Battery-Saving Communication Modes for Wireless Freeway Traffic Sensors

Efficient management of a freeway network requires continuous decision making on the basis of measured traffic conditions. These measurements usually come from fixed-point sensors deployed in a manner that requires communication links that are always on and are polled at regular intervals. Most of the time, when the sensor data are received at a traffic management center (TMC), no action is taken in response to the information. The high power consumption for the frequent transmissions becomes burdensome for wireless sensors that rely on batteries to last through periods without sufficient illumination for solar power or for the entire life span of the sensor if it has no external power supply. Radio transmissions are a large power draw, so each transmission that can be avoided directly translates into longer battery life. To reduce communication frequency without a significant loss in the quality of information, a distributed freeway surveillance system was developed that prefilters the data at the sensor unit. Five communication modes were developed that assess the value of the measurements before the decision is made to send or not send the measurements. Key to this event-driven approach is that the receiving end is an intelligent part of the distributed surveillance system (i.e., given the lack of transmission, the TMC will know how to interpret the evolving traffic state at the sensor location in the context of previously received information).