Deer, moose, and elk don't usually follow the rules of the road. Wandering across a highway, they risk their own lives--as well as those of drivers. With the number of vehicular collisions caused by large wildlife on the rise in the United States, some states are looking to the latest sensor technology to solve the problem.

Researchers at the Western Transportation Institute at Montana State University (MSU) have been testing a number of different approaches for sites with a high intensity of wildlife-vehicle collisions, including one developed by Sensor Technologies and Systems (STS) of Scottsdale, AZ.

The STS system, called the Roadside Animal Detection System (RADS), uses radio sensors to detect large animals approaching a roadway. If one gets too near a road, the sensor activates a warning signal, alerting drivers to be cautious and slow down.

"Animal-vehicle collisions are a growing problem, with growing costs," says Marcel Huijser, head researcher on the RADS project at MSU. In fact, according to the Wildlife Society Bulletin, a quarterly scientific journal, such collisions cause tens of thousands of injuries and more than $1 billion in property damage in the United States each year.

The researchers at MSU have been testing RADS on a highway in Yellowstone National Park for the last four years, as part of a joint pilot project with STS to develop a cost-effective system for reliably detecting large animals.

The RADS design is based on the "break-beam" principle. A pair of sensors--a transmitter and a receiver--is attached to poles up to one-quarter of a mile apart. A low-power microwave radio signal (around 35.5 GHz) is sent between them. When an animal blocks the radio "beam," the receiver signal output is diminished, indicating a detection. This event triggers flashing amber warning lights, which alert approaching drivers.

The detection requires a clear line of sight between the transmitter and receiver. Thus, roadways with curves, slopes, or other "blind spots" will need extra sensors and may require more maintenance. Also, signal interference from tall, wet, or moving vegetation and large vehicles such as 18-wheelers and buses can cause failures and false detections, if the radio signal reflects off these objects and breaks the beam.

"If the driver's confidence in warning signals is eroded because of alarms being constantly triggered, the driver won't respond," says head researcher Huijser. "But, even more importantly, you absolutely cannot have any false negatives--something RADS had very few of."

By evaluating changes in beam intensity, the researchers were better able to determine when the system was actually detecting an animal. These "signal signatures" allowed developers to change the computed thresholds for detection in the digital signal processor and create software filters. While this has greatly reduced the number of false positives, the researchers plan to continue studying the effectiveness of the RADS system.

According to STS developer Terry Wilson, RADS is a lower-cost solution compared with other available methods, such as the building of overpasses for wildlife. And, unlike animal fencing, the RADS system doesn't interfere with migratory routes. "The cost of RADS continues to be lowered as technology continues to shrink and the equipment needed becomes more power efficient," says Wilson.

A collision with a deer costs an average $7,890, according to the MSU researchers. This includes vehicle repairs, injuries to humans, the value of an animal, and the cost of disposing of carcasses. An accident involving an elk can run an average $17,000; for moose, the figure rises to $28,000.