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Street traffic system model to help alleviate bottlenecks

06 Nov 2013

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Massachusetts Institute of Technology professor Berthold Horn used a damped-wave equation to create a framework for an instable traffic system. He also developed an algorithm, which can be considered a Lyapunov function, to address such a traffic situation.

The MIT professor presented the algorithm at the recent IEEE Conference on Intelligent Transport Systems. According to Horn, who is employed at MITs Department of Electrical Engineering and Computer Science, such instabilities arise from sudden brake manoeuvres in dense but otherwise flowing traffic. If one driver brakes, the following driver has to brake even stronger to avoid a collision, and thus the velocity variations build up upstream along a lane of traffic. This effect characterises a chaotic system where positive feedback generates velocity oscillations, according to Horn. Even Adaptive Cruise Control systems (ACC) cannot avoid the occurrence of such instabilities.

Horn's algorithm, in terms of maths, can be described as a Lyapunov function. Variables in this function are driver's reaction times, their desired speed, and their eagerness to reach that travelling speed, which in turn translates into the speed at which they accelerate as soon as they see gaps in front of them.

The algorithm could be implemented in ACC systems. However, to be effective, they will have to be modified. While today's ACCs only measure and control the distance to the vehicle ahead, a system based on Horn's algorithm would also have to take into account the distance to the following car. This means that the vehicles' sensor landscape would have to be extended by an additional backwards-looking radar or lidar sensor. Or, since Horn is expert in computer vision, by a camera-based, backward-looking system.

Horn's system, however, has a downside. It only functions if a high percentage of vehicles are equipped with it. AAC systems still are used predominately to premium class vehicles – radar and lidar sensors still are too expensive to deploy in more affordable vehicles. To overcome this chicken-and-egg problem, Horn suggests that the backward-looking sensor could be implemented as a video camera. Such cameras, Horn said, are readily available at affordable prices.




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