The Physics of Traffic: Empirical Freeway Pattern Features, Engineering Applications, and Theory , Boris S. Kerner Springer-Verlag, New York, 2004. $139.00 (682 pp.). ISBN 3-540-20716-3
I commend Boris Kerner on his pioneering research on a new traffic theory. I have been working on transportation and traffic theories and modeling for 27 years and have been with the Federal Highway Administration’s office of operations R&D since 1987. The Physics of Traffic: Empirical Freeway Pattern Features, Engineering Applications, and Theory is the first book I have read that offers detailed discussions about traffic congestion on freeways. The author has spearheaded the development of the new traffic theory and has paved a way for advancing existing traffic theory. I enjoyed reading the monograph by Kerner, a theoretical physicist at DaimlerChrysler in Germany, and found it interesting and helpful. I believe the book’s contribution to the field, in both theory and application, should be significant.
The three-phase traffic theory provides researchers with a new perspective for advancing existing traffic-flow theory for freeways. However, the three-phase traffic theory still lacks the big, or complete, picture of dynamic and complex traffic phenomena caused by the driver’s instinctive response to roadways, adjacent vehicles, traffic control, and ambient conditions like visibility. Researchers have been studying separately each of the three phases of traffic theory—free flow, synchronized flow, and wide-moving jam—for years. But Kerner integrates these phases into one three-phase traffic theory.
Discussions in the book are qualitative, mainly based on limited data collected.
With more intelligent-transportation-system developments and potential vehicle-infrastructure-integration (VII) deployments in the future, richer micro-traffic data (individual vehicular data at small time intervals) will help researchers and engineers verify, understand, and develop improved traffic theories. Kerner’s traffic theory uses empirical spatiotemporal traffic patterns to provide researchers with an insight into the dynamic process of traffic congestion. The new theory can explain and predict many empirical spatiotemporal traffic patterns that cannot be explained by existing traffic theory. But although the new theory is able to clarify and reproduce characteristics of spatiotemporal features of some major congestion patterns on freeways, many other issues associated with the new qualitative theory remain unresolved and deserve more investigation. For example, researchers need to identify other possible congestion patterns from other theories and empirical data, but how to do that is not covered in the book; gain a better understanding of the patterns, including their formation and evolution; and link the new theory with emerging technologies for potential applications.
As the author states in his concluding chapter, there are also many “black areas” in empirical features of freeway traffic that need to be understood in the future. One challenge will be how to determine critical and threshold traffic variables, find the values of those variables, and predict how traffic flows will evolve from one state to another as described in the new theory (for instance, the Z-shaped and double Z-shaped speed–density relationships as shown in chapter 6). Furthermore, values of critical and threshold traffic variables can be site specific and time dependent and may vary with different traffic management strategies. Quantifying these parameters for potential applications will be challenging. Another difficult issue for researchers will be how to address traffic characteristics found in the new traffic theory and relate them to the mesoscopic and macroscopic traffic theories and models so that consistent results can be obtained from different analysis approaches for different applications.
The new theory applies only to freeway traffic. Researchers need to revisit existing traffic-flow theories for urban streets with traffic signals and explore the possibilities of advancing current theory and analysis methods. I agree with Kerner on the qualitative behavioral assumptions made in the three-phase traffic theory as discussed in chapter 8. But to quantify the theory or to model traffic with these assumptions will be very challenging.
If researchers can reproduce key spatiotemporal congestion patterns based on the new theory, they should be able to control or prevent certain types of congestion from happening or growing, dissolve them at their early stages, or limit them locally. There are potential applications of the new theory to real-time traffic control and management. For example, in addition to the ramp-metering and automatic cruise control discussed in the book, VII can be a potential application of the new traffic theory. Each VII vehicle will be able to communicate with the infrastructure and other VII vehicles to exchange real-time traffic information. Guided by VII and the new traffic theory, the traffic-management center or infrastructure should be able to predict impending congestion and advise drivers to regulate VII vehicles to prevent or mitigate congestion. The reproducible spatiotemporal features will also help research engineers develop new applicable concepts for managing traffic, enhancing vehicle safety, and improving vehicle navigation. I therefore believe that the new traffic theory will contribute significantly to the application of VII.
In addition to VII, with the aid of car-to-car communications through dedicated short-range communication technology, vehicles can collect traffic data from surrounding vehicles and predict the optimal behavior—especially acceleration and deceleration rates and routing—based on the new traffic theory, and the vehicles can act accordingly. The new car-to-car communication technology, combined with the new traffic theory, will improve safety and traffic operations significantly without the help of any external infrastructure.
