Traffic Congestion: A Solvable Problem
More creative use of existing highways and rights-of-way can help us build our way out of gridlock.
All over the world, people are choosing to travel by automobile because this flexible mode of travel best meets their needs. But gridlocked expressways threaten to take the mobile out of automobile. Transportation planners predict that freeways will suffer from unbearable gridlock over the next two decades. Their conventional wisdom maintains that we cannot build our way out of this congestion. Yet the best alternatives that they can offer are to spend billions more on public transport that hardly anyone will use and to try to force people into carpools that do not fit the ways they actually live and work.
The good news is that we can make significant improvements in our roads that will expand mobility for motor vehicles. Don’t worry, I’m not proposing the economically and politically infeasible approach of pushing new freeways through dense and expensive urban landscapes. Rather, I maintain that we can make far more creative use of existing freeways and rights of way to increase capacity and ease congestion.
One way is to provide separate lanes for cars and trucks. Because cars are much smaller, cars-only lanes can be double-decks, either above the road surface or in tunnels beneath high-value real estate. Paris and Los Angeles are developing new urban expressways using these concepts. Special-purpose truck lanes would permit larger, heavier trucks than are now legal in most states and would allow trucks to bypass congested all-purpose lanes, facilitating just-in-time deliveries valued by shippers and receivers.
Although less expensive than creating new rights of ways through highly developed areas, reconstructing freeways with some double-decks and new tunnels will be so costly that it will not be possible as long as we rely only on today’s federal and state fuel taxes. But charging tolls for such expensive new capacity is feasible. New electronic technology makes it possible to vary fees with the time of day and level of congestion and to collect tolls automatically without toll booths.
In short, the combination of innovative highway design, separation of traffic types, toll financing, variable pricing, and electronic toll collection will make it possible to offer drivers real alternatives to gridlocked freeways. Conventional wisdom is wrong. We CAN build our way out of congestion.
The United States is traditionally a can-do nation of problem solvers. But in the matter of traffic, we seem to have lapsed into an uncharacteristic fatalism. It is as if conditions on our city highways are a natural disaster that we must simply endure. Traffic congestion is portrayed as inevitable. Plans for our major metro areas show projections for the year 2020, modeled after funded road improvements, in which average speeds on major arteries continue to decline in rush hours that extend throughout much of the working day.
In its latest draft regional transportation plan, the Southern California Association of Governments says that daily commute times in the Los Angeles area will double by 2020 and “unbearable” present conditions on the freeways will become “even worse.” The plan adds that “the future transportation system clearly will be overwhelmed.” By 2020, drivers are expected to spend 70 percent of their time in stop-and-go traffic, as compared to 56 percent today. Similar predictions have been made for metro areas around the country.
One school of thought favors letting congestion worsen, seeing it as the way to break the automobile’s grip on the U.S. consumer and to persuade people to carpool or take public transit. Supporters of increased mass transit see predictions of gloom and doom on the roads as the most powerful argument for convincing legislators to vote substantial funding for new public conveyances. In effect, a pro-congestion lobby has emerged.
But the notion that public transit is the solution to congestion is wishful thinking. During the past half century, some $340 billion of taxpayer money has been poured into capital and operating costs for such transit. Yet transit is used in less than 2 percent of today’s trips. The average car trip is twice as fast, door to door, as the average transit trip. And it costs less. That combination is impossible to beat, particularly because, with the vast array of equipment available for car users today, people can more easily endure congestion and even be comfortable in it.
Public transit does have certain niche markets. It works well-indeed, it is indispensable-for many work trips from suburbs to central business districts in older cities such as New York, Chicago, Washington, D.C., and San Francisco, where the cost or scarcity of parking almost rules out the use of cars for daily commuting. People who aren’t able or can’t afford to drive their own cars are another natural market for transit. But this carless segment of the population keeps declining, and the old transit-oriented central business districts are declining in importance. Jobs are more and more dispersed, creating a cobweb plan of daily commutes in place of the old hub-and-spoke plan of mass transit.
In addition to pushing transit, governments have made major efforts to create higher vehicle occupancy by encouraging carpooling. Recognizing that the objective is to move people, not vehicles, the federal government has turned its urban highway enhancement funds toward high-occupancy vehicle (HOV) lanes. But there is no sign that this focus has stemmed solo driving either. Forming, operating, and holding together a carpool is tough to manage. It also adds to travel time and robs participants of the ability to depart whenever the driver is ready and to drive directly to the destination. Carpooling imparts to the car some transit-like constraints, such as a schedule and a more circuitous route.
Even with its inconveniences, however, carpooling at least attracts a larger share of commuters than public transit. On an average day, 15 million people carpool, compared to fewer than 6 million in all forms of public transit. (Neither figure, of course, compares favorably to the 84 million who drive alone.) But carpooling, like transit, is in decline. Almost 80 percent of carpool trips are now HOV-2 (driver plus one passenger). HOV-3+ (three occupants or more) declined by nearly half in the past decade. And only a minority of carpoolers are linked through an organized trip-matching system. More than half of carpoolers now appear to be members of one family, most of whom would travel together whether government high-occupancy policies existed or not.
In a few cases (Los Angeles, Houston, and the Washington, D.C., area), carpooling policies seem to have produced reasonable use of HOV lanes. But in general the program has been a disappointment; HOV lanes are heavily underused, in many cases carrying fewer people than adjacent unrestricted lanes. Like transit, carpooling seems to work for declining niche markets-drivers with extremely long commutes from fringe-area communities who work at very large institutions with fixed shifts. And it also works for some low-income workers. But carpooling does not benefit the vast majority of commuters. The statistical probability of finding carpool matches (people with similar origins and destinations at similar times) will continue to diminish with the steady dispersion of jobs and more flexible job hours, just as the probability of finding convenient public transit is declining. Moreover, prosperity has reduced the number of the car-less, which has in turn reduced the number of potential users of both transit and carpools.
Acknowledging the futility of depending on transit and carpooling to dissolve road congestion will be the first step toward more realistic urban transportation policies.
The problem of space
Many people assume that we don’t have space for new roads, and many of the easier ways of widening roads have already been applied. Highways designed with wide grass central medians have generally been paved inward. However, there are still opportunities in many U.S. urban highway corridors to widen outward, replacing slopes with retaining walls. A recent study of the feasibility of widening major freeways in the Los Angeles area found that about 118 miles out of 136 miles had space within the existing reservation or required only small land purchases for the necessary widening.
If going outward is politically impossible or too expensive, one alternative is going down. Freeways entirely above ground may go the way of early elevated transit lines: torn down and replaced by subsurface or fully underground roads. This is already happening in Boston; the underground Central Artery is replacing the elevated John Fitzgerald Expressway. In Brooklyn, the Gowanus Expressway, built atop the abandoned Third Avenue BMT elevated rail line, is the object of discussion and controversy over whether it should be renovated as an elevated highway or torn down and replaced with a tunnel. Such decisions must be made not only road by road but section by section, through the messy and raucous but essential processes of local consultation and argument. In Europe, Asia, and Australia, spectacular examples of inner-city tunnel highways are being built where there is strong objection to land acquisition and construction of surface roads. Major advances in tunneling technologies, which have led to significantly lower tunnel-building costs, will make tunnels an increasingly attractive choice in the future (see sidebar).
Providing separate roadways for trucks and light vehicles is an old idea in the United States, but one that has been ignored for the past 50 years because federal regulations forbid them. Some of the very first grade-separated, controlled-access roads (called parkways) were reserved for cars in the 1920s and 1930s. Many were intersected with low-clearance bridges and tunnels, some as low as 11 feet, so that large trucks cannot drive on them. They usually have short, sharp interchange ramps and narrow lanes, typically 10 feet, compared with the 12 feet that has been standard for mixed traffic lanes on U.S. expressways. The parkways originally had no breakdown shoulders or median barriers. The idea of parkways was to provide city people with links to beaches, parks, and other healthful recreation. They were designed with a special naturalistic quality, and most were not intended for commercial traffic.
Mixed-vehicle highways became standard after the Korean War. Communism was seen as a pressing military threat, and the federal government was keen to accommodate the Pentagon’s desire for new roads able to carry heavy military equipment. The full name of the Eisenhower-initiated 42,000-mile system of interstates was the National System of Interstate and Defense Highways. They had to be built with lane widths of 12 feet; overhead clearances of at least 14 feet; breakdown shoulders of 10 feet; gradients generally a maximum of 3 percent; and bridge and pavement design, sight distances, and curvatures suited to heavy trucks.
The beginnings of a new kind of truck/light vehicle separation are evident in bans on trucks in the inner lanes of roads with five lanes or more. In Los Angeles, a major project for the past six years has been squeezing extra lanes out of the existing pavement by restripping the old standard 12-foot freeway lanes to 11 feet. Studies have shown that speed and safety are unaffected by this lane narrowing. In a standard eight-lane Los Angeles freeway, this change alone contributes eight feet of extra pavement. The rest of the space needed for an extra pair of lanes is usually available in the median or on shoulders. In this “L.A. squeeze,” trucks are usually prohibited in inside lanes. But there is pressure to make lanes wider for trucks. The federal width limit on trucks was increased recently from eight to eight and one-half feet, and newer trucks are able to travel at higher speeds. A number of proposals for new highways provide for truck lanes of 13 feet.
In the United States, as elsewhere, large trucks are a hot-button political issue, with truck lobbies constantly citing the economic advantages of larger, heavier trucks and motorists’ organizations and local activists arguing that larger trucks are dangerous. According to James Ball, a former federal highway official and now a truck toll-road developer, both sides are correct. “On the major truck routes, ” he says, “we need to build separate truck roads where we can cater to the special needs of trucks and provide the most economical mix of roadway dimensions and load-carrying capacity for cargo movement. Yet we have to get the trucks out of lanes in which cars travel. This is the only way to make the major highways safe for small vehicles such as cars.”
Although gut feelings about the dangers of big trucks prevail, U.S. trucks are actually small and light by international standards, so much so that they prevent us from obtaining the maximum economic benefits from our highway system. For example, big Canadian “tridems” (triple-axle trailers forming a 44-metric-ton, 6-axle, 22-wheel rig, compared to the standard tandem-axle trailer of a 36.3-metric-ton, 5-axle, 18-wheel U.S. rig) help Canadian producers undercut U.S. producers of agricultural products and lumber. According to a recent U.S. Department of Transportation study, U.S. freight costs are about $28 billion annually, 12 percent more than they could be if we ran big rigs on a nationwide network of freeways, turnpikes, and special truck lanes, with staging points for making transitions to familiar single tractor-trailer arrangements on local streets.
Designs for right-sized roads
Two West Coast engineers see the segregation of cars and trucks as a possible solution to the problem of building increased capacity in constrained expressway rights of way. Gary Alstot, a transport consultant in Laguna Beach, like many southern Californians, watched in awe as federal money built about three miles of double-deck down the middle of I-110 south of downtown Los Angeles as part of its HOV program. Built as bridgework on giant T posts, the double-deck section of four lanes is generally about 65 feet high because it has to go over the top of interchanges and bridges along the way. That puts the road up three levels. Not only is this height enormously expensive, it is also intrusive. In most places a highway authority couldn’t get away with it. (This I-110 double-deck is in central south Los Angeles, a largely commercial and industrial area that activists don’t much care about.)
Given that more than 80 percent of the traffic consists of light vehicles, it is wasteful to build the entire cross-section of wide urban highways to heavy truck standards. I-110 could have been double-decked under its overpasses instead of over them if the double-deck section had been restricted to cars and the overpasses raised by perhaps three feet or so. Alstot thinks that a 10-foot lane width and seven-foot overhead clearance would be adequate for passenger cars. He points out that the average height of 1992 cars was 46 inches, and two-thirds are less than six feet wide, compared to U.S. truck requirements of 14 feet high and eight and one-half feet wide.
U.S. engineers are following with interest the Cofiroute tunnels for the missing link of the A86 Paris ring road in which similar tubes are planned west of Versailles, one for mixed traffic of two lanes and the other a cars-only tunnel with two decks of three lanes each. The cars-only tunnel, according to cross-sections provided by the French, will have 8.5-foot ceilings and lanes just under 10 feet wide, a little higher and narrower than Alstot’s proposed cross-section.
Independently, Joel K. Marcuson of the Seattle office of Sverdrup Civil Inc., came up with similar ideas while doing research for the federal Automated Highway System project. Heavy trucks and cars have such different acceleration, braking, and other characteristics that it is widely accepted that they will have to be separately handled on future electronic guideways. Who would want to be electronically stuck in a car only a few feet away from a tractor-trailer?
Marcuson suggests that plans for rebuilding U.S. inner-city expressways should include careful study of how to make more efficient use of the available right of way by segregating cars and large vehicles. This would improve conditions now and also help prepare for highway automation. (Most U.S. experiments in hands-off/ feet-off driving are being conducted in barriered, reversible-flow HOV lanes during the off-peak period when they are closed.) “A separate but parallel facility (for high-profile vehicles) would allow for the different operating characteristics of small and large vehicles, allowing different speed limits and different design criteria, both structural and geometric,” he has written.
Marcuson has drawn up a set of highway cross-sections showing how high and low vehicles (trucks and buses versus cars, pickups, and small vans) might usefully be segregated to provide more lanes and better safety in typical wide rights of way. He shows how, by double-decking the light vehicle roadway in the middle, 14 lanes could be achieved in place of the existing eight lanes on a standard Los Angeles right of way.
Other engineers point out that in some places it will make sense to build completely separate truck and car roadways. A truckway might well have a standard two-lane cross-section with occasional passing sections and could then fit into an abandoned railway reservation or alongside major electric transmission lines, or be sunk in a trench or even a tunnel. And a four-lane divided expressway built with 10-foot lanes for light vehicles only, as compared to mixed-traffic 12-foot lanes, would be considerably more compact and less noisy and intrusive to neighbors, and therefore might arouse less local opposition.
The first application of these ideas may come in the Los Angeles area. The Southern California Association of Governments has proposed a network of truck toll lanes through the Los Angeles basin. Five preliminary studies are under way.
The market’s role
Simply building our way out of congestion would be wasteful and far too expensive. What we need is a market mechanism to determine how much motorists value additional road capacity. As long as our highways are paid for mainly by fuel taxes, registration fees, and other general revenues, it will be impossible to make rational decisions about what road space is needed, and we will have no mechanism to manage road space rationally. We could create that market by instituting flexible tolls that would vary with the time of day or, preferably, the level of congestion.
Roads are especially in need of pricing because of the dynamics of traffic flow. Traffic engineers tell us that beyond a certain number of car-equivalent vehicles per traffic lane per hour on a standard expressway, the entry of additional vehicles causes the capacity of the road to decline sharply. Viewed from above, traffic on a highway nearing full capacity starts to exhibit waves of motion similar to a caterpillar’s locomotion. The wave phenomenon develops because, although drivers are comfortable enough being just a few feet from the car ahead when traffic is stopped, they want progressively more space ahead the faster they are going. Somewhere around 2,200 to 2,500 vehicles per lane per hour (the precise number depends on the temperament and skills of drivers, the weather, and the visibility) motorists drive more and more slowly in an attempt to preserve a comfort space ahead. Sometimes many vehicles are forced to stop completely and wait. Other times the flow reaches a low equilibrium speed and all the vehicles crawl for awhile. In either case, the explanation is that just a few extra vehicles have overloaded the road to the point where, instead of accommodating the increased demand, the road is actually carrying fewer vehicles than it is capable of.
Freeway traffic flows are a classic case of an economic externality, where a few extra motorists inadvertently impose on many others much higher costs in the aggregate than they themselves incur individually. Only a managed, flexible pricing mechanism can internalize these costs and allow access to the facility by those who value the trip more than the toll. Such a dynamic market for scarce city highway space will also have other huge benefits. It will generate incentives for highway managers to find efficient ways of enhancing throughput up to the point at which motorists are no longer willing to pay. The market will also signal whether adding capacity (with a widened or parallel roadway, for example) makes sense.
This is well-established economic theory, but it has been technically difficult to implement until recently. Miniaturization and mass production of short-range radio components (byproducts of devices built for the U.S. Air Force for telling friends from foes and then applied in cordless and cellular phones, garage door openers, and the like), together with the development of high-capacity fiber optics and cheap computing power, make it feasible to levy trip charges electronically just by equipping cars with transponders that cost between $15 and $35 and are the size of a cigarette pack. Alternatively, video and pattern-recognition algorithms allow license-plate numbers to be read by a camera on an overhead gantry, and a toll bill can then be sent in the mail. Changing toll rates can be posted on variable-message signs on approaches to the toll lane, or they can be displayed in the vehicle or accessed online from home or office. This technology has been signaling changes in rates (which depend on time of day) in toll lanes of SR-91 Express, the investor-built road in Orange County, California, since the end of 1995 and on highway 407 Express Toll Route (407 ETR) in Toronto since September 1997. The first full-fledged implementation of dynamic tolling, in which toll rates vary with traffic conditions, is being tested in a three-year demonstration project in the high-occupancy/toll (HOT) lanes of I-15 in San Diego.
Road pricing is being introduced into the United States piecemeal. Underused HOV lanes are a good starting place; flexible tolls will allow free-flowing traffic to be maintained by regulating entry on the basis of willingness to pay for the privilege. Right now, a few lanes that have been too successful as HOV-2 may need to become HOV-3 lanes in order to prevent the overloading that threatens their rationale of providing faster travel than the unrestricted lanes. But tightening eligibility from HOV-2 to HOV-3 normally means losing about two-thirds of their users, which would make this formerly heavily traveled lane seriously empty. Without a price, traffic in this lane is either a flood or a drought. By allowing HOV-2 vehicles into HOV-3 lanes on payment of a variable toll, highway managers can avoid throwing all HOV-2s into the unrestricted lanes, worsening congestion there. Pricing gives the road administrator a sensitive tool to manage its use, compared with the crude choice between changing an HOV-2 lane into an HOV-3.
Existing toll facilities such as turnpikes and toll bridges and tunnels in New York City, Chicago, Philadelphia, and San Francisco can also improve traffic flows and their revenues by time- or traffic-variable toll rates. Toll motorways outside Paris have operated differential toll rates on Sundays with success for several years to manage holiday traffic. In Orange County, SR-91 Express was the first to implement tolls on simple on-off express lanes that are part of an existing freeway. The lanes are a popular and political success, having gained three-to-one positive ratings in local opinion surveys since their introduction. Highway 407-ETR in Toronto is the first complete multi-interchange urban motorway system to incorporate remotely collected and variable tolls into its planning from the start. An average of 210,000 motorists per day are currently using it, and its high-tech toll collection system and time-of-day variable tolls are completely accepted and uncontroversial. The road is such an economic and political success that it is being sold by the provincial government to investors.
The best chances for success in introducing road pricing are in situations where congestion is worst; the toll is linked to new capacity (extra lanes or a new road); and some “free” alternatives are retained.
To go faster, pay as you go
In sum, there are several reasonable ways for the United States to build its way out of its unbearable traffic mess, notably separate lanes for cars and trucks, double-deck car lanes, and special-purpose truck lanes and roads. But they are too expensive to build with present highway financing measures. Discovering the market value of a particular trip on a particular road and charging individual drivers accordingly are essential if we are to build our way out of perpetual congestion.
We meter and charge for water and electricity. Utilities managers monitor their use all the time and make capacity adjustments constantly, without fuss. We do not fund an airline monopoly with taxes and offer everyone free plane rides. Yet that is precisely the craziness by which we manage urban highways. It is no wonder they are a mess.
The challenge is to gradually bring our roads into the normal business world, the world where users pay and service providers manage their facilities and fund themselves by satisfying their customers. This idea is gaining increasing acceptance among those who build the roads. A striking example is Parsons Brinckerhoff, the nation’s largest highway engineering firm, which has proposed toll express lanes with variable pricing as the best way to enhance the major highway in Sonoma County, California. Its report observed, “If a roadway facility provides enough economic benefits to justify its development, there usually is an efficient pricing structure that will capture these economic benefits and permit the facility to be largely self-financed.”
The U.S. love affair with the car is not an irrational passion. For most of us, the car is a time-saving machine that makes the humdrum tasks of daily life quicker, easier, and more convenient to accomplish. It allows us to roam widely and to greatly expand our relationships.
We must come to terms with the automobile. The failed effort to pry drivers from their cars has produced vast waste. More important, it has prevented us from adopting measures to fit the motor vehicle into the environment, to make it serve human purposes with fewer unwanted side effects. The problems on the roads must be tackled on the roads.
Advances in tunneling
Tunnels are expensive, but steady advances in tunneling technology have greatly reduced their cost. Many of the new techniques are lumped under the term New Austrian Tunneling Method (NATM). Not very new anymore, NATM is widely credited with producing better bores for the buck.
Prior to NATM, tunnels tended to be of uniform construction throughout their length, and the entire structure was usually designed for the needs of the most difficult section. In other words, these tunnels were overbuilt. NATM emphasizes different techniques for different geologic areas, making maximum use of natural support so as not to waste manmade inverts (horseshoe-shaped frame sections) or other structural supports. NATM also emphasizes moving quickly after excavation to prevent loss of natural support by driving huge bolts into the rock to anchor it in place. Then shortcrete, a stiff, quick-setting concrete mix, is sprayed under pressure onto walls covered with steel mesh. The tunnelers install instruments that will yield reliable measurements of pressures and movements in the natural walls, which permit them to make informed judgments about what further support is necessary.
E.T. Brown, an engineering professor at Imperial College, London, says NATM manages to “mobilize the inherent strength of the ground” through which the tunnel passes, even though it employs relatively cheap rock bolting and Shotcrete. However, he also points out that in some situations what he wryly calls the OETM (Olde English Tunneling Method) of grouted precast rings erected behind a tunneling shield is superior.
There have also been major advances in tunnel-boring machines (TBMs), which were invented by the British engineer Marc Brunel in the 19th century. In the past 20 years, TBMs have become much tougher, more reliable, and capable of boring ever larger diameters. The availability of large TBMs is especially important for highways because they are the largest tunnels in cross section. Until the 1960s, the largest TBMs were about 26 feet in diameter, hence most tunnels had space for only two lanes of traffic. Thanks mainly to Japanese innovation, TBMs 34 feet across are now common, and some are even 46 feet, such as the equipment used on the Trans-Tokyo Bay tunnel, which has room for three lanes of full-size truck traffic.
Once upon a time, the principal challenge in tunneling was breaking up the hard rock and getting the debris out. Now with road headers (relatively simple machines that deploy a large grinder on an arm and a conveyor belt) and with simple mechanical excavators and precise explosives that move the toughest rock, expensive TBMs and large shields are sometimes not even necessary. The greatest challenges are handling water and minimizing cost by choosing right-sized support methods and walling.
Tunnel “jacking” (mechanical forcing) is used increasingly; mechanical forces move (jack) enormous prefabricated tunnel sections horizontally from a pit into the ground beside the pit. Excavators working from inside the safety of the jacked section remove material ahead. This may get to be called BTM, for Boston Tunneling Method, because the Central Artery project is carrying out the world’s largest-ever tunnel jackings.
Another improvement is steel fiber (better described as steel shard) in place of conventional reinforcing rod cages to produce more economical rust-resistant prefabricated concrete sections for tunnels. Sealing and grouting continues to be improved too. Surveying lasers are helping to make sure that two tunnel ends driven toward one another actually meet and match precisely.
Another major advance in tunneling is the invention of the jet fan for ventilation. So named because it looks like the jet engine of an aircraft, a jet fan is hung from the ceiling at intervals along the tunnel and moves the dirty air along it. The air can be vented out one end, taken to vertical exhaust risers, or diverted into treatment channels and replaced, clean, in the tunnel. On all but the very longest tunnels, jet fans allow the tunnel builders to dispense with the plenum, the separate longitudinal ducting above a false ceiling that has traditionally been used to ventilate tunnels. That can reduce the quantity of excavation and construction by 20 percent and thus cut capital costs by comparable amounts.