MIT engineers have developed a state-of-the-art traffic simulator that actually mimics different drivers--aggressive, careless, timid or fast--and how they affect traffic flow.
The simulator enables the researchers to test various traffic management systems for the Central Artery/Harbor Tunnel (CA/T), the big construction project that is relocating the elevated portion of Boston's I-93 below ground and extending the Massachusetts Turnpike (I-90) to Logan Airport via a new tunnel.
The MIT research will help the CA/T system managers maintain a smooth flow of traffic on the new stretch of freeway by allowing them to test traffic management systems under simulated conditions that closely resemble the real thing. As a result, when the actual CA/T opens to the public it is likely that the best traffic management system will already be known and operational.
The traffic simulator, which runs on a workstation, is called the MIcroscopic Traffic SIMulator (MITSIM). Microscopic refers to the treatment of traffic as a set of individual vehicles, or particles, allowing each vehicle to move according to its own characteristics. The more common macroscopic simulator treats traffic like a fluid, assigning one set of characteristics to the entire stream of cars. MITSIM is more lifelike because it allows for differences in vehicles' movements as dictated by drivers' personalities.
"The simulator is designed to be very flexible, making it possible to incorporate various driver behavior models and a wide spectrum of traffic management system designs," said Rabi G. Mishalani, a research associate in the Intelligent Transportation Systems Program of the Center for Transportation Studies. It has more variable components than most other simulators and a graphical user interface (GUI) that allows the operator to watch the cars move through the simulated freeway on the computer screen.
When a researcher starts the simulation and activates the GUI, she sees a bright, multicolored ribbon on the computer screen. When she zooms in closer she sees a section of the new freeway, with layers of tunnels and roads, on- and off-ramps and little moving colored rectangles. Each rectangle represents a vehicle with its assigned peculiarities. They change lanes, exit hurriedly and sometimes even have accidents as they test the potential of a proposed traffic management system.
A good traffic management system controls lane use and traffic volume, determines toll collection placement and methods, and sets speed limits, among other things. It should also include a set of surveillance devices for collecting information about the vehicles, like speed and type, and be comprehensive enough to take into consideration changing weather and traffic conditions. Decisions about each of these components must be made in tandem, because each affects the other. MITSIM allows the system designers to fiddle with the components until they come up with the best combination.
For each traffic management system simulation on MITSIM, data about the number, types and destinations of vehicles are given to the computer program. Highway characteristics, like number of lanes and road surface, are also provided. However, vehicle characteristics, such as desired speed, are determined by mathematical models and assigned by the simulator itself.
"At the fundamental level, vehicle behavior is in the form of a set of mathematical relationships, each one invoked under certain conditions. The conditions under which each should be used are in the form of program instructions," Dr. Mishalani said.
As each vehicle enters the simulated Central Artery, it grabs a packet of vehicle characteristics that determines how it will act in certain circumstances. Not only does each vehicle have a size, type, occupancy level and destination, it also has driver characteristics. These include desired speed, propensity to yield to other vehicles, lane-changing behavior and route decisions. There's even a driver impatience factor that makes each driver's choices more realistic.
If MITSIM, with its little colored rectangles serving as vehicle substitutes, works well in its evaluation of traffic management systems, these MIT researchers will have helped to prepare the CA/T for our eventual use.
This work is sponsored by the Massachusetts Highway Department through Bechtel/Parsons Brinckerhoff. Professor Moshe Ben-Akiva of the Department of Civil and Environmental Engineering is the principal investigator.
A version of this article appeared in MIT Tech Talk on December 13, 1995.