Part 4 (1/2)

For each of the different radio stations for which Hughes reports, he must change the length of his report, as well as the way he says it. One station wants ”upbeat and conversational,” while another wants a precise robotlike diction they call ”traffic formatics.” Some stations have advertis.e.m.e.nts for Hooters Casino, but the Christian stations do not. Some stations actually want him to be someone else. ”Good morning, I'm Jason Kennedy with AM 1150 traffic brought to you by Air New Zealand,” I suddenly hear him say. ”They're sort of competing stations,” he explains sheepishly, ”even though we own them both.” morning, I'm Jason Kennedy with AM 1150 traffic brought to you by Air New Zealand,” I suddenly hear him say. ”They're sort of competing stations,” he explains sheepishly, ”even though we own them both.”

Hughes has an instinctual understanding of Los Angeles' highways. He can tell which way a rainstorm is moving by looking at the real-time traffic-flow highway maps. He knows Fridays heading east out of the city can be particularly bad. ”Everyone's going to Las Vegas-all the way to ten p.m. that'll be backed up.” He knows that people drive slower on highway stretches that have sound barriers to either side. He knows that mornings with heavy rains often lead to lighter afternoon traffic. ”Maybe a lot of people got scared of the rain and disappeared,” he says. He notes that while traffic information is easily available to the public, often the trick is in understanding it. ”It's kind of like The Matrix, The Matrix,” he says. ”You're looking at the map and you can pick out what looks right and what doesn't. I can look at the map now and say, 'Hey, there's something wrong on the 101. A big-rig fire at Highland, probably.'”

There is no limit to the things that can disrupt the flow on Los Angeles highways. ”Do you want to know the number one specific item dropped on the freeway?” asks Claire Sigman, another Airwatch reporter. ”The most recorded item is ladders.” Trucks, just like in the Beverly Hills Cop Beverly Hills Cop movies, also spill avocados and oranges. Portable toilets have been dumped in the middle of the freeway. In 2007, a house, replete with graffiti and a ”For Rent” sign, sat for weeks on the Hollywood Freeway, abandoned during the course of its move after it struck an overpa.s.s (the owner had taken a detour onto an unauthorized route). People hold apocalyptic signs on overpa.s.ses, or threaten to jump. Wildfires break out. Out in the high desert, tumbleweeds cause problems. ”People swerve out of the way, rather than just drive through it,” Hughes says. A computer screen at the Airwatch office ticks off a steady flow of traffic incidents, ranging from the absurd to the horrifying, as recorded by the California Highway Patrol (CHP). Codes are used to disguise the presence of stalled female drivers, who might otherwise be preyed upon by unsavory men listening to police scanners. Not atypical of the stream is incident 0550, which describes a ”WMA,” or white male, wearing a plaid jacket and ”peeing in middle of fwy.” It adds a noteworthy detail: ”No veh in sight.” (Now, where was that wayward Porta Potti?) movies, also spill avocados and oranges. Portable toilets have been dumped in the middle of the freeway. In 2007, a house, replete with graffiti and a ”For Rent” sign, sat for weeks on the Hollywood Freeway, abandoned during the course of its move after it struck an overpa.s.s (the owner had taken a detour onto an unauthorized route). People hold apocalyptic signs on overpa.s.ses, or threaten to jump. Wildfires break out. Out in the high desert, tumbleweeds cause problems. ”People swerve out of the way, rather than just drive through it,” Hughes says. A computer screen at the Airwatch office ticks off a steady flow of traffic incidents, ranging from the absurd to the horrifying, as recorded by the California Highway Patrol (CHP). Codes are used to disguise the presence of stalled female drivers, who might otherwise be preyed upon by unsavory men listening to police scanners. Not atypical of the stream is incident 0550, which describes a ”WMA,” or white male, wearing a plaid jacket and ”peeing in middle of fwy.” It adds a noteworthy detail: ”No veh in sight.” (Now, where was that wayward Porta Potti?) CHP officers are the foot soldiers in the daily battle to keep Los Angeles' traffic from collapsing. The sophisticated computer modeling and fiber-optic cable that the traffic generals in the bunker have at their disposal are of little use when a car has stalled on Interstate 5, as I learned one afternoon when I went out for a patrol in a CHP cruiser with Sergeant Joe Zizi, an easygoing former trooper now doing public relations. CHP patrol officers begin each day by ”cleaning their beat,” or removing any abandoned vehicles or hazards from the road. ”That way there's nothing that people have to look at when they're driving,” Zizi says as he drives along the 101. Something as simple as a couch dumped in a roadside ditch can send minor shudders of curiosity through the traffic flow. A standard-issue black pump-action shotgun sits between the front seats. To enable drivers to carry out their traffic triage duties, patrol cars are outfitted with reinforced b.u.mpers, designed to let them push cars off the road rather than wait for a tow truck. Their trunks are filled with a dizzying array of equipment for dealing with traffic contingencies, ranging from baby-delivering kits (”definitely a spectacle for rubberneckers”) to dog snares.

”For some reason, dogs are attracted to the freeway,” says Zizi. ”They get on there, get completely freaked out, and start running down the center.” According to CHP statistics, these Code 1125-As (traffic hazard-animal) peak on July 5, presumably from dogs scared by the previous night's fireworks. When traffic is moving, CHP officers pa.s.s the time by looking for stolen vehicles (screwdrivers in the ignition are a telltale sign) and, of course, writing traffic tickets. Does Zizi have any advice for beating tickets? ”I have a lot of officers who say that women crying will get them out of tickets, while other officers say that if someone does does cry they're getting the ticket,” he says. ”Of course we have a lot of men who cry trying to get out of tickets, but that really doesn't work on the heartstrings of officers.” cry they're getting the ticket,” he says. ”Of course we have a lot of men who cry trying to get out of tickets, but that really doesn't work on the heartstrings of officers.”

For all the Caltrans cameras and loops wired into the road, for all the CHP officers flagging incidents, the highway system running through Los Angeles is so vast and incomprehensible that, sometimes, the only way to really understand what's happening is to pull way back and view the whole system from above. That is why there is still a place for people like Mike Nolan, KFI's ”eye in the sky,” a longtime L.A. traffic reporter who, twice daily, will take off in his Cessna 182 from Riverside County's Corona Airport and cover a swath of ground from Pasadena to Orange County.

”The learning curve is being able to read a freeway,” he explains, banking his plane over a new subdivision carved into green hillside. ”I know what's normal. I know where it should be slowing down and where it shouldn't. When I see something out of the ordinary, then I investigate it.” Nolan, whose navigational mantra is ”Keep the freeway to your left,” knows traffic patterns like a grizzled fis.h.i.+ng guide knows the best ba.s.s holes. A stalled Volkswagen in East Los Angeles is worse than an overturned oil truck in La Canada (”More spectacular does not necessarily translate into worse,” he says). Mondays, especially during Monday Night Football, Monday Night Football, tend to be a bit lighter. Thursday, congestion-wise, is now looking like the new Friday, traditionally the busy ”getaway day.” There are also strange blips in the pattern, like sunrise slowdowns. ”The very first day of standard time, when we go from daylight saving time to darkness, everybody just locks up,” he says. ”The traffic goes from bad to horrendous.” Rainy days can be bad, but the first rainy day in a while is even worse. ”There's a buildup of oil and rubber if it hasn't rained in a while. It's like driving on ice, literally.” tend to be a bit lighter. Thursday, congestion-wise, is now looking like the new Friday, traditionally the busy ”getaway day.” There are also strange blips in the pattern, like sunrise slowdowns. ”The very first day of standard time, when we go from daylight saving time to darkness, everybody just locks up,” he says. ”The traffic goes from bad to horrendous.” Rainy days can be bad, but the first rainy day in a while is even worse. ”There's a buildup of oil and rubber if it hasn't rained in a while. It's like driving on ice, literally.”

Nolan says people have long been predicting, because of ground sensors and in-vehicle probes that can detect the speed of traffic, that there will no longer be a need for aerial traffic reports. Indeed, on his instrument panel he has attached a TrafficGauge, a Palm Pilotsized device fed by Caltrans data, that shows congestion levels on L.A. freeways. But he says that data rarely tell the whole story, or the correct story. ”In my mind there's no subst.i.tute for looking out the window and telling people what you've got,” he says. ”The sensors in the road are delayed, they're inefficient. They're working half the time, not working half the time. There's no subst.i.tute for saying, 'It's in the right lane, I see it right there, right at the fill-in-the-blank overpa.s.s.' Or that the tow truck is in heavy traffic. The sensor can't tell you the tow truck's a block away, or ready to hook up and pull away. It can't give you the substantive info that comes from looking at it directly.”

Indeed, that afternoon of flying around the city, accompanied by an Airwatch reporter receiving ground reports, seems to be an exercise in chasing ghosts. The jackknifed tractor-trailer on the 710 is not there, or never was there. The blockage on the 405 was a rumor. Nolan is the one who must try to make sense of the strange reports that come in, like the one that announced a dead dog was ”blocking lanes one, two, three, and four.” The most remarkable traffic event he ever saw was during the L.A. riots of 1992. ”I remember seeing people stop at a stoplight in Hollywood. They would get out and loot a store. The light would turn green and they'd get back in and drive away. That was the most incredible thing I've ever seen.”

Flying over a city like Los Angeles, it is easy to glance down and think, for a moment, that the people below, streaming along trails, look like ants. If only it were that simple.

When Slower Is Faster, or How the Few Defeat the Many: Traffic Flow and Human Nature You hit the brakes for a second, just tap them on the freeway, you can literally track the ripple effect of that action across a two-hundred-mile stretch of road, because traffic has a memory. It's amazing. It's like a living organism.

-Mission: Impossible III

At some point you may have come to a highway on-ramp, expecting to join the flow of traffic, only to be stopped by a red light. Such devices are called ramp meters, and they are found from Los Angeles to South Africa to Sydney, Australia.

Ramp meters often seem frustrating because the traffic on the highway appears to be moving just fine. ”People ask me, 'How come you're stopping me at the ramp meter? The freeway is free-flowing,'” says Dawn Helou, the Caltrans engineer. ”The freeway is free-flowing because you're stopping.”

This is one of the most basic, and often overlooked, facts about traffic: That which is best for an individual's interest may not be best for the common good. The game traffic engineers play to fight congestion involves fine-tuning this balance between what is ”user optimal” and what is ”system optimal.” This happens on several different levels, both having to do with congestion: how traffic moves on roads and how larger traffic networks behave (an idea I'll return to in a later chapter).

The reason why highway ramp meters work is, on the face of it, simple once one knows a few basic facts about traffic flow. Engineers have been trying to understand, and model, traffic flow for many decades, but it is a huge and surprisingly wily beast. ”Some puzzles remain unsolved,” declares Carlos Daganzo, an engineer at the University of California, Berkeley. The first efforts merely tried to model the process known as ”car following.” This is based on the simple fact that the way you drive is affected by whether or not someone is in front of you, and how far away or close they are. Like ants responding to the presence of pheromones on the trail, you're influenced by the driver ahead, a constant, unsteady wavering between trying not to get too close and trying not to slip too far back. Now imagine those interactions, plus lane s.h.i.+fts and all the other driving maneuvers, a fluctuating mix of vehicle speeds and sizes, a wide range of driver styles and agendas, a dizzying spectrum of differing lighting and weather and road conditions; then multiply all this by the thousands, and you can begin to appreciate the higher-order complexities of traffic modeling.

Even the most sophisticated models do not fully account for human weirdness and all the ”noise” and ”scatter” in the system. Traffic engineers will offer caveats, like the disclaimer I saw at one traffic conference: ”This model does not account for the heterogeneity of driver behavior.” Do you feel uncomfortable driving next to someone else, and therefore speed up or slow down? Are you sometimes willing, for no apparent reason, to ride quite close to the car in front, before gradually drifting back? All kinds of strange phenomena lie outside easy capture by the traffic sensors. Car following, for instance, is filled with little quirks. A study that looked into how closely pa.s.senger-car drivers followed SUVs found that car drivers, contrary to what they said they did-and despite the fact that the SUV was blocking their view of the traffic ahead-actually drove closer to SUVs than when they followed pa.s.senger cars.

Or take what Daganzo has called the Los Gatos effect, after an uphill stretch of highway in California. You may have experienced this: Drivers seem reluctant to abandon the pa.s.sing lane and join the lane of trucks chugging uphill, even when they are being pressured by other drivers, and even when the other lane is not crowded. What's going on? Drivers may not want to give up the fast lane for fear of having trouble returning to it. They may also be unsure whether the person behind truly wants to go faster or is just keeping a tight s.p.a.ce to prevent someone else from pa.s.sing. A tight ”platoon” forms, but for how long? We all see these odd patterns. One of the idiosyncrasies I have noticed in traffic flow is something I call ”pa.s.sive-aggressive pa.s.sing.” You're in the pa.s.sing lane when suddenly the driver behind you pressures you to move into the slower right-hand lane. After you have done so, they then move into your lane, in front of you, and slow down, thus forcing you you to pa.s.s to pa.s.s them. them.

The basic parameters of how highways perform have been gradually hammered out. One of the key performance measures is volume, also called flow, or the number of vehicles that pa.s.s a buried sensor or some other fixed point on the highway. At four a.m., before rush hour, cars may be zipping along a highway at 75 miles per hour. The volume is measured at 1,700 cars moving past a point in one hour. As rush hour begins, the volume quite naturally begins to rise in an upward curve, reaching a theoretical maximum of 2,400 cars traveling at 55 miles per hour. System-wise, this is traffic nirvana. Then, as additional vehicles enter the highway, the curve begins to drop. Suddenly, the volume is back at 1,700. This time the cars are going 35 miles per hour. ”So you have the two 1,700s,” Helou says. ”Same volume, completely different situation.”

Because traffic moves in time and s.p.a.ce, measurements like volume can be deceiving, as can the highway itself. Solo drivers sitting in a highly congested lane may look to the HOV lane next to them and think that it's empty-a psychological condition so prevalent it even has a name, ”empty lane syndrome.” Many times it just seems empty because of the large headways between vehicles moving at much higher speeds. That lane may actually be achieving the same volume as the lane you are in, but the fact that the drivers might be going upward of 50 miles per hour faster creates an illusion that it's being underused. Of course, neither of these positive or negative individual outcomes-the driver whisking along at 80 miles per hour or the people stuck at 20 miles per hour in the congested lanes-are what's best for the entire system. The ideal highway will move the most cars, most efficiently, at a speed just about halfway.

Even as rush hour kicks in and the speed-flow curve begins to drop, traffic can perk along at what has been called ”synchronized flow,” heavy but steady. But as more vehicles pile onto the highway from on-ramps, the ”density,” or the number of cars actually found in a one-mile stretch (as opposed to pa.s.sing a single spot), begins to thicken. At a certain point, the critical density (the moment, you will recall from before, when the locusts began their coordinated march), the flow begins to break down. Bottlenecks, fixed or moving, squeeze the flow like a narrowing pipe. There are simply too many cars for the road's capacity.

Ramp metering aims to keep the highway's ”main-line flow” below the critical density by not letting the system be flooded with incoming on-ramp cars. ”If you allow unimpeded access, then you have a platoon of vehicles that are entering the main line,” says Helou. This means not only more cars but more cars jockeying to merge. Studies have shown that this is neither predictable nor always cooperative. ”That [merging] eventually breaks down the right lane,” she says. ”This overflows to the next lane, because people try to merge left before they get to it. And then the people in the second lane try to merge to the next lane before they get to it, so you break down the whole freeway.” A line of cars waiting to exit an off-ramp can trigger this same chain reaction, one study showed, even when all the other lanes were flowing nowhere near critical density.

If done properly, ramp metering, by keeping the system below the critical density, finds that sweet spot in which the most vehicles can move at the highest speed through a section of highway. Engineers call this ”throughput maximization.”

A simple way to see this in action involves rice. Take a liter of rice and pour it, all at once, through a funnel and into an empty beaker. Note how long it takes. Next, take the same rice and pour it not all at once but in a smooth, controlled flow, and time that process. Which liter of rice gets through more quickly? In a demonstration of this simple experiment by the Was.h.i.+ngton DOT, it took forty seconds for one liter of rice to pa.s.s through the funnel using the first method. The second method took twenty-seven seconds, nearly one-third less time. What seemed slower was actually faster.

Rice has more to do with traffic than you might think. Many people use water a.n.a.logies when talking about traffic, because it's a great way to describe concepts like volume and capacity. One example, used by Benjamin Coifman, an engineering professor at Ohio State University who specializes in traffic, is to think of a bucket of water with an inch-wide hole in the bottom. If the inflow into the bucket is half an inch in diameter, no water will acc.u.mulate. Raise it to two inches, however, and the water rises, even though some water is still exiting. Whether we drive into a jam (or a jam drives into us) depends on whether the ”water”-that is, the traffic trying to flow through a bottleneck-is draining or rising. ”As a driver, the first thing you encounter is the end of the queue,” Coifman told me. ”The first thing you encounter is wherever the water level happens to be that day.” The bucket metaphor also teaches us something else about traffic: No matter how much capacity there is in the rest of the bucket (or on the roads), the size of the hole (or the bottleneck) dictates what gets through.

At places like bottlenecks, however, traffic acts less like water (it does not speed up as highway ”channels” narrow, for one) and more like rice: Cars, like grains, are discrete objects that act in peculiar ways. Rice is what's called a ”granular media,” a solid that can act like a liquid. Sidney Nagel, a physicist at the University of Chicago and an expert in granular materials, uses the a.n.a.logy of adding a bit of sugar to a spoon. Pour too much, and the pile collapses. The sugar flows like a liquid as it collapses, but it's really a group of interacting objects that do not easily interact. ”They do not attract one another,” says Nagel. ”All they can do is scatter off one another.” Put a bunch of granular materials together, and it is not easy to predict how they will interact. This is why grain silos are the building type most p.r.o.ne to collapse, and it's also why my box of Cascadian Farm Purely O's cereal begins to bow outward at the bottom after several pours.

Why does the rice jam up as you pour it into the funnel? The inflow of rice exceeds the capacity of the funnel opening. The system gets denser and denser. Particles spend more time touching one another. More rice touches more rice. The rice gets ”hung up” from the friction of the funnel walls. Sound familiar? ”That's like cars on the highway,” says Nagel. ”And when you get narrowing of traffic, then that becomes very much stuff trying to flow through the hopper.”

Pouring less rice at a time-or moving fewer cars-keeps more s.p.a.ce, and fewer interactions, between the grains. Things flow faster. As intuitive as the ”slower is faster” idea is, it's not always easy for a driver stuck in traffic to accept. In 1999, a state senator from Minnesota, claiming that ramp metering in the Twin Cities was doing more harm than good, launched a ”Freedom to Drive” proposal that called for, among other things, shutting down the meters. The legislation died, but under another bill a ramp-meter ”holiday” was declared. For two months the meters were turned off. Drivers could enter the highway at will, on so-called sane lanes, unfettered by troublesome red lights. And what happened? The system got worse. Speeds dropped, travel times went up. One study showed that certain highway sections had double the productivity with ramp meters than without. The meters went back on.

The ”slower is faster” idea shows up often in traffic. The cla.s.sic example concerns roundabouts. Many people are under the mistaken impression that roundabouts cause congestion. But a properly designed roundabout can reduce delays by up to 65 percent over an intersection with traffic signals or stop signs. Sure, an individual driver who has a green light may fly through a signalized intersection much more quickly than through a roundabout. Roughly half the time, however, the light will not be green; and even if it is green there is often a rolling queue of vehicles just starting up from the previous red. Add to this such complications as left-turn arrows, which prevent the majority of drivers from moving, not to mention the ”clearance phase,” that capacity-deadening moment when all all lights must be red, to make sure everyone has cleared the intersection. Drivers do have to slow down as they approach a roundabout, but under typical traffic conditions they rarely have to stop. lights must be red, to make sure everyone has cleared the intersection. Drivers do have to slow down as they approach a roundabout, but under typical traffic conditions they rarely have to stop.

In the 1960s, experiments were made at the Holland Tunnel, one of the main arteries for traffic coming into and leaving New York City. When cars were allowed to enter the tunnel in the usual way, with no restrictions, the two-lane tunnel could handle 1,176 cars per hour, at an optimal speed of 19 miles per hour. But in a trial, the tunnel authorities capped the number of cars that could enter the tunnel every two minutes to 44. If that many cars got in before two minutes were up, a police officer made the next group of cars wait ten seconds at the tunnel entrance. The result? The tunnel now handled 1,320 vehicles per hour. (I will explain why shortly.) On streets with traffic signals, engineers set progressions with a certain speed in mind that will enable the driver to hit a line of constant greens. To drive faster than this only ensures that the driver will be forced to come to a stop at the next red light. Each stop requires deceleration and, more important, acceleration, which costs the driver in time and fuel. A queue of drivers stopped at a light is a gathering of ”start-up lost time,” as engineers call it (in an appropriately forlorn echo of Proust). The first cars in a queue squander an average of two seconds each, two seconds that would not have been lost had the car sailed through at the ”saturation-flow” rate. The first driver at a light that turns from red to green, because he must react to the change, make sure that the intersection is empty, and accelerate from a standstill, generates the most ”lost time.” The light is green, but for a moment the intersection is empty. The second driver creates a bit less lost time, the third driver less still, and so on (a.s.suming everyone is reacting as soon as they can, which is not a given). SUVs, because they are longer (on average, 14 percent longer than cars), and take longer to accelerate, can create up to 20 percent more lost time.

Some of the start-up lost time could be ”found” if drivers approached at a slower, more uniform speed that did not require them to come to a stop. (If they came too too slowly, however, time would also be lost, as green signal time would be wasted on an empty intersection.) Much of the time being lost these days is ”clearance lost time,” the time between signals when the intersection is momentarily empty. This is because traffic engineers are increasingly lengthening the ”all-red phase,” meaning that when one direction gets the red, the competing direction has to wait nearly two seconds before getting a green. They do this because more people cannot seem to stop on red. slowly, however, time would also be lost, as green signal time would be wasted on an empty intersection.) Much of the time being lost these days is ”clearance lost time,” the time between signals when the intersection is momentarily empty. This is because traffic engineers are increasingly lengthening the ”all-red phase,” meaning that when one direction gets the red, the competing direction has to wait nearly two seconds before getting a green. They do this because more people cannot seem to stop on red.

Now picture a highway during stop-and-go traffic. Like those drivers stopped at the light, each time we stop and start in a jam we are generating lost time. Unsure of what the drivers ahead are doing, we move in an unsteady way. We are distracted for a moment and do not accelerate. Or we overreact to brake lights, stopping harder than we need to and losing more time. Drivers talking on cell phones may lose still more time through delayed reactions and slower speeds. The closer the vehicles are packed together, the more they affect one another. Everything becomes more unstable. ”All of the excess ability for the system to take in any sort of disturbance is gone,” says Coifman. He uses the metaphor of five croquet b.a.l.l.s. ”If you put them a foot apart and tap one lightly, nothing happens to the other four. If you put them all up against one another and tap one lightly, the far one then moves out. When you get closer to capacity on the roadway, if there's any one little tweak, it impacts a lot of the cars.”

When the first in a group of closely s.p.a.ced cars slows or stops, a ”shock wave” is triggered that moves backward. The first car slows or stops, and the next one slows or stops a little farther back. This wave, whose speed usually seems to register at about 12 miles per hour, could theoretically go on for as long as there was a string of sufficiently dense traffic. Even a single car on a two-lane highway, by simply changing its speed with little rhyme or reason (as people so often seem to do, in what I like to call ”speed-attention-deficit disorder”), can itself pump these waves back down a stream of following vehicles. Furthermore, even if that car's average speed is fairly high, the fluctuations wreak progressive havoc. This was the secret behind the Holland Tunnel experiment: With cars limited to ”platoons” of forty-four vehicles each, the shock waves that were triggered were confined to each group. The platoons were like croquet b.a.l.l.s s.p.a.ced apart.

Many times we find ourselves stuck in traffic that seems to have no visible cause. Or we make it through a jam and begin to speed up, seeming to make progress, only to quickly drive into another jam. ”Phantom jams,” these have been called, to the annoyance of some. ”Phantom jams are in reality nonexistent,” thunders Michael Schreckenberg, a German physics professor at the University of Duisburg-Essen so noted for his traffic studies that he has acquired the epithet ”jam professor” in the German media. There is always a reason for a jam, he says, even if it is not apparent. What seems to be a local disturbance might just be a wave pumped up from downstream in what is in reality a big, wide moving jam. It is wrong, says Schreckenberg, to simply call the whole thing stop-and-go traffic: ”Stop-and-go is the dynamic within within a jam.” a jam.”

We fall for the phantom-jam illusion because traffic happens in both time and s.p.a.ce. You may be driving into a s.p.a.ce where a jam has been. Or you may not be driving into a jam-instead, the jam might be driving into you. ”In my bucket a.n.a.logy,” says Coifman, ”the driver would be a water molecule. If the water level's rising, then the jam's coming to us.” We are also driving into history-or, perhaps more accurately, we are being driven back into history. By the time we actually arrive where something triggered the shock wave, in all likelihood the event will be only a memory. It may have been an accident, now cleared. ”The queue's going to persist for a while as it's dissipating,” says Coifman. ”It's that water sitting in the bucket. In this case you've enlarged the hole in the bucket, but it does not disappear instantaneously.”

Or the hiccup in heavy traffic that pa.s.ses through you might be the echo of someone who, forward in s.p.a.ce and backward in time, did something as simple as change lanes. The car that changes lanes moves, eating up capacity in the new lane and causing the driver behind to slow; it also frees up capacity in the lane it has left, which triggers a bit of acceleration in that lane. These actions ripple backward in a kind of seesaw effect. This is why, if you pick one car in the neighboring lane as your benchmark, you will often find yourself pa.s.sing that car and being pa.s.sed by that car continuously. This is equilibrium a.s.serting itself, the accordion of traffic flow stretching and compressing, the lingering chain reaction of everyone who thought they could get a better deal.

Since it takes so long for traffic to resume flowing freely once it has plunged past the critical density, it would seem the best way to avoid the ill effects of a jam would be not to drive into it, or let it drive into you, in the first place. This is the thought that occurred one afternoon a few years ago to Bill Beatty, a self-described ”amateur traffic physicist” who works in the physics laboratory at the University of Was.h.i.+ngton. Beatty was on State Highway 202, returning from a state fair. The road, a ”little four-lane,” was thronged with traffic from the fair. The traffic was ”completely periodic,” as he describes it. ”You'd drive real fast and then almost get to sixty and then you'd slow down and come to a stop, for almost two minutes,” he says.

So Beatty decided to try an experiment: He would drive only 35 miles per hour. Rather than let the waves drive into him, he would ”eat the waves,” or subdue the wildly varying oscillations of stop-and-go traffic. Instead of tailgating and constantly braking, he would try to drive at a uniform speed, leaving a large gap between himself and the car ahead. When he looked in his rearview mirror, he saw a revelation in the pattern of headlights: Those behind him looked to be in a regular pattern, while the other lane had cl.u.s.ters of clumped stop-and-go vehicles. He had ”damped” the wave, leveled off the extremes. ”It cuts off the mountains and puts them in the valley,” he says of his technique. ”So instead of getting to drive at sixty miles per hour briefly, you're forced to drive at thirty-five miles per hour. But you don't have to stop, either.”

Without a.n.a.lyzing the total traffic flow of the highway, it would be hard to know for sure what good Beatty's experiment did. People may have just merged in front of him, pus.h.i.+ng him back (if he wanted to keep the same following distance), while those behind him who thought he was going too slow may have jumped into the next lane, causing additional disturbance. But even if Beatty's technique did little more than take a tightly congested traffic jam and stretch it backward, so that a car spent the same amount of time traveling a section of road, it would still save fuel and reduce the risk of rear-end accidents-two added benefits for the same price. Only how do you get everyone to cooperate? How do you prevent people, as so often seems to happen, from simply consuming the s.p.a.ce you have left open? How, in essence, can we simulate ant-trail behavior on the highway?