Download KSEA Letters 40-3, Apr. 2012 [pdf] - Korean-American ...

TECHNICAL ARTICLES
COOPERATIVE CONTROL AT TRAFFIC LIGHT HARNESSED BY WIRELESS CARS
[Byungkyu “Brian” Park, KSEA Publication Director I]
Associate Professor and Graduate Program Director, Civil and Environmental Engineering
University of Virginia
INTRODUCTION
Figure 1. Two Schools of Fish Moving
through “Fish Intersection”
Everyone hates waiting at traffic lights. As more cars travel through the urban streets, it
is inevitable that cars have to stop at the intersection – this happens whenever demand
exceeds supply as we understand from the Economics 101 class. This is common at the
intersections in metropolitan areas where an intersection has to serve cars from multiple
approaches. Does it happen to fish? Not really! As shown in Figure 1, anyone enjoys scuba
diving would have seen how two schools of fish moves through “fish intersection” without
collision (IEEE, 2004). One might wonder why cars cannot cooperatively move just like the
fish! Obviously, it is not likely for cars unless human drivers completely give up their egos
and cooperative control becomes possible. To ensure cooperative control, cars should be
able to effectively communicate and an algorithm should be devised. A recent US Department
of Transportation’s initiative on Connected Vehicle technology presents a possibility
of such cooperative control based on wireless communications among vehicles and infrastructure
(US Department of Transportation).
METHODS
A driver may travel through an intersection without stopping when the intersection is controlled by YIELD sign and there is sufficient
gap available. This happens when there are few cars. As more cars try to cross the intersection, drivers have to stop at the intersection
and have to look for safe gaps. What if wireless cars that are capable of communicating each other within a certain distance
are abundant waiting for cooperative control? Then, one can predict car’s trajectory to the intersection area where collision might
occur. As shown in Figure 2, a collision between two conflicting trajectories (i.e., cars movements) at the intersection is about to
happen (see the two trajectories cross at lw area, representing intersection area), but the collision could be avoided with trajectory
adjustments. An easy option would be that one car goes faster and the other car goes slower than their current speeds. The trajectory
adjustments mean that cars will cooperatively adjust their acceleration rates to avoid collision. This approach can be formulated as a
nonlinear constrained optimization (Lee & Park, 2012). The objective function minimizes overlaps among conflicting cars within the
intersection collision area, while constraints include minimum and maximum acceleration rates, minimum and maximum speeds,
and minimum headway between cars.
Figure 2. Concept of Cooperative Control Algorithm
16 KSEA LETTERS Vol. 40 No. 3 April 2012