Programs & Labs
Winter 2002
Institute researcher seeks to increase highway capacity
Dr. Rajamani discussed his work on ACC systems at an Advanced Transportation Technologies seminar in December, 2001
The highways around major metropolitan areas become more crowded every day, as annual increases in traffic demand outpace all efforts to increase highway capacity. Where new roadway construction is feasible, costs in metropolitan areas may be as high as $30 million per mile of new roadway.
If roadway expansion alone cannot stop congestion, many researchers believe the solution may lie in advanced technologies for the passenger vehicle. Rajesh Rajamani, Nelson Assistant Professor in the Department of Mechanical Engineering, is exploring an emerging vehicle-based technology that may increase the carrying capacity of existing urban highways.
How close is too close?
As an example of an in-vehicle technology that may play a part in reducing congestion in the future, Rajamani points to Adaptive Cruise Control (ACC) systems, which use radar sensors to monitor the gap between the ACC-equipped vehicle and the vehicle ahead. Although ACC is marketed and used as a convenient enhancement to standard cruise control, that role may change as ACC becomes as ubiquitous as regular cruise control is today. In theory, ACC has the capacity to "squeeze" more vehicles onto a roadway by enabling them to operate closer together than they could using human control alone.
Rajamani, with Professor Panos Michalopoulos and Assistant Professor David Levinson of Civil Engineering, asked what would happen if the time gaps between vehicles were reduced. Today's ACC systems use a policy known as "constant time-gap" to control vehicle spacing—vehicles maintain a constant spacing of roughly two seconds between themselves and the vehicle ahead, which corresponds to a distance of roughly 60 meters at highway speeds. If this time gap could be reduced to one second or less while maintaining safety, highway capacity would increase substantially. However, the team found significant barriers to implementing a shorter constant time-gap.
In simulations of traffic flow, the flow rate of vehicles through a highway segment increases as density of vehicles on the roadway increases. If vehicles operated at a single, constant speed, this simple linear relationship would point the way to increasing capacity. But vehicles do not behave this way in the real world. In addition to braking and acceleration in response to changing traffic patterns or unanticipated movements by other vehicles, vehicles will change speeds due to the ACC system's continual readjustments as it receives sensor data and recomputes the desired spacing.
Over the length of a hypothetical roadway segment, the "stability" of the traffic flow depends on whether these "errors"—deviations from constant speed—are magnified or diminished as they propagate through the flow of traffic. If a small slowdown by one vehicle leads to greater and greater decelerations by other vehicles as they adjust to it and to each other, the system will rapidly become unstable and may reach a state in which vehicles on the same stretch of roadway are operating at radically different speeds, despite the best efforts of their computerized speed controllers and their human operators. The researchers have shown that the constant time-gap policy is stable on a discrete, circular "highway," but unstable under many other boundary conditions.
Human reaction time is also a factor that must be accounted for. With less space between cars, it becomes more difficult for the driver to react to changing traffic conditions quickly enough to avoid a crash. The solution to this dilemma is to give the ACC system the authority to initiate hard braking and work down to zero speed without input from the driver. But this solution makes it imperative that the ACC system be totally reliable.
Next-generation ACC
In fact, the relationship of flow rate to traffic density under the constant time-gap regime reaches a maximum and then begins to reverse, as further increases in density result in greater and greater decreases in traffic flow volume. Plotting this relationship on a graph reveals a curve that rises to a maximum and then falls off as density increases. However, the team's research has established that it is possible to design new spacing policies that are unconditionally stable over a wide range of operating densities—a result of great interest to the developers of next-generation adaptive cruise control systems for future highways.
By a process of "reverse engineering," it is possible to start with a graph of the desired relationship between traffic density and flow rate and work backwards to derive the formula that describes the curve. This formula may then serve as the basis for a new algorithm governing spacing between vehicles. As ACC systems become more common, this research may help keep highways flowing freely even in heavy traffic.
Ed. note – For more information on this project, see the synopsis of Dr. Rajamani's seminar, part of the Advanced Transportation Technologies Seminar Series.