voelcker, in his article “A Critical Review of the Legal Penalties for Drivers Who Kill Cyclists or Pedestrians,” makes the useful point that “Health and Safety regulations would not permit thousands of one-tonne steel and glass machines with exposed moving parts to repeatedly pass feet or inches away from unprotected workers on the shop floor at well over 10 m/s (HSE 1998, Sect. 11). Yet this is the situation in our towns and cities today. Why are drivers allowed to impose this danger on pedestrians without more strict prosecution of liability?” Retrieved from
www.jake-v.co.uk/cycling
.
no posts: V. P. Kallberg, “Reflector Posts—Signs of Danger?”
Transportation Research Record,
vol. 1403, pp. 57–66.
than when it is not: See, for example, S. Comte, A. Várhelyi, and J. Santos, “The Effects of ATT and Non-ATT Systems and Treatments on Driver Speed Behaviour,” Working Paper R 3.1.1 in the MASTER project, VTT Communities & Infrastructure (VTT, Finland), August 1997.
it confuses traffic people too: See Raymond A. Krammes, Kay Fitzpatrick, Joseph D. Blaschke, and Daniel B. Fambro,
Speed: Understanding Design, Operating, and Posted Speed,
Report No. 1465–1 (Austin, TX: Texas Dept. of Transportation, March 1996).
“time-consuming effort”: See David Shinar,
Psychology on the Road: The Human Factor in Traffic Safety
(New York: Wiley, 1978), p. 87.
in the period studied: Neal E. Wood, “Shoulder Rumble Strips: A Method to Alert ‘Drifting’ Drivers,” Pennsylvania Turnpike Commission, Harrisburg, Pennsylvania, January 1994.
nor is it always easy to locate: Think for a moment about when you come across a hill on a freeway. It is rarely very steep, but then again, it is not flat. Notice how the road gently unspools before you as you near the crest of the hill. The road has been designed so that if there is an unexpected obstacle lurking over that hill, the average driver should be able to see it and have enough time to react and stop. This seems like a smart idea. But how high should the imaginary obstacle be? What would make the road “safe”? Ezra Hauer, a retired Canadian professor of engineering, has observed that early on, highway engineers settled on a four-inch obstacle—a hypothetical “dead dog.” They did not know whether three-inch obstacles might also be dangerous, or even how many people were hitting four-inch roadkill as they came over a hill. All they really knew was that building the road so that drivers could stop in time for a three-inch obstacle would require more excavating, and thus more money. Little decisions like this may seem trivial, but in a larger sense they literally help shape the way our world looks to us (and how people behave in it). And so for every highway on a hill in America, the road was designed so the average driver could stop in time for a four-inch “dead dog.” In the absence of real information about how, why, and when obstacles on the road lead to crashes, this was, at the very least, prudent engineering, Hauer argues, based on the most efficient construction costs. But over time, something strange happened. Cars began to get lower. Suddenly, drivers could not see the four-inch obstacle in the given time. So the “dead dog” grew two inches taller—even though, Hauer says, “no link has been found between the risk of collisions with small fixed objects on crest curves and the available sight distance.” New roads were built with the new standard (and on existing roads, the driver had just better pray there were no four-inch dogs lying around). Things have since gotten even more complicated. The popularity of SUVs and pickup trucks in the United States means there is “some evidence,” as Ray Krammes told me, that cars are now getting
higher.
Is it time to lower the dead dog? See Ezra Hauer, “Safety in Geometric Design Standards,” Toronto, Ontario, 1999. Retrieved from
http://ca.geocities.com/[email protected]/Pubs/SafetyinGeometricDesign.pdf
.
in the United States alone): Steve Moler, “Stop. You’re Going the Wrong Way!”
Public Roads,
vol. 66, no. 2 (September–October 2002).
moving smoothly
triples:
The literature on weaving sections is surprisingly enormous, but for a good summary of weaving-section research and dynamics, see Richard Glad, John C. Milton, and David K. Olson,
Weave Analysis and Performance: The Washington State Case Study
(Olympia, Wash.: 2001).
safer and more efficient: See, for example, Richard W. Glad, Milton, and Olson, ibid.
be
less
safe: This information comes from an unpublished paper by Ezra Hauer, “Lane Width and Safety” (review of literature for the Interactive Highway Safety Design Model, 2000); accessed at
http://ca.geocities.com/[email protected]/download.htm
).
not statistically significant. See Karin M. Bauer, Douglas W. Harwood, Karen R. Richard, and Warren E. Hughes, “Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways,”
Transportation Research Record: Journal of the Transportation Research Board,
vol. 1897 (2004). On a side note concerning the phrase “statistically significant,” Ezra Hauer cautions that statisticians and policy makers often take the phrase “statistically not significant,” when referring to a traffic-safety study, to mean there would be no cost or benefit to implementing or not implementing some policy or another. Hauer points as one example to a series of studies examining the adoption of “right turn on red” laws; all showed higher crash rates after right turn on red was adopted. None were “statistically significant,” but all “pointed in the same direction”: Allowing right turns on red led to more crashes. See Hauer, “The Harm Done by Tests of Significance,”
Accident Analysis & Prevention,
vol. 36 (2004), pp. 495–500.
“to the road they see”: See Hauer, “Lane Width and Safety,” op cit.
already on the road: See Robert E. Dewar and Paul L. Olson,
Human Factors in Traffic Safety
(Tucson: Lawyers and Judges Publishing, 2002), p. 429. David Shinar writes of the “double jeopardy” of misidentified signing. “Misidentified signs compromise safety by taking more time from the driving task
and
leading drivers to make incorrect decisions. But signs that are interpreted as opposite of their intended meaning mislead the drivers who seem to respond to them as quickly as they do to signs that they identify correctly. Indicating that in these infrequent cases the drivers are sure, but wrong.” See Shinar,
Traffic Safety and Human Behavior
(Amsterdam: Elsevier, 2007), p. 168.