The use of non-embedded traffic sensors for freeway management, signal control, and traffic monitoring continues to expand. Many different technologies are available. The line up for non-embedded traffic sensors typically fall into two categories: active sensors and passive sensors. Active sensors send out a signal and receive a response or echo of the signal. These types of sensors include radar based sensors, active infrared sensors, ultra-sonic sensors, and even scanning lasers. Passive sensors detect the environment being monitored without sending a signal. These types of sensors include cameras, acoustic sensors, and passive infrared sensors. Most of these sensors are intended to replace embedded loops in the roadway. Embedded loops are widely used but have many operational and maintenance issues. If you compare the life cycle costs of embedded loops to non-embedded sensors, the embedded loops can be much more expensive. If you only compare the capital cost of loops to non-embedded sensors, loops can appear to be cheaper. The significant cost savings in using non-embedded sensors comes on the operations and maintenance side.
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