August 2000
The Study of Transportation is Paved with Science
Human curiosity and the desire to explain how the world around us behaves drive a fertile application area of operations research
By Randolph W. Hall
In operations research, it is not unusual to see the word "science" affixed to a discipline, as in management science, manufacturing science or organizational science. Cynically, one might view this as clever marketing, for science is often equated with goodness, quality or purity. And try as we may to make OR scientific, it certainly is not all that the profession is about. A good practitioner of "management science" needs many skills and talents, most of which have little to do with science.
Cynicism aside, a scientific thread does seem to underlie operations research. More than a century ago, Stanley Jevons wrote in "The Principles of Science":
"The whole value of science consists in the power which it confers upon us of applying to one object the knowledge acquired from like objects." [1]
By aspiring to do just this — to understand, design and operate systems in a manner that gains knowledge from like systems — we behave scientifically.
The "Thought-Chain of Science"
Jevons' insights provided motivation for "The Handbook of Transportation Science," recently published by Kluwer. [2] The premise for our book is that transportation can be defined as a scientific discipline that transcends transportation technology and methods. Whether by car, truck, airplane — or perhaps a mode of transit not yet conceived — transportation obeys fundamental properties. The science of transportation defines these properties and demonstrates how knowledge of one mode of transportation can be used to explain the behavior of another.
Like any of the natural sciences, transportation science as a discipline arose out of human curiosity and the desire for explanations for how the world around us behaves. In the words of famed physicist Max Planck,
"The beginning of every act of knowing, and therefore the starting point of every science, must be in our own personal experience ... They form the first and most real hook on which we fasten the thought-chain of science." [3]
And so is the case for transportation science. When we look back to the earliest publications on the subject from the 1950s and early 1960s, we see first a desire to understand the dynamics of roadway traffic. Then and now, there is hardly a person in the profession who does not view a trip on the highway as a scientific experiment, seeking to understand why traffic flows as it does, how bottlenecks appear and disappear, and what causes the myriad of driving behaviors.
Transportation Phenomena
Transportation scientists are motivated by the desire to explain spatial interactions that result in movement of people or objects from place to place. Transportation science's heritage includes research in the fields of geography, economics and location theory. Its methodologies draw from operations research, probability and control theory. It is fundamentally a quantitative discipline, relying on mathematical models and optimization algorithms to explain the phenomena of transportation.
Transportation science also draws from the natural sciences, for transportation does not just appear in human-built systems. Transportation naturally occurs in blood circulation, bird migration, ant navigation, rivers and currents, atmospheric flows, refraction of light, orbits and animal territories. Long before humans began inventing technologies to facilitate transportation, the world was a dynamic place with objects and organisms in constant motion, not just obeying the laws of physics, but also obeying principles of intelligent transportation design. Many early transportation researchers were, in fact, trained in natural sciences, and cleverly combined knowledge of natural phenomena, such as thermodynamics and fluid mechanics, with their observations on traffic flow.
Transportation science recognizes that all modes of transportation have the same essential elements — vehicles, guideways and terminals — operating under some control policy. Vehicles comprise mobile resources that accompany persons or shipments (P/S, discrete or continuous) as they travel. They provide the motive power to propel P/Ss on their trips, and the carrying space to ensure a safe and comfortable journey. Guideways are stationary resources that define feasible paths of travel and provide the physical infrastructure to support vehicles and P/Ss. They add safety by restricting movements to defined paths, and they provide an efficient surface for movement.
Terminals are stationary resources that reside at discrete locations. They offer the capability to sort vehicles, persons and objects among incoming and outgoing transportation routes. Lastly, control represents the rules, regulations and algorithms that determine movements and trajectories within transportation systems.
Evolution of Transportation
Many years ago, transportation occurred by human, animal and natural (e.g., wind, currents, gravity) power, in simple vehicles (or none at all), on guideways that required little construction. Terminals, if they could be called that, were market towns, caravansaries or trading posts, and control was executed through the minds of individual travelers. By contrast, today most movement depends on propulsion by motors or engines, built guideways and terminals, and, to some degree, computer control. Supporting communication technology is also undergoing rapid change, through Internet purchasing, wireless data communication and mobile computing. So in many respects, one might say that transportation of the late 21st century has little in common with its ancestors.
Nevertheless, similarities abound. For any given mode of transportation, vehicles, guideways, terminals and control are configured to perform several basic functions. All modes provide the capability to propel, brake and steer. Most (even animal and human) provide mechanisms to store energy for propulsion, to sort persons and objects at terminals, to couple shipments together into efficient loads, and to contain these shipments as they travel from place to place. How a mode of transportation accomplishes these functions may be unique, but the basic tasks are the same. [4]
Branches of Transportation Research
Transportation science in part describes how humans and systems behave when making transportation decisions, and in part prescribes how decisions ought to be made when optimizing a transportation objective. On a day-to-day basis, individuals are presented with a plethora of transportation choices, some of which are determined by ingrained habits and circumstances; others of which result from deliberation.
At the most routine level, driving behavior is reflected in a continuous stream of decisions defining speed and direction of travel. The route followed, time of travel and, to some degree, the choice of destination and mode are all daily decisions, constituting short-term traveler behavior. These decisions are imbedded within the broader context of how we plan and organize our activities, constituting long-term behavior. Where we reside and where we work, and how human activity is organized in built environments (cities, towns, residential developments, business districts, etc.) are examples of human decisions with long-term consequences. Collectively, transportation behavior constitutes one of the main branches of research in transportation science.
Another branch of transportation science focuses on flows and movement along guideways. The essential characteristic is that interactions occur among entities, both along the path and at points where paths intersect, split or combine. As flow rates increase, speeds tend to decline, density tends to increase and queueing may occur. These phenomena have been studied in great depth in the traffic flow literature, which is foremost descriptive in nature.
Control policies for flow networks, on the other hand, are prescriptive. They are used to optimize movement along guideways. Policies can include localized controls, regulating trajectories of individual vehicles; segment based controls, regulating groups of vehicles passing through intersections or segments; or global controls affecting entry or exit to/from the network or network routing.
Routing — optimizing the path(s) followed by entities as they move from place to place — is also an area of prescriptive transportation science. The three basic tasks of routing are assignment (determining which resources perform which pieces of work), sequencing (the order in which work is completed) and navigation (the path followed from one assignment to the next). Routing methods are needed not just for vehicles but for all types of mobile resources, including containers, trailers and crews that operate the vehicles. Managing the flows of resources, while satisfying constraints on work rules, is one challenge in vehicle routing.
One more area of prescriptive transportation science is design, including network design and location. There are natural patterns to how a network should be constructed. A tree-like structure is found in most naturally occurring transportation networks, such as rivers, blood circulation and plants, as well as many distribution and supply networks. But human-built transportation networks tend toward a denser structure, offering redundant paths, while nevertheless following familiar patterns such as grid, ring/radial or hub-and-spoke. Design of these networks to facilitate efficient movement forms another body of research.
Application or Theory?
Perhaps the simplest — and most confusing — way to classify research is to place it in one of two categories: "applied" or "theoretical." In OR, the latter inevitably seems to begin with a set of mathematically stated assumptions and leads to proof of theorems that are derived from these statements. Applied research, by contrast, begins with a problem statement (presumably representative of a real organization), and proceeds to a solution through the application of known algorithms or other mathematical techniques.
Examples of "theoretical" and "applied" research exist in transportation science. Clear evidence of the applicability of operations research and management science to transportation can be seen in past issues of OR/MS Today, and plenty of transportation related theorems can be found in other INFORMS publications. Yet following the simple definitions, most transportation research could only be classified as "neither." While one might say transportation is by definition an application (it is certainly not an abstraction), a grounding in the real world should not preclude theory. Physics certainly is not an "applied science," even though it is a real-world application of mathematics. It is just as natural to be a theoretical transportation researcher as a theoretical physicist.
One strength of transportation science is the relatively high status given to empirically based research and to theories induced from data on real-world phenomena (e.g., theory without provable theorems). In this way, transportation is different from some OR disciplines, perhaps because "the problem" is not always the centerpiece of the research. Transportation phenomena can be studied without having the goal of optimizing some objective function, and without the intention of serving the business needs of a private company. Another likely reason is that data are more readily available for transportation than for other phenomena studied in OR. Also, transportation simply invites observation. Nearly everyone experiences transportation, and thus has the ability to form empirically founded theories for its behavior. It stimulates, in Max Planck's words, the "thought-chain of science" through personal experience.
On the other hand, transportation does not lend itself to as high a degree of precision as other parts of OR. Theories are tested, but they are less likely to be "proven," and they seldom predict with total accuracy. But proof and total accuracy are not demands of science; they are demands of mathematics. As philosopher Karl Popper stated:
"Science never pursues the illusory aim of making its answers final, or even probable. Its advance is, rather, towards the infinite yet attainable aim of ever discovering new, deeper and more general problems, and of subjecting its ever tentative answers to ever renewed and ever more rigorous tests." [5]
Transportation offers an example of how OR can be used to build a lasting body of scientific knowledge centered on real-world phenomena. It should be no surprise that transportation is also one of the most fertile application areas of operations research. It is our fundamental understanding of transportation, through scientific research, that has allowed us to make transportation better. This is the promise for operations research.
References
- Jevons, W.S., 1958, "The Principles of Science," Dover Publishing, New York, p.1.
- Hall, R.W. (editor), 1999, "Handbook of Transportation Science," Kluwer Academic Publishers, Norwell, Mass.
- Planck, M., 1932, "Where is Science Going," W.W. Norton and Company, New York, p. 66.
- Hall, R.W., 1995, "The architecture of transportation systems," Transportation Research, C, 3, pp. 129-142.
- Popper, K.R., 1959, "The Logic of Scientific Discovery," Basic Books, New York, p. 281.
Randolph Hall is a professor of Industrial and Systems Engineering and director of the METRANS Center at University of Southern California. He is the editor of "Handbook of Transportation Science" and author of "Queueing Methods for Services and Manufacturing."
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